Design Goal: All cookies MUST be double-keyed to the URL bar origin and third-party origin. There exists a Mozilla bug that contains a prototype patch, but it lacks UI, and does not apply to modern Firefox versions.

Implementation Status: As a stopgap to satisfy our design requirement of unlinkability, we currently entirely disable 3rd party cookies by setting network.cookie.cookieBehavior to 1. We would prefer that third party content continue to function, but we believe the requirement for unlinkability trumps that desire.

In Firefox, there are actually two distinct caching mechanisms: One for general content (HTML, Javascript, CSS), and one specifically for images. The content cache is isolated to the URL bar domain by altering each cache key to include an additional ID that includes the URL bar domain. This functionality can be observed by navigating to about:cache and viewing the key used for each cache entry. Each third party element should have an additional "id=string" property prepended, which will list the FQDN that was used to source it.

Additionally, because the image cache is a separate entity from the content cache, we had to patch Firefox to also isolate this cache per URL bar domain.

HTTP Authorization headers can be used to encode silent third party tracking identifiers. To prevent this, we remove HTTP authentication tokens for third party elements through a patch to nsHTTPChannel.

DOM storage for third party domains MUST be isolated to the URL bar origin, to prevent linkability between sites. This functionality is provided through a patch to Firefox.

Design Goal: Users should be able to click-to-play flash objects from trusted sites. To make this behavior unlinkable, we wish to include a settings file for all platforms that disables flash cookies using the Flash settings manager.

Implementation Status: We are currently having difficulties causing Flash player to use this settings file on Windows, so Flash remains difficult to enable.

Design Goal: TLS session resumption tickets and SSL Session IDs MUST be limited to the URL bar origin.

Implementation Status: We currently clear SSL Session IDs upon New Identity, we disable TLS Session Tickets via the Firefox Pref security.enable_tls_session_tickets. We disable SSL Session IDs via a patch to Firefox. To compensate for the increased round trip latency from disabling these performance optimizations, we also enable TLS False Start via the Firefox Pref security.ssl.enable_false_start.

Tor circuits and HTTP connections from a third party in one URL bar origin MUST NOT be reused for that same third party in another URL bar origin.

This isolation functionality is provided by the combination of a Firefox patch to allow SOCKS usernames and passwords, as well as a Torbutton component that sets the SOCKS username and password for each request. The Tor client has logic to prevent connections with different SOCKS usernames and passwords from using the same Tor circuit. Firefox has existing logic to ensure that connections with SOCKS proxies do not re-use existing HTTP Keep-Alive connections unless the proxy settings match. We extended this logic to cover SOCKS username and password authentication, providing us with HTTP Keep-Alive unlinkability.

SharedWorkers are a special form of Javascript Worker Threads that have a shared scope between all threads from the same Javascript origin.

Design Goal: SharedWorker scope MUST be isolated to the URL bar domain. A SharedWorker launched from a third party from one URL bar domain MUST NOT have access to the objects created by that same third party loaded under another URL bar domain.

Implementation Status: For now, we disable SharedWorkers via the pref dom.workers.sharedWorkers.enabled.

The URL.createObjectURL API allows a site to load arbitrary content into a random UUID that is stored in the user's browser, and this content can be accessed via a URL of the form blob:UUID from any other content element anywhere on the web. While this UUID value is neither under control of the site nor predictable, it can still be used to tag a set of users that are of high interest to an adversary.

URIs created with URL.createObjectURL MUST be limited in scope to the first party URL bar domain that created them. We provide this isolation in Tor Browser via a direct patch to Firefox and disable URL.createObjectURL in the WebWorker context as a stopgap, due to an edge case with enforcing this isolation in WebWorkers.

Because SPDY can store identifiers, it is disabled through the Firefox preference network.http.spdy.enabled.

Design Goal: To prevent attacks aimed at subverting the Cross-Origin Identifier Unlinkability privacy requirement, the browser MUST NOT store any identifiers (cookies, cache, DOM storage, HTTP auth, etc) for cross-origin redirect intermediaries that do not prompt for user input. For example, if a user clicks on a bit.ly URL that redirects to a doubleclick.net URL that finally redirects to a cnn.com URL, only cookies from cnn.com should be retained after the redirect chain completes.

Non-automated redirect chains that require user input at some step (such as federated login systems) SHOULD still allow identifiers to persist.

Implementation status: There are numerous ways for the user to be redirected, and the Firefox API support to detect each of them is poor. We have a trac bug open to implement what we can.

window.name is a magical DOM property that for some reason is allowed to retain a persistent value for the lifespan of a browser tab. It is possible to utilize this property for identifier storage.

In order to eliminate non-consensual linkability but still allow for sites that utilize this property to function, we reset the window.name property of tabs in Torbutton every time we encounter a blank Referer. This behavior allows window.name to persist for the duration of a click-driven navigation session, but as soon as the user enters a new URL or navigates between HTTPS/HTTP schemes, the property is cleared.

We disable the password saving functionality in the browser as part of our Disk Avoidance requirement. However, since users may decide to re-enable disk history records and password saving, we also set the signon.autofillForms preference to false to prevent saved values from immediately populating fields upon page load. Since Javascript can read these values as soon as they appear, setting this preference prevents automatic linkability from stored passwords.

An extreme (but not impossible) attack to mount is the creation of HSTS supercookies. Since HSTS effectively stores one bit of information per domain name, an adversary in possession of numerous domains can use them to construct cookies based on stored HSTS state.

Design Goal: There appears to be three options for us: 1. Disable HSTS entirely, and rely instead on HTTPS-Everywhere to crawl and ship rules for HSTS sites. 2. Restrict the number of HSTS-enabled third parties allowed per URL bar origin. 3. Prevent third parties from storing HSTS rules. We have not yet decided upon the best approach.

Implementation Status: Currently, HSTS state is cleared by New Identity, but we don't defend against the creation of these cookies between New Identity invocations.

End-user configuration details are by far the most severe threat to fingerprinting, as they will quickly provide enough information to uniquely identify a user. We believe it is essential to avoid exposing platform configuration details to website content at all costs. We also discourage excessive fine-grained customization of Tor Browser by minimizing and aggregating user-facing privacy and security options, as well as by discouraging the use of additional addons. When it is necessary to expose configuration details in the course of providing functionality, we strive to do so only on a per-site basis via site permissions, to avoid linkability.

Device and hardware characteristics can be determined in three ways: they can be reported explicitly by the browser, they can be inferred through browser functionality, or they can be extracted through statistical measurements of system performance. We are most concerned with the cases where this information is either directly reported or can be determined via a single use of an API or feature, and prefer to place such APIs either behind site permissions, alter their functionality to prevent exposing the most variable aspects of these characteristics, or disable them entirely.

On the other hand, because statistical inference of system performance requires many iterations to achieve accuracy in the face of noise and concurrent activity, we are less concerned with this mechanism of extracting this information. We also expect that reducing the resolution of Javascript's time sources will significantly increase the duration of execution required to extract accurate results, and thus make statistical approaches both unattractive and highly noticeable due to excessive resource consumption.

Operating system vendor and version differences permeate many different aspects of the browser. While it is possible to address these issues with some effort, the relative lack of diversity in operating systems causes us to primarily focus our efforts on passive operating system fingerprinting mechanisms at this point in time. For the purposes of protecting user anonymity, it is not strictly essential that the operating system be completely concealed, though we recognize that it is useful to reduce this differentiation ability where possible, especially for cases where the specific version of a system can be inferred.

While somewhat outside the scope of browser fingerprinting, for completeness it is important to mention that users themselves theoretically might be fingerprinted through their behavior while interacting with a website. This behavior includes e.g. keystrokes, mouse movements, click speed, and writing style. Basic vectors such as keystroke and mouse usage fingerprinting can be mitigated by altering Javascript's notion of time. More advanced issues like writing style fingerprinting are the domain of other tools.

Due to vast differences in feature set and implementation behavior even between different versions of the same browser, browser vendor and version differences are simply not possible to conceal in any realistic way. It is only possible to minimize the differences among different installations of the same browser vendor and version. We make no effort to mimic any other major browser vendor, and in fact most of our fingerprinting defenses serve to differentiate Tor Browser users from normal Firefox users. Because of this, any study that lumps browser vendor and version differences into its analysis of the fingerprintability of a population is largely useless for evaluating either attacks or defenses. Unfortunately, this includes popular large-scale studies such as Panopticlick and Am I Unique.

Value spoofing can be used for simple cases where the browser directly provides some aspect of the user's configuration details, devices, hardware, or operating system directly to a website. It becomes less useful when the fingerprinting method relies on behavior to infer aspects of the hardware or operating system, rather than obtain them directly.

In cases where simple spoofing is not enough to properly conceal underlying device characteristics or operating system details, the underlying subsystem that provides the functionality for a feature or API may need to be completely reimplemented. This is most common in cases where customizable or version-specific aspects of the user's operating system are visible through the browser's featureset or APIs, usually because the browser directly exposes OS-provided implementations of underlying features. In these cases, such OS-provided implementations must be replaced by a generic implementation, or at least an implementation wrapper that makes effort to conceal any user-customized aspects of the system.

Virtualization is needed when simply reimplementing a feature in a different way is insufficient to fully conceal the underlying behavior. This is most common in instances of device and hardware fingerprinting, but since the notion of time can also be virtualized, virtualization also can apply to any instance where an accurate measurement of wall clock time is required for a fingerprinting vector to attain high accuracy.

In the event that reimplementation or virtualization is too expensive in terms of performance or engineering effort, and the relative expected usage of a feature is rare, site permissions can be used to prevent the usage of a feature for cross-site tracking. Unfortunately, site permissions become less effective once a feature is already widely overused and abused by many websites, since warning fatigue typically sets in for most users after just a few permission requests.

Due to the current bias in favor of invasive APIs that expose the maximum amount of platform information, some features and APIs are simply not salvageable in their current form. When such invasive features serve only a narrow domain or use case, or when there are alternate ways of accomplishing the same task, these features and/or certain aspects of their functionality may be simply removed.

While many end-user configuration details that the browser currently exposes may be safely replaced by false information, randomization of these details must be just as exhaustive as an approach that seeks to make these behaviors uniform. When confronting either strategy, the adversary can still make use of any details which have not been altered to be either sufficiently uniform or sufficiently random.

Furthermore, the randomization approach seems to break down when it is applied to deeper issues where underlying system functionality is directly exposed. In particular, it is not clear how to randomize the capabilities of hardware attached to a computer in such a way that it either convincingly behaves like other hardware, or such that the exact properties of the hardware that vary from user to user are sufficiently randomized. Similarly, truly concealing operating system version differences through randomization may require multiple reimplementations of the underlying operating system functionality to ensure that every operating system version is covered by the range of possible behaviors.

The fact that randomization causes behaviors to differ slightly with every site visit makes it appealing at first glance, but this same property makes it very difficult to objectively measure its effectiveness. By contrast, an implementation that strives for uniformity is very simple to measure. Despite their current flaws, a properly designed version of Panopticlick or Am I Unique could report the entropy and uniqueness rates for all users of a single user agent version, without the need for complicated statistics about the variance of the measured behaviors.

Randomization (especially incomplete randomization) may also provide a false sense of security. When a fingerprinting attempt makes naive use of randomized information, a fingerprint will appear unstable, but may not actually be sufficiently randomized to prevent a dedicated adversary. Sophisticated fingerprinting mechanisms may either ignore randomized information, or incorporate knowledge of the distribution and range of randomized values into the creation of a more stable fingerprint (by either removing the randomness, modeling it, or averaging it out).

When randomization is introduced to features that affect site behavior, it can be very distracting for this behavior to change between visits of a given site. For the simplest cases, this will lead to minor visual nuisances. However, when this information affects reported functionality or hardware characteristics, sometimes a site will function one way on one visit, and another way on a subsequent visit.

Randomizing involves performance costs. This is especially true if the fingerprinting surface is large (like in a modern browser) and one needs more elaborate randomizing strategies (including randomized virtualization) to ensure that the randomization fully conceals the true behavior. Many calls to a cryptographically secure random number generator during the course of a page load will both serve to exhaust available entropy pools, as well as lead to increased computation while loading a page.

Improper randomization might introduce a new fingerprinting vector, as the process of generating the values for the fingerprintable attributes could be itself susceptible to side-channel attacks, analysis, or exploitation.

Plugins add to fingerprinting risk via two main vectors: their mere presence in window.navigator.plugins (because they are optional, end-user installed third party software), as well as their internal functionality.

Design Goal: All plugins that have not been specifically audited or sandboxed MUST be disabled. To reduce linkability potential, even sandboxed plugins should not be allowed to load objects until the user has clicked through a click-to-play barrier. Additionally, version information should be reduced or obfuscated until the plugin object is loaded. For flash, we wish to provide a settings.sol file to disable Flash cookies, and to restrict P2P features that are likely to bypass proxy settings. We'd also like to restrict access to fonts and other system information (such as IP address and MAC address) in such a sandbox.

Implementation Status: Currently, we entirely disable all plugins in Tor Browser. However, as a compromise due to the popularity of Flash, we allow users to re-enable Flash, and flash objects are blocked behind a click-to-play barrier that is available only after the user has specifically enabled plugins. Flash is the only plugin available, the rest are entirely blocked from loading by the Firefox patches mentioned in the Proxy Obedience section. We also set the Firefox preference plugin.expose_full_path to false, to avoid leaking plugin installation information.

After plugins and plugin-provided information, we believe that the HTML5 Canvas is the single largest fingerprinting threat browsers face today. Initial studies show that the Canvas can provide an easy-access fingerprinting target: The adversary simply renders WebGL, font, and named color data to a Canvas element, extracts the image buffer, and computes a hash of that image data. Subtle differences in the video card, font packs, and even font and graphics library versions allow the adversary to produce a stable, simple, high-entropy fingerprint of a computer. In fact, the hash of the rendered image can be used almost identically to a tracking cookie by the web server.

In some sense, the canvas can be seen as the union of many other fingerprinting vectors. If WebGL is normalized through software rendering, system colors were standardized, and the browser shipped a fixed collection of fonts (see later points in this list), it might not be necessary to create a canvas permission. However, until then, to reduce the threat from this vector, we have patched Firefox to prompt before returning valid image data to the Canvas APIs, and for access to isPointInPath and related functions. If the user hasn't previously allowed the site in the URL bar to access Canvas image data, pure white image data is returned to the Javascript APIs.

In Firefox, by using either WebSockets or XHR, it is possible for remote content to enumerate the list of TCP ports open on 127.0.0.1, as well as on any other machines on the local network. In other browsers, this can be accomplished by DOM events on image or script tags. This open vs filtered vs closed port list can provide a very unique fingerprint of a machine, because it essentially enables the detection of many different popular third party applications and optional system services (Skype, Bitcoin, Bittorrent and other P2P software, SSH ports, SMB and related LAN services, CUPS and printer daemon config ports, mail servers, and so on). It is also possible to determine when ports are closed versus filtered/blocked (and thus probe custom firewall configuration).

In Tor Browser, we prevent access to 127.0.0.1/localhost by ensuring that even these requests are still sent by Firefox to our SOCKS proxy (ie we set network.proxy.no_proxies_on to the empty string). The local Tor client then rejects them, since it is configured to proxy for internal IP addresses by default. Access to the local network is forbidden via the same mechanism.

Both NTLM and SPNEGO authentication mechanisms can leak the hostname, and in some cases the current username. The only reason why these aren't a more serious problem is that they typically involve user interaction, and likely aren't an attractive vector for this reason. However, because it is not clear if certain carefully-crafted error conditions in these protocols could cause them to reveal machine information and still fail silently prior to the password prompt, these authentication mechanisms should either be disabled, or placed behind a site permission before their use. We simply disable them.

The GamePad API provides web pages with the USB device id, product id, and driver name of all connected game controllers, as well as detailed information about their capabilities. This API should be behind a site permission in Private Browsing Modes, or should present a generic controller type (perhaps a two button controller that can be mapped to the keyboard) in all cases. We simply disable it via the pref dom.gamepad.enabled.

According to the Panopticlick study, fonts provide the most linkability when they are provided as an enumerable list in file system order, via either the Flash or Java plugins. However, it is still possible to use CSS and/or Javascript to query for the existence of specific fonts. With a large enough pre-built list to query, a large amount of fingerprintable information may still be available, especially given that additional fonts often end up installed by third party software and for multilingual support.

Design Goal: The sure-fire way to address font linkability is to ship the browser with a font for every language, typeface, and style, and to only use those fonts at the exclusion of system fonts. We are currently investigating this approach, and our current favorite font sets for this purpose are the Droid fonts, the Nanum fonts, and Lohit fonts. The Droid font set is fairly complete by itself, but Nanum and Lohit have smaller versions of many South Asian languages. When combined in a way that chooses the smallest font implementations for each locale, these three font sets provide coverage for the all languages used on Wikipedia with more than 10,000 articles, and several others as well, in approximately 3MB of compressed overhead. The Noto font set is another font set that aims for complete coverage, but is considerably larger than the combination of the Droid, Nanum, and Lohit fonts.

Implementation Status: In the meantime while we investigate shipping our own fonts, we disable plugins, which prevents font name enumeration. Additionally, we limit both the number of font queries from CSS, as well as the total number of fonts that can be used in a document with a Firefox patch. We create two prefs, browser.display.max_font_attempts and browser.display.max_font_count for this purpose. Once these limits are reached, the browser behaves as if browser.display.use_document_fonts was set.

To improve rendering, we exempt remote @font-face fonts from these counts, and if a font-family CSS rule lists a remote font (in any order), we use that font instead of any of the named local fonts.

Both CSS and Javascript have access to a lot of information about the screen resolution, usable desktop size, OS widget size, toolbar size, title bar size, and OS desktop widget sizing information that are not at all relevant to rendering and serve only to provide information for fingerprinting. Since many aspects of desktop widget positioning and size are user configurable, these properties yield customized information about the computer, even beyond the monitor size.

Design Goal: Our design goal here is to reduce the resolution information down to the bare minimum required for properly rendering inside a content window. We intend to report all rendering information correctly with respect to the size and properties of the content window, but report an effective size of 0 for all border material, and also report that the desktop is only as big as the inner content window. Additionally, new browser windows are sized such that their content windows are one of a few fixed sizes based on the user's desktop resolution. In addition, to further reduce resolution-based fingerprinting, we are investigating zoom/viewport-based mechanisms that might allow us to always report the same desktop resolution regardless of the actual size of the content window, and simply scale to make up the difference. Until then, the user should also be informed that maximizing their windows can lead to fingerprintability under this scheme.

Implementation Status: We automatically resize new browser windows to a 200x100 pixel multiple using a window observer based on desktop resolution. To minimize the effect of the long tail of large monitor sizes, we also cap the window size at 1000 pixels in each direction. Additionally, we patch Firefox to use the client content window size for window.screen, and to report a window.devicePixelRatio of 1.0. Similarly, we patch DOM events to return content window relative points. We also force popups to open in new tabs (via browser.link.open_newwindow.restriction), to avoid full-screen popups inferring information about the browser resolution. In addition, we prevent auto-maximizing on browser start, and inform users that maximized windows are detrimental to privacy in this mode.

Beyond simple resolution information, a large amount of so-called "Media" information is also exported to content. Even without Javascript, CSS has access to a lot of information about the device orientation, system theme colors, and other desktop and display features that are not at all relevant to rendering and also user configurable. Most of this information comes from CSS Media Queries, but Mozilla has exposed several user and OS theme defined color values to CSS as well.

Design Goal: CSS should not be able infer anything that the user has configured about their computer. Additionally, it should not be able to infer machine-specific details such as screen orientation or type.

Implementation Status: We patch Firefox to report a fixed set of system colors to content window CSS, and prevent detection of font smoothing on OSX. We also always report landscape-primary for the screen orientation.

WebGL is fingerprintable both through information that is exposed about the underlying driver and optimizations, as well as through performance fingerprinting.

Because of the large amount of potential fingerprinting vectors and the previously unexposed vulnerability surface, we deploy a similar strategy against WebGL as for plugins. First, WebGL Canvases have click-to-play placeholders (provided by NoScript), and do not run until authorized by the user. Second, we obfuscate driver information by setting the Firefox preferences webgl.disable-extensions and webgl.min_capability_mode, which reduce the information provided by the following WebGL API calls: getParameter(), getSupportedExtensions(), and getExtension().

Another option for WebGL might be to use software-only rendering, using a library such as Mesa. The use of such a library would avoid hardware-specific rendering differences.

Design Goal: All Tor Browser users MUST provide websites with an identical user agent and HTTP header set for a given request type. We omit the Firefox minor revision, and report a popular Windows platform. If the software is kept up to date, these headers should remain identical across the population even when updated.

Implementation Status: Firefox provides several options for controlling the browser user agent string which we leverage. We also set similar prefs for controlling the Accept-Language and Accept-Charset headers, which we spoof to English by default. Additionally, we remove content script access to Components.interfaces, which can be used to fingerprint OS, platform, and Firefox minor version.

In Tor Browser, we provide non-English users the option of concealing their OS and browser locale from websites. It is debatable if this should be as high of a priority as information specific to the user's computer, but for completeness, we attempt to maintain this property.

Implementation Status: We set the fallback character set to set to windows-1252 for all locales, via intl.charset.default. We also patch Firefox to allow us to instruct the JS engine to use en-US as its internal C locale for all Date, Math, and exception handling.

While the latency in Tor connections varies anywhere from milliseconds to a few seconds, it is still possible for the remote site to detect large differences between the user's clock and an official reference time source.

Design Goal: All Tor Browser users MUST report the same timezone to websites. Currently, we choose UTC for this purpose, although an equally valid argument could be made for EDT/EST due to the large English-speaking population density (coupled with the fact that we spoof a US English user agent). Additionally, the Tor software should detect if the users clock is significantly divergent from the clocks of the relays that it connects to, and use this to reset the clock values used in Tor Browser to something reasonably accurate. Alternatively, the browser can obtain this clock skew via a mechanism similar to that used in tlsdate.

Implementation Status: We set the timezone using the TZ environment variable, which is supported on all platforms.

Javascript performance fingerprinting is the act of profiling the performance of various Javascript functions for the purpose of fingerprinting the Javascript engine and the CPU.

Design Goal: We have several potential mitigation approaches to reduce the accuracy of performance fingerprinting without risking too much damage to functionality. Our current favorite is to reduce the resolution of the Event.timeStamp and the Javascript Date() object, while also introducing jitter. We believe that Javascript time resolution may be reduced all the way up to the second before it seriously impacts site operation. Our goal with this quantization is to increase the amount of time it takes to mount a successful attack. Mowery et al found that even with the default precision in most browsers, they required up to 120 seconds of amortization and repeated trials to get stable results from their feature set. We intend to work with the research community to establish the optimum trade-off between quantization+jitter and amortization time, as well as identify highly variable Javascript operations. As long as these attacks take several seconds or more to execute, they are unlikely to be appealing to advertisers, and are also very likely to be noticed if deployed against a large number of people.

Implementation Status: Currently, our mitigation against performance fingerprinting is to disable Navigation Timing through the Firefox preference dom.enable_performance, and to disable the Mozilla Video Statistics API extensions via the preference media.video_stats.enabled.

Keystroke fingerprinting is the act of measuring key strike time and key flight time. It is seeing increasing use as a biometric.

Design Goal: We intend to rely on the same mechanisms for defeating Javascript performance fingerprinting: timestamp quantization and jitter.

Implementation Status: We have no implementation as of yet.

As we mentioned in the introduction of this section, OS type fingerprinting is currently considered a lower priority, due simply to the numerous ways that characteristics of the operating system type may leak into content, and the comparatively low contribution of OS to overall entropy. In particular, there are likely to be many ways to measure the differences in widget size, scrollbar size, and other rendered details on a page. Also, directly exported OS routines (such as those from the standard C math library) expose differences in their implementations through their return values.

Design Goal: We intend to reduce or eliminate OS type fingerprinting to the best extent possible, but recognize that the effort for reward on this item is not as high as other areas. The entropy on the current OS distribution is somewhere around 2 bits, which is much lower than other vectors which can also be used to fingerprint configuration and user-specific information. You can see the major areas of OS fingerprinting we're aware of using the tbb-fingerprinting-os tag on our bug tracker.

Implementation Status: At least three HTML5 features have different implementation status across the major OS vendors and/or the underlying hardware: the Battery API, the Network Connection API, and the Sensor API. We disable these APIs through the Firefox preferences dom.battery.enabled, dom.network.enabled, and device.sensors.enabled.