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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Torbutton Design Documentation</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /></head><body><div class="article" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="design"></a>Torbutton Design Documentation</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Mike</span> <span class="surname">Perry</span></h3><div class="affiliation"><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:mikeperry.fscked/org">mikeperry.fscked/org</a>&gt;</code></p></div></div></div></div><div><p class="pubdate">July 4 2008</p></div></div><hr /></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="#id2970568">1. Introduction</a></span></dt><dd><dl><dt><span class="sect2"><a href="#adversary">1.1. Adversary Model</a></span></dt><dt><span class="sect2"><a href="#requirements">1.2. Torbutton Requirements</a></span></dt><dt><span class="sect2"><a href="#layout">1.3. Extension Layout</a></span></dt></dl></dd><dt><span class="sect1"><a href="#id2980698">2. Components</a></span></dt><dd><dl><dt><span class="sect2"><a href="#id3000781">2.1. Hooked Components</a></span></dt><dt><span class="sect2"><a href="#id2988472">2.2. New Components</a></span></dt></dl></dd><dt><span class="sect1"><a href="#id2981568">3. Chrome</a></span></dt><dd><dl><dt><span class="sect2"><a href="#browseroverlay">3.1. Browser Overlay - torbutton.xul</a></span></dt><dt><span class="sect2"><a href="#id2984229">3.2. Preferences Window - preferences.xul</a></span></dt><dt><span class="sect2"><a href="#id2988730">3.3. Other Windows</a></span></dt></dl></dd><dt><span class="sect1"><a href="#id2986171">4. Toggle Code Path</a></span></dt><dd><dl><dt><span class="sect2"><a href="#id2990959">4.1. Button Click</a></span></dt><dt><span class="sect2"><a href="#id2984082">4.2. Proxy Update</a></span></dt><dt><span class="sect2"><a href="#id3001325">4.3. Settings Update</a></span></dt></dl></dd><dt><span class="sect1"><a href="#id2984248">5. Description of Options</a></span></dt><dd><dl><dt><span class="sect2"><a href="#id2980079">5.1. Test Settings</a></span></dt><dt><span class="sect2"><a href="#plugins">5.2. Disable plugins on Tor Usage (crucial)</a></span></dt><dt><span class="sect2"><a href="#id2978605">5.3. Isolate Dynamic Content to Tor State (crucial)</a></span></dt><dt><span class="sect2"><a href="#jshooks">5.4. Hook Dangerous Javascript (crucial)</a></span></dt><dt><span class="sect2"><a href="#id2992126">5.5. Resize windows to multiples of 50px during Tor usage (recommended)</a></span></dt><dt><span class="sect2"><a href="#id3004184">5.6. Disable Updates During Tor (recommended)</a></span></dt><dt><span class="sect2"><a href="#id2997514">5.7. Disable Search Suggestions during Tor (recommended)</a></span></dt><dt><span class="sect2"><a href="#id3000110">5.8. Block Tor/Non-Tor access to network from file:// urls (recommended)</a></span></dt><dt><span class="sect2"><a href="#id2998307">5.9. Close all Tor/Non-Tor tabs and windows on toggle (optional)</a></span></dt><dt><span class="sect2"><a href="#id2996566">5.10. Isolate Access to History navigation to Tor state (crucial)</a></span></dt><dt><span class="sect2"><a href="#id2998342">5.11. History Access Settings</a></span></dt><dt><span class="sect2"><a href="#id2957709">5.12. Clear History During Tor Toggle (optional)</a></span></dt><dt><span class="sect2"><a href="#id2962370">5.13. Block Password+Form saving during Tor/Non-Tor</a></span></dt><dt><span class="sect2"><a href="#id2962437">5.14. Block Tor disk cache and clear all cache on Tor Toggle</a></span></dt><dt><span class="sect2"><a href="#id2962492">5.15. Block disk and memory cache during Tor</a></span></dt><dt><span class="sect2"><a href="#id2962549">5.16. Clear Cookies on Tor Toggle</a></span></dt><dt><span class="sect2"><a href="#id2962603">5.17. Store Non-Tor cookies in a protected jar</a></span></dt><dt><span class="sect2"><a href="#id2962662">5.18. Store both Non-Tor and Tor cookies in a protected jar (dangerous)</a></span></dt><dt><span class="sect2"><a href="#id2962702">5.19. Manage My Own Cookies (dangerous)</a></span></dt><dt><span class="sect2"><a href="#id2962718">5.20. Disable DOM Storage during Tor usage (crucial)</a></span></dt><dt><span class="sect2"><a href="#id2962826">5.21. Clear HTTP Auth on Tor Toggle (recommended)</a></span></dt><dt><span class="sect2"><a href="#id3005721">5.22. Clear cookies on Tor/Non-Tor shutdown</a></span></dt><dt><span class="sect2"><a href="#id3005775">5.23. Reload cookie jar/clear cookies on Firefox crash</a></span></dt><dt><span class="sect2"><a href="#id3005850">5.24. On crash recovery or session restored startup, restore via: Tor, Non-Tor</a></span></dt><dt><span class="sect2"><a href="#id3005910">5.25. On normal startup, set state to: Tor, Non-Tor, Shutdown State</a></span></dt><dt><span class="sect2"><a href="#id3005958">5.26. Prevent session store from saving Non-Tor/Tor-loaded tabs</a></span></dt><dt><span class="sect2"><a href="#id3006023">5.27. Set user agent during Tor usage (crucial)</a></span></dt><dt><span class="sect2"><a href="#id3006210">5.28. Spoof US English Browser</a></span></dt><dt><span class="sect2"><a href="#id3006297">5.29. Don't send referrer during Tor Usage</a></span></dt><dt><span class="sect2"><a href="#id3006338">5.30. Store SSL/CA Certs in separate jars for Tor/Non-Tor (recommended)</a></span></dt></dl></dd><dt><span class="sect1"><a href="#FirefoxBugs">6. Relevant Firefox Bugs</a></span></dt><dd><dl><dt><span class="sect2"><a href="#FirefoxSecurity">6.1. Bugs impacting security</a></span></dt><dt><span class="sect2"><a href="#FirefoxWishlist">6.2. Bugs blocking functionality</a></span></dt><dt><span class="sect2"><a href="#FirefoxMiscBugs">6.3. Low Priority Bugs</a></span></dt></dl></dd><dt><span class="sect1"><a href="#TestPlan">7. Testing</a></span></dt><dd><dl><dt><span class="sect2"><a href="#Categories">7.1. Single state testing</a></span></dt><dt><span class="sect2"><a href="#id3007257">7.2. Multi-state testing</a></span></dt><dt><span class="sect2"><a href="#id3007328">7.3. Active testing (aka How to Hack Torbutton)</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2970568"></a>1. Introduction</h2></div></div></div><p>

This document describes the goals, operation, and testing procedures of the
Torbutton Firefox extension. It is current as of Torbutton 1.2.0rc5.

</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="adversary"></a>1.1. Adversary Model</h3></div></div></div><p>

A Tor web browser adversary has a number of goals, capabilities, and attack
types that can be used to guide us towards a set of requirements for the
Torbutton extension. Let's start with the goals.

</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2997298"></a>Adversary Goals</h4></div></div></div><div class="orderedlist"><ol type="1"><li><span class="command"><strong>Bypassing proxy settings</strong></span><p>The adversary's primary goal is direct compromise and bypass of 
Tor, causing the user to directly connect to an IP of the adversary's
choosing.</p></li><li><span class="command"><strong>Correlation of Tor vs Non-Tor Activity</strong></span><p>If direct proxy bypass is not possible, the adversary will likely
happily settle for the ability to correlate something a user did via Tor with
their non-Tor activity. This can be done with cookies, cache identifiers,
javascript events, and even CSS. Sometimes the fact that a user uses Tor may
be enough for some authorities.</p></li><li><span class="command"><strong>History disclosure</strong></span><p>
The adversary may also be interested in history disclosure: the ability to
query a user's history to see if they have issued certain censored search
queries, or visited censored sites.
     </p></li><li><span class="command"><strong>Location information</strong></span><p>

Location information such as timezone and locality can be useful for the
adversary to determine if a user is in fact originating from one of the
regions they are attempting to control, or to zero-in on the geographical
location of a particular dissident or whistleblower.

</p></li><li><span class="command"><strong>Miscellaneous anonymity set reduction</strong></span><p>

Anonymity set reduction is also useful in attempting to zero in on a
particular individual. If the dissident or whistleblower is using a rare build
of Firefox for an obscure operating system, this can be very useful
information for tracking them down, or at least <a class="link" href="#fingerprinting">tracking their activities</a>.

</p></li><li><span class="command"><strong>History records and other on-disk
information</strong></span><p>
In some cases, the adversary may opt for a heavy-handed approach, such as
seizing the computers of all Tor users in an area (especially after narrowing
the field by the above two pieces of information). History records and cache
data are the primary goals here.
     </p></li></ol></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2970954"></a>Adversary Capabilities - Positioning</h4></div></div></div><p>
The adversary can position themselves at a number of different locations in
order to execute their attacks.
    </p><div class="orderedlist"><ol type="1"><li><span class="command"><strong>Exit Node or Upstream Router</strong></span><p>
The adversary can run exit nodes, or alternatively, they may control routers
upstream of exit nodes. Both of these scenarios have been observed in the
wild.
     </p></li><li><span class="command"><strong>Adservers and/or Malicious Websites</strong></span><p>
The adversary can also run websites, or more likely, they can contract out
ad space from a number of different adservers and inject content that way. For
some users, the adversary may be the adservers themselves. It is not
inconceivable that adservers may try to subvert or reduce a user's anonymity 
through Tor for marketing purposes.
     </p></li><li><span class="command"><strong>Local Network/ISP/Upstream Router</strong></span><p>
The adversary can also inject malicious content at the user's upstream router
when they have Tor disabled, in an attempt to correlate their Tor and Non-Tor
activity.
     </p></li><li><span class="command"><strong>Physical Access</strong></span><p>
Some users face adversaries with intermittent or constant physical access.
Users in Internet cafes, for example, face such a threat. In addition, in
countries where simply using tools like Tor is illegal, users may face
confiscation of their computer equipment for excessive Tor usage or just
general suspicion.
     </p></li></ol></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2972854"></a>Adversary Capabilities - Attacks</h4></div></div></div><p>
The adversary can perform the following attacks from a number of different 
positions to accomplish various aspects of their goals.
    </p><div class="orderedlist"><ol type="1"><li><span class="command"><strong>Inserting Javascript</strong></span><p>
Javascript allows the adversary the opportunity to accomplish a number of
their goals. If not properly disabled, Javascript event handlers and timers
can cause the browser to perform network activity after Tor has been disabled,
thus allowing the adversary to correlate Tor and Non-Tor activity. Javascript
also allows the adversary to execute <a class="ulink" href="http://gemal.dk/browserspy/css.html" target="_top">history disclosure attacks</a>:
to query the history via the different attributes of 'visited' links. Finally,
Javascript can be used to query the user's timezone via the
<code class="function">Date()</code> object, and to reduce the anonymity set by querying
the <code class="function">navigator</code> object for operating system, CPU, and user
agent information.
     </p></li><li><span class="command"><strong>Inserting Plugins</strong></span><p>

Plugins are abysmal at obeying the proxy settings of the browser. Every plugin
capable of performing network activity that the author has
investigated is also capable of performing network activity independent of
browser proxy settings - and often independent of its own proxy settings.
In addition, plugins can be used to store unique identifiers that are more
difficult to clear than standard cookies. 
<a class="ulink" href="http://epic.org/privacy/cookies/flash.html" target="_top">Flash-based
cookies</a> fall into this category, but there are likely numerous other
examples.

</p></li><li><span class="command"><strong>Inserting CSS</strong></span><p>

CSS can also be used to correlate Tor and Non-Tor activity, via the usage of
<a class="ulink" href="http://www.tjkdesign.com/articles/css%20pop%20ups/" target="_top">CSS
popups</a> - essentially CSS-based event handlers that fetch content via
CSS's onmouseover attribute. If these popups are allowed to perform network
activity in a different Tor state than they were loaded in, they can easily
correlate Tor and Non-Tor activity and reveal a user's IP address. In
addition, CSS can also be used without Javascript to perform <a class="ulink" href="http://ha.ckers.org/weird/CSS-history.cgi" target="_top">CSS-only history disclosure
attacks</a>.
     </p></li><li><span class="command"><strong>Read and insert cookies</strong></span><p>

An adversary in a position to perform MITM content alteration can inject
document content elements to both read and inject cookies for
arbitrary domains. In fact, many "SSL secured" websites are vulnerable to this
sort of <a class="ulink" href="http://seclists.org/bugtraq/2007/Aug/0070.html" target="_top">active
sidejacking</a>.

</p></li><li><span class="command"><strong>Create arbitrary cached content</strong></span><p>

Likewise, the browser cache can also be used to <a class="ulink" href="http://crypto.stanford.edu/sameorigin/safecachetest.html" target="_top">store unique
identifiers</a>. Since by default the cache has no same-origin policy,
these identifiers can be read by any domain, making them an ideal target for
adserver-class adversaries.

</p></li><li><a id="fingerprinting"></a><span class="command"><strong>Fingerprint users based on browser
attributes</strong></span><p>

There is an absurd amount of information available to websites via attributes
of the browser. This information can be used to reduce anonymity set, or even
<a class="ulink" href="http://0x000000.com/index.php?i=520&amp;bin=1000001000" target="_top">uniquely
fingerprint individual users</a>. </p><p>
For illustration, let's perform a
back-of-the-envelope calculation on the number of anonymity sets for just the
resolution information available in the <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window" target="_top">window</a> and
<a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.screen" target="_top">window.screen</a>
objects. Browser window resolution information provides something like
(1280-640)*(1024-480)=348160 different anonymity sets. Desktop resolution
information contributes about another factor of 5 (for about 5 resolutions in
typical use). In addition, the dimensions and position of the desktop taskbar
are available, which can reveal hints on OS information. This boosts the count
by a factor of 5 (for each of the major desktop taskbars - Windows, OSX, KDE
and Gnome, and None). Subtracting the browser content window
size from the browser outer window size provide yet more information.
Firefox toolbar presence gives about a factor of 8 (3 toolbars on/off give
2<sup>3</sup>=8). Interface effects such as titlebar fontsize
and window manager settings gives a factor of about 9 (say 3 common font sizes
for the titlebar and 3 common sizes for browser GUI element fonts).
Multiply this all out, and you have (1280-640)*(1024-480)*5*5*8*9 ~=
2<sup>29</sup>, or a 29 bit identifier based on resolution
information alone. </p><p>

Of course, this space is non-uniform and prone to incremental changes.
However, if a bit vector space consisting of the above extracted attributes
were used instead of the hash approach from <a class="ulink" href="http://0x000000.com/index.php?i=520&amp;bin=1000001000" target="_top">The Hacker
Webzine article above</a>, minor changes in browser window resolution will
no longer generate totally new identifiers.

</p><p>

To add insult to injury, <a class="ulink" href="http://pseudo-flaw.net/content/tor/torbutton/" target="_top">chrome URL disclosure
attacks</a> mean that each and every extension on <a class="ulink" href="https://addons.mozilla.org" target="_top">addons.mozilla.org</a> adds another bit
to that 2<sup>29</sup>. With hundreds of popular extensions
and thousands of extensions total, it is easy to see that this sort of
information is an impressively powerful identifier if used properly by a
competent and determined adversary such as an ad network.  Again, a
nearest-neighbor bit vector space approach here would also gracefully handle
incremental changes to installed extensions.

</p></li><li><span class="command"><strong>Remotely or locally exploit browser and/or
OS</strong></span><p>
Last, but definitely not least, the adversary can exploit either general 
browser vulnerabilities, plugin vulnerabilities, or OS vulnerabilities to
install malware and surveillance software. An adversary with physical access
can perform similar actions. Regrettably, this last attack capability is
outside of Torbutton's ability to defend against, but it is worth mentioning
for completeness.
     </p></li></ol></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="requirements"></a>1.2. Torbutton Requirements</h3></div></div></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>

Since many settings satisfy multiple requirements, this design document is
organized primarily by Torbutton components and settings. However, if you are
the type that would rather read the document from the requirements
perspective, it is in fact possible to search for each of the following
requirement phrases in the text to find the relevant features that help meet
that requirement.

</div><p>

From the above Adversary Model, a number of requirements become clear.

</p><div class="orderedlist"><ol type="1"><li><a id="proxy"></a><span class="command"><strong>Proxy Obedience</strong></span><p>The browser
MUST NOT bypass Tor proxy settings for any content.</p></li><li><a id="isolation"></a><span class="command"><strong>Network Isolation</strong></span><p>Pages MUST NOT perform any network activity in a Tor state different
 from the state they were originally loaded in.</p></li><li><a id="state"></a><span class="command"><strong>State Separation</strong></span><p>Browser state (cookies, cache, history, 'DOM storage'), accumulated in
 one Tor state MUST NOT be accessible via the network in
 another Tor state.</p></li><li><a id="undiscoverability"></a><span class="command"><strong>Tor Undiscoverability</strong></span><p>With
the advent of bridge support in Tor 0.2.0.x, there are now a class of Tor
users whose network fingerprint does not obviously betray the fact that they
are using Tor. This should extend to the browser as well - Torbutton MUST NOT 
reveal its presence while Tor is disabled.</p></li><li><a id="disk"></a><span class="command"><strong>Disk Avoidance</strong></span><p>The browser SHOULD NOT write any Tor-related state to disk, or store it
 in memory beyond the duration of one Tor toggle.</p></li><li><a id="location"></a><span class="command"><strong>Location Neutrality</strong></span><p>The browser SHOULD NOT leak location-specific information, such as
 timezone or locale via Tor.</p></li><li><a id="setpreservation"></a><span class="command"><strong>Anonymity Set
Preservation</strong></span><p>The browser SHOULD NOT leak any other anonymity set reducing information 
 (such as user agent, extension presence, and resolution information)
automatically via Tor. The assessment of the attacks above should make it clear
that anonymity set reduction is a very powerful method of tracking and
eventually identifying anonymous users.
</p></li><li><a id="updates"></a><span class="command"><strong>Update Safety</strong></span><p>The browser
SHOULD NOT perform unauthenticated updates or upgrades via Tor.</p></li><li><a id="interoperate"></a><span class="command"><strong>Interoperability</strong></span><p>Torbutton SHOULD interoperate with third-party proxy switchers that
 enable the user to switch between a number of different proxies. It MUST
 provide full Tor protection in the event a third-party proxy switcher has
 enabled the Tor proxy settings.</p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="layout"></a>1.3. Extension Layout</h3></div></div></div><p>Firefox extensions consist of two main categories of code: 'Components' and
'Chrome'. Components are a fancy name for classes that implement a given
interface or interfaces. In Firefox, components <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/creatingcomps.html" target="_top">can be
written</a> in C++,
Javascript, or a mixture of both. Components have two identifiers: their
'<a class="ulink" href="http://www.mozilla.org/projects/xpcom/book/cxc/html/quicktour2.html#1005005" target="_top">Contract
ID</a>' (a human readable path-like string), and their '<a class="ulink" href="http://www.mozilla.org/projects/xpcom/book/cxc/html/quicktour2.html#1005329" target="_top">Class
ID</a>' (a GUID hex-string). In addition, the interfaces they implement each have a hex
'Interface ID'. It is possible to 'hook' system components - to reimplement
their interface members with your own wrappers - but only if the rest of the
browser refers to the component by its Contract ID. If the browser refers to
the component by Class ID, it bypasses your hooks in that use case.
Technically, it may be possible to hook Class IDs by unregistering the
original component, and then re-registering your own, but this relies on
obsolete and deprecated interfaces and has proved to be less than
stable.</p><p>'Chrome' is a combination of XML and Javascript used to describe a window.
Extensions are allowed to create 'overlays' that are 'bound' to existing XML
window definitions, or they can create their own windows. The DTD for this XML
is called <a class="ulink" href="http://developer.mozilla.org/en/docs/XUL_Reference" target="_top">XUL</a>.</p></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2980698"></a>2. Components</h2></div></div></div><p>

Torbutton installs components for two purposes: hooking existing components to
reimplement their interfaces; and creating new components that provide
services to other pieces of the extension.
 
  </p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3000781"></a>2.1. Hooked Components</h3></div></div></div><p>Torbutton makes extensive use of Contract ID hooking, and implements some
of its own standalone components as well.  Let's discuss the hooked components
first.</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="sessionstore"></a><a class="ulink" href="http://developer.mozilla.org/en/docs/nsISessionStore" target="_top">@mozilla.org/browser/sessionstore;1</a> -
<a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/nsSessionStore2.js" target="_top">components/nsSessionStore2.js</a>
and <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/nsSessionStore3.js" target="_top">components/nsSessionStore3.js</a></h4></div></div></div><p>These components address the <a class="link" href="#disk">Disk Avoidance</a>
requirements of Torbutton. As stated in the requirements, Torbutton needs to
prevent Tor tabs from being written to disk by the Firefox session store for a
number of reasons, primary among them is the fact that Firefox can crash at
any time, and a restart can cause you to fetch tabs in the incorrect Tor
state.</p><p>These components illustrate a complication with Firefox hooking: you can
only hook member functions of a class if they are published in an
interface that the class implements. Unfortunately, the sessionstore has no
published interface that is amenable to disabling the writing out of Tor tabs
in specific. As such, Torbutton had to include the <span class="emphasis"><em>entire</em></span>
nsSessionStore from both Firefox 2 and Firefox 3, 
with a couple of modifications to prevent tabs that were loaded with Tor
enabled from being written to disk, and some version detection code to
determine which component to load. The <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/nsSessionStore3.diff" target="_top">diff against the original session
store</a> is included in the SVN repository.</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2985696"></a><a class="ulink" href="http://lxr.mozilla.org/seamonkey/source/browser/components/sessionstore/src/nsSessionStartup.js" target="_top">@mozilla.org/browser/sessionstartup;1</a> -
    <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/crash-observer.js" target="_top">components/crash-observer.js</a></h4></div></div></div><p>This component wraps the Firefox Session Startup component that is in
charge of <a class="ulink" href="http://developer.mozilla.org/en/docs/Session_store_API" target="_top">restoring saved
sessions</a>. The wrapper's only job is to intercept the
<code class="function">doRestore()</code> function, which is called by Firefox if it is determined that the
browser crashed and the session needs to be restored. The wrapper notifies the
Torbutton chrome that the browser crashed by setting the pref
<span class="command"><strong>extensions.torbutton.crashed</strong></span>, or that it is a normal
startup via the pref <span class="command"><strong>extensions.torbutton.noncrashed</strong></span>. The Torbutton Chrome <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIPrefBranch2.html#method_addObserver" target="_top">listens for a
preference change</a> for this value and then does the appropriate cleanup. This
includes setting the Tor state to the one the user selected for crash recovery
in the preferences window (<span class="command"><strong>extensions.torbutton.restore_tor</strong></span>), and
restoring cookies for the corresponding cookie jar, if it exists.</p><p>By performing this notification, this component assists in the 
<a class="link" href="#proxy">Proxy Obedience</a>, and <a class="link" href="#isolation">Network Isolation</a> requirements.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2979678"></a><a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_browserglobalhistory2.html" target="_top">@mozilla.org/browser/global-history;2</a>
- <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/ignore-history.js" target="_top">components/ignore-history.js</a></h4></div></div></div><p>This component was contributed by <a class="ulink" href="http://www.collinjackson.com/" target="_top">Collin Jackson</a> as a method for defeating
CSS and Javascript-based methods of history disclosure. The global-history
component is what is used by Firefox to determine if a link was visited or not
(to apply the appropriate style to the link). By hooking the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIGlobalHistory2.html#method_isVisited" target="_top">isVisited</a>
and <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIGlobalHistory2.html#method_addURI" target="_top">addURI</a>
methods, Torbutton is able to selectively prevent history items from being
added or being displayed as visited, depending on the Tor state and the user's
preferences.
</p><p>
This component helps satisfy the <a class="link" href="#state">State Separation</a>
and <a class="link" href="#disk">Disk Avoidance</a> requirements of Torbutton.
</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2988472"></a>2.2. New Components</h3></div></div></div><p>Torbutton creates four new components that are used throughout the
extension. These components do not hook any interfaces, nor are they used
anywhere besides Torbutton itself.</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2981164"></a><a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cookie-jar-selector.js" target="_top">@stanford.edu/cookie-jar-selector;2
- components/cookie-jar-selector.js</a></h4></div></div></div><p>The cookie jar selector (also based on code from <a class="ulink" href="http://www.collinjackson.com/" target="_top">Collin
Jackson</a>) is used by the Torbutton chrome to switch between
Tor and Non-Tor cookies. Its operations are simple: sync cookies to disk, then
move the current cookies.txt file to the appropriate backup location
(cookies-tor.txt or cookies-nontor.txt), and then moving the other cookie jar
into place.</p><p>
This component helps to address the <a class="link" href="#state">State
Isolation</a> requirement of Torbutton.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id2995031"></a><a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/torbutton-logger.js" target="_top">@torproject.org/torbutton-logger;1
- components/torbutton-logger.js</a></h4></div></div></div><p>The torbutton logger component allows on-the-fly redirection of torbutton
logging messages to either Firefox stderr
(<span class="command"><strong>extensions.torbutton.logmethod=0</strong></span>), the Javascript error console
(<span class="command"><strong>extensions.torbutton.logmethod=1</strong></span>), or the DebugLogger extension (if
available - <span class="command"><strong>extensions.torbutton.logmethod=2</strong></span>). It also allows you to
change the loglevel on the fly by changing
<span class="command"><strong>extensions.torbutton.loglevel</strong></span> (1-5, 1 is most verbose).
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="windowmapper"></a><a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/window-mapper.js" target="_top">@torproject.org/content-window-mapper;1
- components/window-mapper.js</a></h4></div></div></div><p>Torbutton tags Firefox <a class="ulink" href="http://www.xulplanet.com/references/elemref/ref_tabbrowser.html" target="_top">tabs</a> with a special variable that indicates the Tor
state the tab was most recently used under to fetch a page. The problem is
that for many Firefox events, it is not possible to determine the tab that is
actually receiving the event. The Torbutton window mapper allows the Torbutton
chrome and other components to look up a <a class="ulink" href="http://www.xulplanet.com/references/elemref/ref_tabbrowser.html" target="_top">browser
tab</a> for a given <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIDOMWindow.html" target="_top">HTML content
window</a>. It does this by traversing all windows and all browsers, until it
finds the browser with the requested <a class="ulink" href="http://www.xulplanet.com/references/elemref/ref_browser.html#prop_contentWindow" target="_top">contentWindow</a> element. Since the content policy
and page loading in general can generate hundreds of these lookups, this
result is cached inside the component.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="contentpolicy"></a><a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cssblocker.js" target="_top">@torproject.org/cssblocker;1
- components/cssblocker.js</a></h4></div></div></div><p>This is a key component to Torbutton's security measures. When Tor is
toggled, Javascript is disabled, and pages are instructed to stop loading.
However, CSS is still able to perform network operations by loading styles for
onmouseover events and other operations. In addition, favicons can still be
loaded by the browser. The cssblocker component prevents this by implementing
and registering an <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIContentPolicy.html" target="_top">nsIContentPolicy</a>.
When an nsIContentPolicy is registered, Firefox checks every attempted network
request against its <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIContentPolicy.html#method_shouldLoad" target="_top">shouldLoad</a>
member function to determine if the load should proceed. In Torbutton's case,
the content policy looks up the appropriate browser tab using the <a class="link" href="#windowmapper" title="@torproject.org/content-window-mapper;1 - components/window-mapper.js">window mapper</a>,
and checks that tab's load tag against the current Tor state. If the tab was
loaded in a different state than the current state, the fetch is denied.
Otherwise, it is allowed.</p> This helps to achieve the <a class="link" href="#isolation">Network
Isolation</a> requirements of Torbutton.

<p>In addition, the content policy also blocks website javascript from
<a class="ulink" href="http://pseudo-flaw.net/content/tor/torbutton/" target="_top">querying for
versions and existence of extension chrome</a> while Tor is enabled, and
also masks the presence of Torbutton to website javascript while Tor is
disabled. </p><p>

Finally, some of the work that logically belongs to the content policy is
instead handled by the <span class="command"><strong>torbutton_http_observer</strong></span> and
<span class="command"><strong>torbutton_weblistener</strong></span> in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.js" target="_top">torbutton.js</a>. These two objects handle blocking of
Firefox 3 favicon loads, popups, and full page plugins, which for whatever
reason are not passed to the Firefox content policy itself (see Firefox Bugs 
<a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=437014" target="_top">437014</a> and 
<a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=401296" target="_top">401296</a>).

</p><p>

This helps to fulfill both the <a class="link" href="#setpreservation">Anonymity Set Preservation</a> and the <a class="link" href="#undiscoverability">Tor Undiscoverability</a> requirements of
Torbutton.</p></div></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2981568"></a>3. Chrome</h2></div></div></div><p>The chrome is where all the torbutton graphical elements and windows are
located. Each window is described as an <a class="ulink" href="http://developer.mozilla.org/en/docs/XUL_Reference" target="_top">XML file</a>, with zero or more Javascript
files attached. The scope of these Javascript files is their containing
window.</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="browseroverlay"></a>3.1. Browser Overlay - <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.xul" target="_top">torbutton.xul</a></h3></div></div></div><p>The browser overlay, torbutton.xul, defines the toolbar button, the status
bar, and events for toggling the button. The overlay code is in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.js" target="_top">chrome/content/torbutton.js</a>.
It contains event handlers for preference update, shutdown, upgrade, and
location change events.</p><p>The <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_docloaderservice1.html" target="_top">location
change</a> <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIWebProgressListener.html" target="_top">webprogress
listener</a>, <span class="command"><strong>torbutton_weblistener</strong></span> is perhaps the
most important part of the chrome from a security standpoint. It is a <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIWebProgressListener.html" target="_top">web
progress listener</a> that handles
receiving an event every time a page load or iframe load occurs. This class
eventually calls down to <code class="function">torbutton_update_tags()</code> and 
<code class="function">torbutton_hookdoc()</code>, which apply the browser Tor load state tags, plugin
permissions, and install the Javascript hooks to hook the <a class="ulink" href="http://phrogz.net/objJob/object.asp?id=224" target="_top">Date</a> object and
the <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.navigator" target="_top">navigator</a> object (for timezone and platform information,
respectively).</p><p>
The browser overlay helps to satisfy a number of Torbutton requirements. These
are better enumerated in each of the Torbutton preferences below. However,
there are also a number of Firefox preferences set in
<code class="function">torbutton_update_status()</code> that aren't governed by any
Torbutton setting. These are:
</p><div class="orderedlist"><ol type="1"><li><a class="ulink" href="http://kb.mozillazine.org/Browser.bookmarks.livemark_refresh_seconds" target="_top">browser.bookmarks.livemark_refresh_seconds</a><p>
This pref is set in an attempt to disable the fetching of LiveBookmarks via
Tor. Since users can potentially collect a large amount of live bookmarks to
very personal sites (blogs of friends, wikipedia articles they maintain,
comment feeds of their own blog), it is not possible to cleanly isolate these
fetches and they are simply disabled during Tor usage.
This helps to address the <a class="link" href="#state">State Separation</a> requirement.
Unfortunately <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=436250" target="_top">Firefox Bug
436250</a> prevents this from
functioning completely correctly.
</p></li><li><a class="ulink" href="http://kb.mozillazine.org/Network.security.ports.banned" target="_top">network.security.ports.banned</a><p>
Torbutton sets this setting to add ports 8123, 8118, 9050 and 9051 (which it
reads from <span class="command"><strong>extensions.torbutton.banned_ports</strong></span>) to the list
of ports Firefox is forbidden to access. These ports are Polipo, Privoxy, Tor,
and the Tor control port, respectively. This is set for both Tor and Non-Tor
usage, and prevents websites from attempting to do http fetches from these
ports to see if they are open, which addresses the <a class="link" href="#undiscoverability">Tor Undiscoverability</a> requirement.
 </p></li><li><a class="ulink" href="http://kb.mozillazine.org/Browser.send_pings" target="_top">browser.send_pings</a><p>
This setting is currently always disabled. If anyone ever complains saying
that they *want* their browser to be able to send ping notifications to a
page or arbitrary link, I'll make this a pref or Tor-only. But I'm not holding
my breath. I haven't checked if the content policy is called for pings, but if
not, this setting helps with meeting the <a class="link" href="#isolation">Network
Isolation</a> requirement.
 </p></li><li><a class="ulink" href="http://kb.mozillazine.org/Browser.safebrowsing.remoteLookups" target="_top">browser.safebrowsing.remoteLookups</a><p>
Likewise for this setting. I find it hard to imagine anyone who wants to ask
Google in real time if each URL they visit is safe, especially when the list
of unsafe URLs is downloaded anyway. This helps fulfill the <a class="link" href="#disk">Disk Avoidance</a> requirement, by preventing your entire
browsing history from ending up on Google's disks.
 </p></li><li><a class="ulink" href="http://kb.mozillazine.org/Browser.safebrowsing.enabled" target="_top">browser.safebrowsing.enabled</a><p>
Safebrowsing does <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=360387" target="_top">unauthenticated
updates under Firefox 2</a>, so it is disabled during Tor usage. 
This helps fulfill the <a class="link" href="#updates">Update
Safety</a> requirement. Firefox 3 has the fix for that bug, and so
safebrowsing updates are enabled during Tor usage.
 </p></li><li><a class="ulink" href="http://kb.mozillazine.org/Network.protocol-handler.warn-external.%28protocol%29" target="_top">network.protocol-handler.warn-external.(protocol)</a><p>
If Tor is enabled, we need to prevent random external applications from
launching without at least warning the user. This group of settings only
partially accomplishes this, however. Applications can still be launched via
plugins. The mechanisms for handling this are described under the "Disable
Plugins During Tor Usage" preference. This helps fulfill the <a class="link" href="#proxy">Proxy Obedience</a> requirement, by preventing external
applications from accessing network resources at the command of Tor-fetched
pages.
 </p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2984229"></a>3.2. Preferences Window - <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/preferences.xul" target="_top">preferences.xul</a></h3></div></div></div><p>The preferences window of course lays out the Torbutton preferences, with
handlers located in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/preferences.js" target="_top">chrome/content/preferences.js</a>.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2988730"></a>3.3. Other Windows</h3></div></div></div><p>There are additional windows that describe popups for right clicking on
the status bar, the toolbutton, and the about page.</p></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2986171"></a>4. Toggle Code Path</h2></div></div></div><p>

The act of toggling is connected to <code class="function">torbutton_toggle()</code>
via the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.xul" target="_top">torbutton.xul</a>
and <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/popup.xul" target="_top">popup.xul</a>
overlay files. Most of the work in the toggling process is present in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.js" target="_top">torbutton.js</a>

</p><p>

Toggling is a 3 stage process: Button Click, Proxy Update, and
Settings Update. These stages are reflected in the prefs
<span class="command"><strong>extensions.torbutton.tor_enabled</strong></span>,
<span class="command"><strong>extensions.torbutton.proxies_applied</strong></span>, and
<span class="command"><strong>extensions.torbutton.settings_applied</strong></span>. The reason for the
three stage preference update is to ensure immediate enforcement of <a class="link" href="#isolation">Network Isolation</a> via the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a>. Since the content window
javascript runs on a different thread than the chrome javascript, it is
important to properly convey the stages to the content policy to avoid race
conditions and leakage, especially with <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409737" target="_top">Firefox Bug 
409737</a> unfixed. The content policy does not allow any network activity
whatsoever during this three stage transition.

</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2990959"></a>4.1. Button Click</h3></div></div></div><p>

This is the first step in the toggling process. When the user clicks the
toggle button or the toolbar, <code class="function">torbutton_toggle()</code> is
called. This function checks the current Tor status by comparing the current
proxy settings to the selected Tor settings, and then sets the proxy settings
to the opposite state, and sets the pref
<span class="command"><strong>extensions.torbutton.tor_enabled</strong></span> to reflect the new state.
It is this proxy pref update that gives notification via the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIPrefBranch2.html#method_addObserver" target="_top">pref
observer</a>
<span class="command"><strong>torbutton_unique_pref_observer</strong></span> to perform the rest of the
toggle.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2984082"></a>4.2. Proxy Update</h3></div></div></div><p>

When Torbutton receives any proxy change notifications via its
<span class="command"><strong>torbutton_unique_pref_observer</strong></span>, it calls
<code class="function">torbutton_set_status()</code> which checks against the Tor
settings to see if the Tor proxy settings match the current settings. If so,
it calls <code class="function">torbutton_update_status()</code>, which determines if
the Tor state has actually changed, and sets
<span class="command"><strong>extensions.torbutton.proxies_applied</strong></span> to the appropriate Tor
state value, and ensures that
<span class="command"><strong>extensions.torbutton.tor_enabled</strong></span> is also set to the correct
value. This is decoupled from the button click functionalty via the pref
observer so that other addons (such as SwitchProxy) can switch the proxy
settings between multiple proxies.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3001325"></a>4.3. Settings Update</h3></div></div></div><p>

The next stage is also handled by
<code class="function">torbutton_update_status()</code>. This function sets scores of
Firefox preferences, saving the original values to prefs under
<span class="command"><strong>extensions.torbutton.saved.*</strong></span>, and performs the history
clearing, cookie jaring, and ssl certificate jaring work of Torbutton. At the
end of its work, it sets
<span class="command"><strong>extensions.torbutton.settings_applied</strong></span>, which signifies the
completion of the toggle operation to the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a>.

</p></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2984248"></a>5. Description of Options</h2></div></div></div><p>This section provides a detailed description of Torbutton's options. Each
option is presented as the string from the preferences window, a summary, the
preferences it touches, and the effect this has on the components, chrome, and
browser properties.</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2980079"></a>5.1. Test Settings</h3></div></div></div><p>
This button under the Proxy Settings tab provides a way to verify that the 
proxy settings are correct, and actually do route through the Tor network. It
performs this check by issuing an <a class="ulink" href="http://developer.mozilla.org/en/docs/XMLHttpRequest" target="_top">XMLHTTPRequest</a>
for <a class="ulink" href="https://check.torproject.org/?TorButton=True" target="_top">https://check.torproject.org/?Torbutton=True</a>.
This is a special page that returns very simple, yet well-formed XHTML that
Torbutton can easily inspect for a hidden link with an id of
<span class="command"><strong>TorCheckResult</strong></span> and a target of <span class="command"><strong>success</strong></span>
or <span class="command"><strong>failure</strong></span> to indicate if the
user hit the page from a Tor IP, a non-Tor IP. This check is handled in
<code class="function">torbutton_test_settings()</code> in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.js" target="_top">torbutton.js</a>.
Presenting the results to the user is handled by the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/preferences.xul" target="_top">preferences
window</a>
callback <code class="function">torbutton_prefs_test_settings()</code> in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/preferences.js" target="_top">preferences.js</a>.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="plugins"></a>5.2. Disable plugins on Tor Usage (crucial)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.no_tor_plugins</strong></span></p><p>Enabling this preference causes the above mentioned Torbutton chrome web progress
 listener <span class="command"><strong>torbutton_weblistener</strong></span> to disable Java via <span class="command"><strong>security.enable_java</strong></span> and to disable
 plugins via the browser <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIDocShell.html" target="_top">docShell</a>
 attribute <span class="command"><strong>allowPlugins</strong></span>. These flags are set every time a new window is
 created (<code class="function">torbutton_tag_new_browser()</code>), every time a web
load
event occurs
 (<code class="function">torbutton_update_tags()</code>), and every time the tor state is changed
 (<code class="function">torbutton_update_status()</code>). As a backup measure, plugins are also
 prevented from loading by the content policy in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cssblocker.js" target="_top">@torproject.org/cssblocker;1</a> if Tor is
 enabled and this option is set.
 </p><p>Even all this turns out to be insufficient if the user directly
 clicks on a plugin-handled mime-type. <a class="ulink" href="http://www.janusvm.com/goldy/pdf/" target="_top">In this case</a> (and also <a class="ulink" href="http://www.janusvm.com/goldy/side-channels/frames/" target="_top">this
one</a>), the browser decides that
 maybe it should ignore all these other settings and load the plugin anyways,
 because maybe the user really did want to load it (never mind this same
 load-style could happen automatically  with meta-refresh or any number of
 other ways..). To handle these cases, Torbutton stores a list of plugin-handled
 mime-types, and sets the pref
<span class="command"><strong>plugin.disable_full_page_plugin_for_types</strong></span> to this list.
Additionally, (since nothing can be assumed when relying on Firefox
preferences and internals) if it detects a load of one of them from the web progress
 listener, it cancels the request, tells the associated DOMWindow 
to stop loading, clears the document, AND throws an exception. Anything short 
of all this and
 the plugin managed to find some way to load.
 </p><p>
 All this could be avoided, of course, if Firefox would either <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=401296" target="_top">obey
 allowPlugins</a> for directly visited URLs, or notify its content policy for such
 loads either <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=309524" target="_top">via</a> <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=380556" target="_top">shouldProcess</a> or shouldLoad. The fact that it does not is
 not very encouraging. 
 </p><p>

Since most plugins completely ignore browser proxy settings, the actions
performed by this setting are crucial to satisfying the <a class="link" href="#proxy">Proxy Obedience</a> requirement.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2978605"></a>5.3. Isolate Dynamic Content to Tor State (crucial)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.isolate_content</strong></span></p><p>Enabling this preference is what enables the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cssblocker.js" target="_top">@torproject.org/cssblocker;1</a> content policy
mentioned above, and causes it to block content load attempts in pages an
opposite Tor state from the current state. Freshly loaded <a class="ulink" href="http://www.xulplanet.com/references/elemref/ref_tabbrowser.html" target="_top">browser
tabs</a> are tagged 
with a <span class="command"><strong>__tb_load_state</strong></span> member in
<code class="function">torbutton_update_tags()</code> and this
value is compared against the current tor state in the content policy.</p><p>It also kills all Javascript in each page loaded under that state by
toggling the <span class="command"><strong>allowJavascript</strong></span> <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIDocShell.html" target="_top">docShell</a> property, and issues a
<a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIWebNavigation.html#method_stop" target="_top">webNavigation.stop(webNavigation.STOP_ALL)</a> to each browser tab (the
equivalent of hitting the STOP button).</p><p>

Unfortunately, <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409737" target="_top">Firefox bug
409737</a> prevents <span class="command"><strong>docShell.allowJavascript</strong></span> from killing
all event handlers, and event handlers registered with <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:element.addEventListener" target="_top">addEventListener()</a>
are still able to execute. The <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">Torbutton Content
Policy</a> should prevent such code from performing network activity within
the current tab, but activity that happens via a popup window or via a
Javascript redirect can still slip by. For this reason, Torbutton blocks
popups by checking for a valid <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.opener" target="_top">window.opener</a>
attribute in <code class="function">torbutton_check_progress()</code>. If the window
has an opener from a different Tor state, its load is blocked. The content
policy also takes similar action to prevent Javascript redirects. This also
has the side effect/feature of preventing the user from following any links
from a page loaded in an opposite Tor state.

</p><p>
This setting is responsible for satisfying the <a class="link" href="#isolation">Network Isolation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="jshooks"></a>5.4. Hook Dangerous Javascript (crucial)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.kill_bad_js</strong></span></p><p>This setting enables injection of the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/jshooks.js" target="_top">Javascript
hooking code</a>. Javascript is injected into
pages to hook the <a class="ulink" href="http://phrogz.net/objJob/object.asp?id=224" target="_top">Date
class</a> to mask your timezone. This is done in the chrome in
<code class="function">torbutton_hookdoc()</code>, which is called ultimately by both the 
<a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIWebProgressListener.html" target="_top">webprogress
listener</a> <span class="command"><strong>torbutton_weblistener</strong></span> and the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a> (the latter being a hack to handle
javascript: urls). This behavior helps to satisfy the <a class="link" href="#location">Location Neutrality</a> requirement.

</p><p>

In addition, this setting also hooks various resolution properties of the
<a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window" target="_top">window</a>,
<a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.screen" target="_top">window.screen</a>,
and <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.navigator" target="_top">window.navigator</a>
to mask window size information and user agent properties not handled by the
standard Firefox user agent override settings. The resolution hooks
effectively make the Firefox browser window appear to websites as if the renderable area
takes up the entire desktop, has no toolbar or other GUI element space, and
the desktop itself has no toolbars.
These hooks drastically reduce the amount of information available to do <a class="link" href="#fingerprinting">anonymity set reduction attacks</a> and help to
meet the <a class="link" href="#setpreservation">Anonymity Set Preservation</a>
requirements.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2992126"></a>5.5. Resize windows to multiples of 50px during Tor usage (recommended)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.resize_windows</strong></span></p><p>

This option drastically cuts down on the number of distinct anonymity sets
that divide the Tor web userbase. Without this setting, the dimensions for a
typical browser window range from 600-1200 horizontal pixels and 400-1000
vertical pixels, or about 600x600 = 360000 different sets. Resizing the
browser window to multiples of 50 on each side reduces the number of sets by
50^2, bringing the total number of sets to 144. Of course, the distribution
among these sets are not uniform, but scaling by 50 will improve the situation
due to this non-uniformity for users in the less common resolutions.
Obviously the ideal situation would be to lie entirely about the browser
window size, but this will likely cause all sorts of rendering issues, and is
also not implementable in a foolproof way from extension land.

</p><p>

The implementation of this setting is spread across a couple of different
locations in the Torbutton javascript <a class="link" href="#browseroverlay" title="3.1. Browser Overlay - torbutton.xul">browser
overlay</a>. Since resizing minimized windows causes them to be restored,
and since maximized windows remember their previous size to the pixel, windows
must be resized before every document load (at the time of browser tagging)
via <code class="function">torbutton_check_round()</code>, called by
<code class="function">torbutton_update_tags()</code>. To prevent drift, the extension
tracks the original values of the windows and uses this to perform the
rounding on document load. In addition, to prevent the user from resizing a
window to a non-50px multiple, a resize listener
(<code class="function">torbutton_do_resize()</code>) is installed on every new browser
window to record the new size and round it to a 50px multiple while Tor is
enabled. In all cases, the browser's contentWindow.innerWidth and innerHeight
are set. This ensures that there is no discrepancy between the 50 pixel cutoff
and the actual renderable area of the browser (so that it is not possible to
infer toolbar size/presence by the distance to the nearest 50 pixel roundoff).

</p><p>
This setting helps to meet the <a class="link" href="#setpreservation">Anonymity Set Preservation</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3004184"></a>5.6. Disable Updates During Tor (recommended)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.no_updates</strong></span></p><p>This setting causes Torbutton to disable the four <a class="ulink" href="http://wiki.mozilla.org/Update:Users/Checking_For_Updates#Preference_Controls_and_State" target="_top">Firefox
update settings</a> during Tor
  usage: <span class="command"><strong>extensions.update.enabled</strong></span>,
<span class="command"><strong>app.update.enabled</strong></span>,
  <span class="command"><strong>app.update.auto</strong></span>, and
<span class="command"><strong>browser.search.update</strong></span>.  These prevent the
  browser from updating extensions, checking for Firefox upgrades, and
  checking for search plugin updates while Tor is enabled.
  </p><p>
This setting satisfies the <a class="link" href="#updates">Update Safety</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2997514"></a>5.7. Disable Search Suggestions during Tor (recommended)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.no_search</strong></span></p><p>
This setting causes Torbutton to disable <a class="ulink" href="http://kb.mozillazine.org/Browser.search.suggest.enabled" target="_top"><span class="command"><strong>browser.search.suggest.enabled</strong></span></a>
during Tor usage.
This governs if you get Google search suggestions during Tor
usage. Your Google cookie is transmitted with google search suggestions, hence
this is recommended to be disabled.

These settings prevent file urls from performing network operations during the
respective Tor states. Firefox 2's implementation of same origin policy allows
file urls to read and <a class="ulink" href="http://www.gnucitizen.org/blog/content-disposition-hacking/" target="_top">submit
arbitrary files from the local filesystem</a> to arbitrary websites. To
make matters worse, the 'Content-Disposition' header can be injected
arbitrarily by exit nodes to trick users into running arbitrary html files in
the local context. These preferences cause the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a> to block access to any network
resources from File urls during the appropriate Tor state.

</p><p>

This preference helps to ensure Tor's <a class="link" href="#isolation">Network
Isolation</a> requirement, by preventing file urls from executing network
operations in opposite Tor states. Also, allowing pages to submit arbitrary
files to arbitrary sites just generally seems like a bad idea.
 
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2998307"></a>5.9. Close all Tor/Non-Tor tabs and windows on toggle (optional)</h3></div></div></div><p>Options: 
   </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.close_nontor</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.close_tor</strong></span></td></tr></table><p>
  </p><p>

These settings cause Torbutton to enumerate through all windows and close all
tabs in each window for the appropriate Tor state. This code can be found in
<code class="function">torbutton_update_status()</code>.  The main reason these settings
exist is as a backup mechanism in the event of any Javascript or content policy
leaks due to <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409737" target="_top">Firefox Bug
409737</a>.  Torbutton currently tries to block all Javascript network
activity via the content policy, but until that bug is fixed, there is some
risk that there are alternate ways to bypass the policy. This option is
available as an extra assurance of <a class="link" href="#isolation">Network
Isolation</a> for those who would like to be sure that when Tor is toggled
all page activity has ceased. It also serves as a potential future workaround
in the event a content policy failure is discovered, and provides an additional
level of protection for the <a class="link" href="#disk">Disk Avoidance</a>
protection so that browser state is not sitting around waiting to be swapped
out longer than necessary.

</p><p>
While this setting doesn't satisfy any Torbutton requirements, the fact that
cookies are transmitted for partially typed queries does not seem desirable
for Tor usage.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2996566"></a>5.10. Isolate Access to History navigation to Tor state (crucial)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.block_js_history</strong></span></p><p>
This setting determines if Torbutton installs an <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsISHistoryListener.html" target="_top">nsISHistoryListener</a>
attached to the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsISHistory.html" target="_top">sessionHistory</a> of 
of each browser's <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_webshell1.html" target="_top">webNavigatator</a>.
The nsIShistoryListener is instantiated with a reference to the containing
browser window and blocks the back, forward, and reload buttons on the browser
navigation bar when Tor is in an opposite state than the one to load the
current tab. In addition, Tor clears the session history during a new document
load if this setting is enabled.

</p><p>

This is marked as a crucial setting in part
because Javascript access to the history object is indistinguishable from 
user clicks, and because
<a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409737" target="_top">Firefox Bug
409737</a> allows javascript to execute in opposite Tor states, javascript
can issue reloads after Tor toggle to reveal your original IP. Even without
this bug, however, Javascript is still able to access previous pages in your
session history that may have been loaded under a different Tor state, to
attempt to correlate your activity.

</p><p>

This setting helps to fulfill Torbutton's <a class="link" href="#state">State
Separation</a> and (until Bug 409737 is fixed) <a class="link" href="#isolation">Network Isolation</a>
requirements.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2998342"></a>5.11. History Access Settings</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.block_thread</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.block_nthread</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.block_thwrite</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.block_nthwrite</strong></span></td></tr></table><p>
  </p><p>These four settings govern the behavior of the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/ignore-history.js" target="_top">components/ignore-history.js</a>
history blocker component mentioned above. By hooking the browser's view of
the history itself via the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_browserglobalhistory2.html" target="_top">mozilla.org/browser/global-history;2</a>
component, this mechanism defeats all document-based <a class="ulink" href="http://gemal.dk/browserspy/css.html" target="_top">history disclosure
attacks</a>, including <a class="ulink" href="http://ha.ckers.org/weird/CSS-history.cgi" target="_top">CSS-only attacks</a>.
</p><p>

On Firefox 3, the history write settings also govern if Torbutton sets
<span class="command"><strong>browser.history_expire_days</strong></span> to 0 on the appropriate Tor
state, which <a class="ulink" href="http://developer.mozilla.org/en/docs/index.php?title=nsINavHistoryService#Attributes" target="_top">should
disable</a> all <a class="ulink" href="http://developer.mozilla.org/en/docs/Places" target="_top">Places</a> database
writes.

</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> and <a class="link" href="#disk">Disk Avoidance</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2957709"></a>5.12. Clear History During Tor Toggle (optional)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.clear_history</strong></span></p><p>This setting governs if Torbutton calls
<a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIBrowserHistory.html#method_removeAllPages" target="_top">nsIBrowserHistory.removeAllPages</a>
and <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsISHistory.html#method_PurgeHistory" target="_top">nsISHistory.PurgeHistory</a>
for each tab on Tor toggle.</p><p>
This setting is an optional way to help satisfy the <a class="link" href="#state">State Separation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962370"></a>5.13. Block Password+Form saving during Tor/Non-Tor</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.block_tforms</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.block_ntforms</strong></span></td></tr></table><p>
  </p><p>These settings govern if Torbutton disables
<span class="command"><strong>browser.formfill.enable</strong></span>
and <span class="command"><strong>signon.rememberSignons</strong></span> during Tor and Non-Tor usage.
Since form fields can be read at any time by Javascript, this setting is a lot
more important than it seems.
</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> and <a class="link" href="#disk">Disk Avoidance</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962437"></a>5.14. Block Tor disk cache and clear all cache on Tor Toggle</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.clear_cache</strong></span>
  </p><p>This option causes Torbutton to call <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsICacheService.html#method_evictEntries" target="_top">nsICacheService.evictEntries(0)</a>
on Tor toggle to remove all entries from the cache. In addition, this setting
causes Torbutton to set <a class="ulink" href="http://kb.mozillazine.org/Browser.cache.disk.enable" target="_top">browser.cache.disk.enable</a> to false.
</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> and <a class="link" href="#disk">Disk Avoidance</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962492"></a>5.15. Block disk and memory cache during Tor</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.block_cache</strong></span></p><p>This setting
causes Torbutton to set <a class="ulink" href="http://kb.mozillazine.org/Browser.cache.memory.enable" target="_top">browser.cache.memory.enable</a>,
<a class="ulink" href="http://kb.mozillazine.org/Browser.cache.disk.enable" target="_top">browser.cache.disk.enable</a> and
<a class="ulink" href="http://kb.mozillazine.org/Network.http.use-cache" target="_top">network.http.use-cache</a> to false during tor usage.
</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> and <a class="link" href="#disk">Disk Avoidance</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962549"></a>5.16. Clear Cookies on Tor Toggle</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.clear_cookies</strong></span>
  </p><p>

This setting causes Torbutton to call <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsICookieManager.html#method_removeAll" target="_top">nsICookieManager.removeAll()</a> on
every Tor toggle. In addition, this sets <a class="ulink" href="http://kb.mozillazine.org/Network.cookie.lifetimePolicy" target="_top">network.cookie.lifetimePolicy</a>
to 2 for Tor usage, which causes all cookies to be demoted to session cookies,
which prevents them from being written to disk.

This setting causes Torbutton to use <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cookie-jar-selector.js" target="_top">@stanford.edu/cookie-jar-selector;2</a> to store
non-tor cookies in a cookie jar during Tor usage, and clear the Tor cookies
before restoring the jar.
</p><p>
This setting also sets <a class="ulink" href="http://kb.mozillazine.org/Network.cookie.lifetimePolicy" target="_top">network.cookie.lifetimePolicy</a>
to 2 for Tor usage, which causes all cookies to be demoted to session cookies,
which prevents them from being written to disk.

This setting causes Torbutton to use <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cookie-jar-selector.js" target="_top">@stanford.edu/cookie-jar-selector;2</a> to store
both Tor and Non-Tor cookies into protected jars.
</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962702"></a>5.19. Manage My Own Cookies (dangerous)</h3></div></div></div><p>Options: None</p><p>This setting disables all Torbutton cookie handling by setting the above
cookie prefs all to false.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962718"></a>5.20. Disable DOM Storage during Tor usage (crucial)</h3></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962720"></a>5.20.1. Do not write Tor/Non-Tor cookies to disk</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.tor_memory_jar</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.nontor_memory_jar</strong></span></td></tr></table><p>
  </p><p>
These settings (contributed by arno) cause Torbutton to set <a class="ulink" href="http://kb.mozillazine.org/Network.cookie.lifetimePolicy" target="_top">network.cookie.lifetimePolicy</a>
to 2 during the appropriate Tor state, and to store cookies acquired in that
state into a Javascript
<a class="ulink" href="http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Guide:Processing_XML_with_E4X" target="_top">E4X</a>
object as opposed to writing them to disk.
</p><p>
This allows Torbutton to provide an option to preserve a user's 
cookies while still satisfying the <a class="link" href="#disk">Disk Avoidance</a>
requirement.
</p></div><p>Option: <span class="command"><strong>extensions.torbutton.disable_domstorage</strong></span>
  </p><p>

This setting causes Torbutton to toggle <span class="command"><strong>dom.storage.enabled</strong></span> during Tor
usage to prevent 
<a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:Storage" target="_top">DOM Storage</a> from
  being used to store persistent information across Tor states.</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2962826"></a>5.21. Clear HTTP Auth on Tor Toggle (recommended)</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.clear_http_auth</strong></span>
  </p><p>
This setting causes Torbutton to call <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIHttpAuthManager.html#method_clearAll" target="_top">nsIHttpAuthManager.clearAll()</a>
every time Tor is toggled.
</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3005721"></a>5.22. Clear cookies on Tor/Non-Tor shutdown</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.shutdown_method</strong></span>
  </p><p> This option variable can actually take 3 values: 0, 1, and 2. 0 means no
cookie clearing, 1 means clear only during Tor-enabled shutdown, and 2 means
clear for both Tor and Non-Tor shutdown. When set to 1 or 2, Torbutton listens
for the <a class="ulink" href="http://developer.mozilla.org/en/docs/Observer_Notifications#Application_shutdown" target="_top">quit-application-granted</a> event in
<code class="function">torbutton_uninstall_observer()</code> and use <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cookie-jar-selector.js" target="_top">@stanford.edu/cookie-jar-selector;2</a>
to clear out all cookies and all cookie jars upon shutdown.  </p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3005775"></a>5.23. Reload cookie jar/clear cookies on Firefox crash</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.reload_crashed_jar</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.crashed</strong></span></td></tr></table><p>
  </p><p>This is no longer a user visible option, and is enabled by default. In
the event of a crash, the Torbutton <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/crash-observer.js" target="_top">components/crash-observer.js</a> 
  component will notify the Chrome (via the
  <span class="command"><strong>extensions.torbutton.crashed</strong></span> pref and a <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/ifaces/nsIPrefBranch2.html#method_addObserver" target="_top">pref
observer</a> in
the chrome that listens for this update), and Torbutton will load the
  correct jar for the current Tor state via the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/cookie-jar-selector.js" target="_top">@stanford.edu/cookie-jar-selector;2</a>
  component.</p><p>
This setting helps to satisfy the <a class="link" href="#state">State Separation</a> requirement in the event of Firefox
crashes.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3005850"></a>5.24. On crash recovery or session restored startup, restore via: Tor, Non-Tor</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.restore_tor</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.crashed</strong></span></td></tr></table><p>
  </p><p>This option works with the Torbutton <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/crash-observer.js" target="_top">crash-observer.js</a> 
  to set the Tor state after a crash is detected (via the 
  <span class="command"><strong>extensions.torbutton.crashed</strong></span> pref)</p><p>

Since the Tor state after a Firefox crash is unknown/indeterminate, this
setting helps to satisfy the <a class="link" href="#state">State Separation</a>
requirement in the event of Firefox crashes by ensuring all cookies,
settings and saved sessions are reloaded from a fixed Tor state.
 
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3005910"></a>5.25. On normal startup, set state to: Tor, Non-Tor, Shutdown State</h3></div></div></div><p>Options:
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.startup_state</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.noncrashed</strong></span></td></tr></table><p>
  </p><p>This option also works with the Torbutton <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/crash-observer.js" target="_top">crash-observer.js</a> 
  to set the Tor state after a normal startup is detected (via the 
  <span class="command"><strong>extensions.torbutton.noncrashed</strong></span> pref)</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3005958"></a>5.26. Prevent session store from saving Non-Tor/Tor-loaded tabs</h3></div></div></div><p>Options: 
  </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.nonontor_sessionstore</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.notor_sessionstore</strong></span></td></tr></table><p>
  </p><p>If these options are enabled, the <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/components/nsSessionStore3.js" target="_top">replacement nsSessionStore.js</a>
  component checks the <span class="command"><strong>__tb_tor_fetched</strong></span> tag of tabs before writing them
  out. If the tag is from a blocked Tor state, the tab is not written to disk.
  </p><p>
This setting helps to satisfy the <a class="link" href="#disk">Disk Avoidance</a>
requirement, and also helps to satisfy the <a class="link" href="#state">State Separation</a> requirement in the event of Firefox
crashes.

</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3006023"></a>5.27. Set user agent during Tor usage (crucial)</h3></div></div></div><p>Options:
   </p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.set_uagent</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.oscpu_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.platform_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.productsub_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.appname_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.appversion_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.useragent_override</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.useragent_vendor</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.useragent_vendorSub</strong></span></td></tr></table><p>
   </p><p>On face, user agent switching appears to be straight-forward in Firefox.
It provides several options for controlling the browser user agent string:
<span class="command"><strong>general.appname.override</strong></span>,
<span class="command"><strong>general.appversion.override</strong></span>,
<span class="command"><strong>general.platform.override</strong></span>,
<span class="command"><strong>general.useragent.override</strong></span>,
<span class="command"><strong>general.useragent.vendor</strong></span>, and
<span class="command"><strong>general.useragent.vendorSub</strong></span>. If
the Torbutton preference <span class="command"><strong>extensions.torbutton.set_uagent</strong></span> is
true, Torbutton copies all of the other above prefs into their corresponding
browser preferences during Tor usage.</p><p>However, this is not the whole story. Additionally, even with the above
prefs set, the <span class="command"><strong>oscpu</strong></span>, <span class="command"><strong>buildID</strong></span>, and <span class="command"><strong>productSub</strong></span> fields of the
<a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.navigator" target="_top">navigator</a> object are not changed appropriately by the above prefs.
Javascript hooks implemented in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/jshooks.js" target="_top">chrome/content/jshooks.js</a> are installed as part of the
same mechanism that hooks the date object.
</p><p>

It also turns out that it is possible to detect the original Firefox version
by <a class="ulink" href="http://0x000000.com/index.php?i=523&amp;bin=1000001011" target="_top">inspecting
certain resource:// files</a>. These cases are handled by Torbutton's
<a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a>.

</p><p>
This setting helps to satisfy the <a class="link" href="#setpreservation">Anonymity Set Preservation</a> requirement.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3006210"></a>5.28. Spoof US English Browser</h3></div></div></div><p>Options:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.spoof_english</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.spoof_charset</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.spoof_language</strong></span></td></tr></table><p>
</p><p> This option causes Torbutton to set
<span class="command"><strong>general.useragent.locale</strong></span>,
<span class="command"><strong>intl.accept_charsets</strong></span> and
<span class="command"><strong>intl.accept_languages</strong></span> to the value specified in
<span class="command"><strong>extensions.torbutton.spoof_locale</strong></span>,
<span class="command"><strong>extensions.torbutton.spoof_charset</strong></span> and
<span class="command"><strong>extensions.torbutton.spoof_language</strong></span> during Tor usage.  </p><p>
This setting helps to satisfy the <a class="link" href="#setpreservation">Anonymity Set Preservation</a> and <a class="link" href="#location">Location Neutrality</a> requirements.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3006297"></a>5.29. Don't send referrer during Tor Usage</h3></div></div></div><p>Option: <span class="command"><strong>extensions.torbutton.disable_referer</strong></span>
</p><p> 
This option causes Torbutton to set <a class="ulink" href="http://kb.mozillazine.org/Network.http.sendSecureXSiteReferrer" target="_top">network.http.sendSecureXSiteReferrer</a> and
<a class="ulink" href="http://kb.mozillazine.org/Network.http.sendRefererHeader" target="_top">network.http.sendRefererHeader</a> during Tor usage.</p><p>
This setting also does not directly satisfy any Torbutton requirement, but
some may desire to mask their referrer for general privacy concerns.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3006338"></a>5.30. Store SSL/CA Certs in separate jars for Tor/Non-Tor (recommended)</h3></div></div></div><p>Options:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><span class="command"><strong>extensions.torbutton.jar_certs</strong></span></td></tr><tr><td><span class="command"><strong>extensions.torbutton.jar_ca_certs</strong></span></td></tr></table><p>
</p><p>

These settings govern if Torbutton attempts to isolate the user's SSL
certificates into separate jars for each Tor state. This isolation is
implemented in <code class="function">torbutton_jar_certs()</code> in <a class="ulink" href="https://tor-svn.freehaven.net/svn/torbutton/trunk/src/chrome/content/torbutton.js" target="_top">chrome/content/torbutton.js</a>,
which calls <code class="function">torbutton_jar_cert_type()</code> and
<code class="function">torbutton_unjar_cert_type()</code> for each certificate type in
the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_securitynsscertcache1.html" target="_top">@mozilla.org/security/nsscertcache;1</a>.
Certificates are deleted from and imported to the <a class="ulink" href="http://www.xulplanet.com/references/xpcomref/comps/c_securityx509certdb1.html" target="_top">@mozilla.org/security/x509certdb;1</a>.
</p><p>
The first time this pref is used, a backup of the user's certificates is
created in their profile directory under the name
<code class="filename">cert8.db.bak</code>. This file can be copied back to
<code class="filename">cert8.db</code> to fully restore the original state of the
user's certificates in the event of any error.
</p><p>
Since exit nodes and malicious sites can insert content elements sourced to
specific SSL sites to query if a user has a certain certificate,
this setting helps to satisfy the <a class="link" href="#state">State
Separation</a> requirement of Torbutton. Unfortunately, <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=435159" target="_top">Firefox Bug
435159</a> prevents it from functioning correctly in the event of rapid Tor toggle, so it
is currently not exposed via the preferences UI.

</p></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="FirefoxBugs"></a>6. Relevant Firefox Bugs</h2></div></div></div><p>

</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="FirefoxSecurity"></a>6.1. Bugs impacting security</h3></div></div></div><p>

Torbutton has to work around a number of Firefox bugs that impact its
security. Most of these are mentioned elsewhere in this document, but they
have also been gathered here for reference. Several of these have fixes in
Firefox3.0/trunk, but are listed because they still have not been backported
to FF2.0. In order of decreasing severity, they are:

</p><div class="orderedlist"><ol type="1"><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=392274" target="_top">Bug 392274 - Timezone
config/chrome API</a><p>
The lack of a config or API to configure the timezone requires Torbutton to
<a class="link" href="#jshooks" title="5.4. Hook Dangerous Javascript (crucial)">insert client content window javascript</a> to hook
the Date object. Additionally, a way to <a class="ulink" href="http://pseudo-flaw.net/tor/torbutton/unmask-date.html" target="_top">remove the Date
hooks</a> was discovered by Greg Fleischer. Worse, on Firefox 3,
javascript sandboxing prevents most of the javascript hooks from being
installed, including the Date hooks. On Windows and Linux, you can set the TZ
environment variable to "UTC" as a workaround. Firefox will obey this
environment variable for your Timezone on those platforms, but on Windows this
does not take effect until browser restart. 
   </p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=436250" target="_top">Bug 436250 - Livemarks can't be
disabled at runtime</a><p>

The RSS Feed based "Livemarks"/"Live Bookmarks" update frequency is controlled
by the pref <span class="command"><strong>browser.bookmarks.livemark_refresh_seconds</strong></span>.
However, changing this preference does not cancel any pending timers, which
means that at least one livemarks pref fetch will happen over Tor, and once
this pref is set to disable livemarks for Tor, changing it back will never
cause the service to start back up again.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=435159" target="_top">Bug 435159 -
nsNSSCertificateDB::DeleteCertificate has race conditions</a><p>

In Torbutton 1.2.0rc1, code was added to attempt to isolate SSL certificates
the user has installed. Unfortunately, the method call to delete a certificate
from the current certificate database acts lazily: it only sets a variable
that marks a cert for deletion later, and it is not cleared if that
certificate is re-added. This means that if the Tor state is toggled quickly,
that certificate could remain present until it is re-inserted (causing an
error dialog), and worse, it would still be deleted after that.  The lack of
this functionality is considered a Torbutton security bug because cert
isolation is considered a <a class="link" href="#state">State Separation</a>
feature.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409737" target="_top">Bug 409737 -
javascript.enabled and docShell.allowJavascript do not disable all event
handlers</a><p>

This bug allows pages to execute javascript via addEventListener and perhaps
other callbacks. In order to prevent this bug from enabling an attacker to
break the <a class="link" href="#isolation">Network Isolation</a> requirement,
Torbutton 1.1.13 began blocking popups and history manipulation from different
Tor states.  So long as there are no ways to open popups or redirect the user
to a new page, the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">Torbutton content
policy</a> should block Javascript network access. However, if there are
ways to open popups or perform redirects such that Torbutton cannot block
them, pages may still have free reign to break that requirement and reveal a
user's original IP address.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=405652" target="_top">Bug 405652 - In the
TLS ClientHello message the gmt_unix_time is incorrect</a><p>

It turns out that Firefox's SSL implementation sends the machine uptime as the
current time. This essentially is a unique identifier that can be used for
the duration of your machine uptime. The issue has been fixed in Firefox 3.0,
but it has as of yet not been backported to 2.0.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=143220" target="_top">Bug 143220 - Script can get the value of a file control, including the path</a><p>

Javascript can query the .value field of file input dialogs to retrieve
username and sometimes hostname/workgroup information. This is obviously very
dangerous for people who are attempting to submit files anonymously via
webforms (ie whistleblowers and anonymous publishers). It is also fixed in
Firefox 3.0, but has not yet been backported to 2.0.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=418119" target="_top">Bug 418119 - nsIContentPolicy not called for external DTDs of XML documents</a><p>

XML documents can source chrome and resource URLs in their DTDs without a call
to nsIContentPolicy::shouldLoad. Enumerating chrome URLs gives websites and
exit nodes a lot of information. They can use it to probe for vulnerable
versions of extensions, and can also use it to build an <a class="link" href="#fingerprinting">identifier for tracking purposes</a>.  This bug
makes it impossible for extensions such as Adblock and Torbutton to prevent
chrome inspection and enumeration.  There is no workaround for this bug as of
yet.

</p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="FirefoxWishlist"></a>6.2. Bugs blocking functionality</h3></div></div></div><p>
The following bugs impact Torbutton and similar extensions' functionality.
   </p><div class="orderedlist"><ol type="1"><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=439384" target="_top">Bug 439384 -
"profile-do-change" event does not cause cookie table reload</a><p>

In Firefox 3, the change to the new sqlite database for cookie storage has a
bug that prevents Torbutton's cookie jaring from working properly. The
"profile-do-change" observer event no longer properly causes either a sync or
reload of the cookie database from disk after it is copied into place.
Torbutton currently works around this by issuing the SQLLite queries manually
to store and rebuild the cookie database.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=417869" target="_top">Bug 417869 -
Browser context is difficult to obtain from many XPCOM callbacks</a><p>

It is difficult to determine which tabbrowser many XPCOM callbacks originate
from, and in some cases absolutely no context information is provided at all.
While this doesn't have much of an effect on Torbutton, it does make writing
extensions that would like to do per-tab settings and content filters (such as
FoxyProxy) difficult to impossible to implement securely.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=418321" target="_top">Bug 418321 -
Components do not expose disk interfaces</a><p>

Several components currently provide no way of reimplementing their disk
access to easily satisfy Torbutton's <a class="link" href="#disk">Disk
Avoidance</a> requirements. Workarounds exist, but they are <a class="link" href="#sessionstore" title="@mozilla.org/browser/sessionstore;1 - components/nsSessionStore2.js and components/nsSessionStore3.js">clunky</a>, and
some of them involve disabling functionality during Tor usage.

</p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="FirefoxMiscBugs"></a>6.3. Low Priority Bugs</h3></div></div></div><p>
The following bugs have an effect upon Torbutton, but are superseded by more
practical and more easily fixable variant bugs above; or have stable, simple
workarounds.
  </p><div class="orderedlist"><ol type="1"><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=435151" target="_top">Bug 435151 - XPCSafeJSObjectWrapper breaks evalInSandbox</a><p>

Under Firefox 3, the XPCSafeJSObjectWrapper breaks when you try to use
constructors of classes defined from within the scope of the sandbox, among
other things. This prevents Torbutton from applying the Timezone hooks under
Firefox 3, but a better solution for Torbutton's specific date hooking needs 
would be a fix for the above mentioned Bug 392274. Of course, many more
extensions may be interested in the sandbox hooking functionality working
properly though.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=437014" target="_top">Bug 437014 -
nsIContentPolicy::shouldLoad no longer called for favicons</a><p>

Firefox 3.0 stopped calling the shouldLoad call of content policy for favicon
loads. Torbutton had relied on this call to block favicon loads for opposite
Tor states. The workaround it employs for Firefox 3 is to cancel the request
when it arrives in the <span class="command"><strong>torbutton_http_observer</strong></span> used for
blocking full page plugin loads. This seems to work just fine, but is a bit
dirty.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=437016" target="_top">Bug 437016 -
nsIContentPolicy::shouldLoad not called for livemarks</a><p>

An alternative fix for the livemarks bug above would be to block livemarks
fetches from the content policy. Unfortunately shouldLoad is not called for
livemarks fetches.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=418986" target="_top">Bug 418986 - window.screen
provides a large amount of identifiable information</a><p>

As <a class="link" href="#fingerprinting">mentioned above</a>, a large amount of
information is available from <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:window.screen" target="_top">window.screen</a>.
Currently, there is no way to obscure this information without Javascript
hooking. This bug is a feature request to provide some other method to change
these values.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=309524" target="_top">Bug 309524</a>
and <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=380556" target="_top">Bug
380556</a> - nsIContentPolicy::shouldProcess is not called.
     <p>

This is a call that would be useful to develop a better workaround for the
allowPlugins issue above. If the content policy were called before a URL was
handed over to a plugin or helper app, it would make the workaround for the
above allowPlugins bug a lot cleaner. Obviously this bug is not as severe as
the others though, but it might be nice to have this API as a backup.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=401296" target="_top">Bug 401296 - docShell.allowPlugins
not honored for direct links</a> (Perhaps subset of <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=282106" target="_top">Bug 282106</a>?)
     <p>

Similar to the javascript plugin disabling attribute, the plugin disabling
attribute is also not perfect — it is ignored for direct links to plugin
handled content, as well as meta-refreshes to plugin handled content.  This
requires Torbutton to listen to a number of different http events to intercept
plugin-related mime type URLs and cancel their requests. Again, since plugins
are quite horrible about obeying proxy settings, loading a plugin pretty much
ensures a way to break the <a class="link" href="#isolation">Network Isolation</a>
requirement and reveal a user's original IP address. Torbutton's code to
perform this workaround has been subverted at least once already by Kyle
Williams.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=418983" target="_top">Bug 41893 - Scoping
issues with window.__defineGetter__()</a><p>

For some reason, defining getters off of window seems to mess with the
implicit window scoping in some documents. There is a workaround for this bug,
so it is barely relevant. It would be far more useful to eliminate the need
for Javascript hooking in the first place by addressing the above bugs. This
bug is just listed for completeness.

</p></li><li><a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=419598" target="_top">Bug 419598 - 'var
Date' is deletable</a><p>

Based on Page 62 of the <a class="ulink" href="http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf" target="_top">ECMA-262
Javascript spec</a>, it seems like it should be possible to do something
like the following to prevent the Date object from being unmasked:
</p><pre class="screen">
with(window) {
    var Date = fakeDate;
    var otherVariable = 42;
}

delete window.Date; // Should fail. Instead succeeds, revealing original Date.
delete window.otherVariable; // Fails, leaving window.otherVariable set to 42.
</pre><p>

From the ECMA-262 spec:

</p><div class="blockquote"><blockquote class="blockquote">
If the variable statement occurs inside a FunctionDeclaration, the variables
are defined with function-local scope in that function, as described in
s10.1.3. Otherwise, they are defined with global scope (that is, they are
created as members of the global object, as described in 10.1.3) using
property attributes { DontDelete }. Variables are created when the execution
scope is entered. A Block does not define a new execution scope. Only Program
and FunctionDeclaration produce a new scope. Variables are initialized to
undefined when created. A variable with an Initialiser is assigned the value
of its AssignmentExpression when the VariableStatement is executed, not when
the variable is created.
</blockquote></div><p>

In fact, this is exactly how the with statement with a variable declaration
behaves <span class="emphasis"><em>for all other variables other than ones that shadow system
variables</em></span>. Some variables (such as
<span class="command"><strong>window.screen</strong></span>, and <span class="command"><strong>window.history</strong></span>) can't
even be shadowed in this way, and give an error about lacking a setter. If
such shadowing were possible, it would greatly simplify the Javascript hooking
code, which currently relies on undocumented semantics of
<span class="command"><strong>__proto__</strong></span> to copy the original values in the event of a
delete. This <span class="command"><strong>__proto__</strong></span> hack unfortunately does not work for
the Date object though.

</p></li></ol></div></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="TestPlan"></a>7. Testing</h2></div></div></div><p>

The purpose of this section is to cover all the known ways that Tor browser
security can be subverted from a testing and penetration perspective. The hope
is that it will be useful both for creating a "Tor Safety Check"
page, and for developing novel tests and actively attacking Torbutton with the
goal of finding vulnerabilities in either it or the Mozilla components,
interfaces and settings upon which it relies.
 
  </p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="Categories"></a>7.1. Single state testing</h3></div></div></div><p>
The following tests can be run from a single web page in one visit without
toggling Tor state or requiring user interaction. Currently they exist as their
own individual tests, but conceivably a single "Tor Safety Check"
page can be devised that contains all of these attacks. 
All of these tests are currently known to pass, but that does not mean that
consolidating them into an easy to run test page is pointless. Torbutton is a
complicated piece of software. During development, changes to one component
can affect a whole slough of unrelated features. Having easy-to-verify
comprehensive test pages would make it much easier to fix other issues as they
present themselves without introducing regressions.

</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007076"></a>Java and Plugin Decloaking</h4></div></div></div><p>
As <a class="link" href="#plugins" title="5.2. Disable plugins on Tor Usage (crucial)">mentioned above</a>, Java and plugins <a class="ulink" href="http://java.sun.com/j2se/1.5.0/docs/api/java/net/class-use/NetworkInterface.html" target="_top">can query</a> the <a class="ulink" href="http://www.rgagnon.com/javadetails/java-0095.html" target="_top">local IP
address</a> and report it back to the
remote site. They can also <a class="ulink" href="http://metasploit.com/research/misc/decloak/index.htm" target="_top">bypass proxy settings</a> and directly connect to a
remote site without Tor. Every browser plugin we have tested with Firefox has
some form of network capability, and every one ignores proxy settings or worse - only
partially obeys them. This includes but is not limited to:
QuickTime, Windows Media Player, RealPlayer, mplayerplug-in, AcroRead, and
Flash. In addition, 
<a class="ulink" href="http://www.janusvm.com/goldy/pdf/" target="_top">issues have been
discovered</a> with the browsers handling of
<a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=401296" target="_top">direct links to plugin-handled
content</a> as well as meta-refreshes to plugin content. To make matters
worse, <a class="ulink" href="http://www.janusvm.com/goldy/side-channels/side-channels.html" target="_top">externally
handled mime types and urls</a> can also cause direct non-Tor connections
as well.
    </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007174"></a>History Disclosure attacks</h4></div></div></div><p>
The browser's history can also be queried by a remote site to inspect for
Google queries, visits to sites that contain usernames in the URLs, or
other anonymity set reducing information. This can be done by either
<a class="ulink" href="http://gemal.dk/browserspy/css.html" target="_top">Javascript</a>, or by 
<a class="ulink" href="http://ha.ckers.org/weird/CSS-history.cgi" target="_top">CSS</a> without any scripting involved.

</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007200"></a>User agent, extension, resolution and OS information</h4></div></div></div><p>

As mentioned above, these properties can be combined to greatly reduce
anonymity set and even build a potentially <a class="link" href="#fingerprinting">globally unique identifier</a> for
users. <a class="ulink" href="http://0x000000.com/index.php?i=520&amp;bin=1000001000" target="_top">Examples of this
in the wild</a> rely on <a class="ulink" href="http://gemal.dk/browserspy/basic.html" target="_top">user agent and OS
information</a> as well as <a class="ulink" href="http://pseudo-flaw.net/content/tor/torbutton/" target="_top">chrome disclosure
information</a>.

</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007238"></a>Timezone and Location Information</h4></div></div></div><p>
<a class="ulink" href="http://gemal.dk/browserspy/date.html" target="_top">Time and Timezone</a>
should be obscured to be GMT-only, and by the browser should present itself
with an US English locale.
    </p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3007257"></a>7.2. Multi-state testing</h3></div></div></div><p>

The tests in this section are geared towards a page that would instruct the
user to toggle their Tor state after the fetch and perform some operations:
mouseovers, stray clicks, and potentially reloads.

</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007269"></a>Cookies and Cache Correlation</h4></div></div></div><p>
The most obvious test is to set a cookie, ask the user to toggle tor, and then
have them reload the page. The cookie should no longer be set if they are
using the default Torbutton settings. In addition, it is possible to leverage
the cache to <a class="ulink" href="http://crypto.stanford.edu/sameorigin/safecachetest.html" target="_top">store unique
identifiers</a>. The default settings of Torbutton should also protect
against these from persisting across Tor Toggle.

</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007292"></a>Javascript timers and event handlers</h4></div></div></div><p>

Javascript can set timers and register event handlers in the hopes of fetching
URLs after the user has toggled Torbutton. 
    </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007305"></a>CSS Popups and non-script Dynamic Content</h4></div></div></div><p>

Even if Javascript is disabled, CSS is still able to 
<a class="ulink" href="http://www.tjkdesign.com/articles/css%20pop%20ups/" target="_top">create popup-like
windows</a>
via the 'onmouseover' CSS attribute, which can cause arbitrary browser
activity as soon as the mouse enters into the content window. It is also
possible for meta-refresh tags to set timers long enough to make it likely
that the user has toggled Tor before fetching content.

</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id3007328"></a>7.3. Active testing (aka How to Hack Torbutton)</h3></div></div></div><p>

The idea behind active testing is to discover vulnerabilities in Torbutton to
bypass proxy settings, run script in an opposite Tor state, store unique
identifiers, leak location information, or otherwise violate <a class="link" href="#requirements" title="1.2. Torbutton Requirements">its requirements</a>. Torbutton has ventured out
into a strange and new security landscape. It depends on Firefox mechanisms
that haven't necessarily been audited for security, certainly not for the
threat model that Torbutton seeks to address. As such, it and the interfaces
it depends upon still need a 'trial by fire' typical of new technologies. This
section of the document was written with the intention of making that period
as fast as possible. Please help us get through this period by considering
these attacks, playing with them, and reporting what you find (and potentially
submitting the test cases back to be run in the standard batch of Torbutton
tests.

</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id3007358"></a>Some suggested vectors to investigate</h4></div></div></div><p>
    </p><div class="itemizedlist"><ul type="disc"><li>Strange ways to register Javascript <a class="ulink" href="http://en.wikipedia.org/wiki/DOM_Events" target="_top">events</a> and <a class="ulink" href="http://www.devshed.com/c/a/JavaScript/Using-Timers-in-JavaScript/" target="_top">timeouts</a> should
be verified to actually be ineffective after Tor has been toggled.</li><li>Other ways to cause Javascript to be executed after
<span class="command"><strong>javascript.enabled</strong></span> has been toggled off.</li><li>Odd ways to attempt to load plugins. Kyle Williams has had
<a class="ulink" href="http://www.janusvm.com/goldy/pdf/" target="_top">some
success</a> with direct loads/meta-refreshes of plugin-handled URLs.</li><li>The Date and Timezone hooks should be verified to work with
crazy combinations of iframes, nested iframes, iframes in frames, frames in
iframes, and popups being loaded and
reloaded in rapid succession, and/or from one another. Think race conditions and deep, 
parallel nesting, involving iframes from both <a class="ulink" href="http://en.wikipedia.org/wiki/Same_origin_policy" target="_top">same-origin and
non-same-origin</a> domains.</li><li>In addition, there may be alternate ways and other
methods to query the timezone, or otherwise use some of the Date object's
methods in combination to deduce the timezone offset. Of course, the author
tried his best to cover all the methods he could foresee, but it's always good
to have another set of eyes try it out.</li><li>Similarly, is there any way to confuse the <a class="link" href="#contentpolicy" title="@torproject.org/cssblocker;1 - components/cssblocker.js">content policy</a>
mentioned above to cause it to allow certain types of page fetches? For
example, it was recently discovered that favicons are not fetched by the
content, but the chrome itself, hence the content policy did not look up the
correct window to determine the current Tor tag for the favicon fetch. Are
there other things that can do this? Popups? Bookmarklets? Active bookmarks? </li><li>Alternate ways to store and fetch unique identifiers. For example, <a class="ulink" href="http://developer.mozilla.org/en/docs/DOM:Storage" target="_top">DOM Storage</a>
caught us off guard. 
It was
also discovered by <a class="ulink" href="http://pseudo-flaw.net" target="_top">Gregory
Fleischer</a> that <a class="ulink" href="http://pseudo-flaw.net/content/tor/torbutton/" target="_top">content window access to
chrome</a> can be used to build <a class="link" href="#fingerprinting">unique
identifiers</a>. 
Are there any other
arcane or experimental ways that Firefox provides to create and store unique
identifiers? Or perhaps unique identifiers can be queried or derived from
properties of the machine/browser that Javascript has access to? How unique
can these identifiers be?
     </li><li>Is it possible to get the browser to write some history to disk
(aside from swap) that can be retrieved later? By default, Torbutton should
write no history, cookie, or other browsing activity information to the
harddisk.</li><li>Do popup windows make it easier to break any of the above
behavior? Are javascript events still canceled in popups? What about recursive
popups from Javascript, data, and other funky URL types? What about CSS
popups? Are they still blocked after Tor is toggled?</li><li>Chrome-escalation attacks. The interaction between the
Torbutton chrome Javascript and the client content window javascript is pretty
well-defined and carefully constructed, but perhaps there is a way to smuggle
javascript back in a return value, or otherwise inject network-loaded
javascript into the chrome (and thus gain complete control of the browser).
</li></ul></div><p>

</p></div></div></div></div></body></html>

tor-webwml.git

Inital move of torbutton website into website trunk.