Tor blimey, guv'nor: Firefox to try on privacy tool's Canvas gloves to leave fewer fingerprints

Browser maker turns to anonymizing network for anti-identification technique

By Thomas Claburn in San Francisco


Mozilla has incorporated a privacy protection option pioneered by The Tor Project into Firefox's code, but plans to make the feature available only through the browser's nightly builds.

For the past four years, Mozilla developers have been mulling how the Tor browser, which is based on Firefox ESR, prevents the use of the Canvas API for browser fingerprinting, a method for secretly tracking and potentially unmasking browser users online.

Modern browsers can be directed to write hidden data to the Canvas graphics layer and to then hash the data to generate a distinct identifier. Identifiers of this sort can be used to distinguish one unauthenticated user from another across websites, in what amounts to a denial-of-privacy attack.

Unlike tracking methods that rely on accepting a file from a server – one-pixel images, cookie files, and the like – browser fingerprinting isn't easily blocked because it relies on information made available by the client.

Tracking of this sort can be done without the Canvas API, by analyzing the collected technical details automatically disclosed by browsers. Together, these details are often enough to create a set of browser characteristics that, without additional information, can distinguish website visitors from one another. Tracked characteristics may include configuration settings like installed fonts, whether JavaScript is enabled, the specified timezone, the operating system, the language, and the browser's HTTP user-agent string.

There are extensions that can mitigate such data collection but the extensions may themselves be used as a form of identifying mark. In general, the best defense against browser profiling is to have one's system appear like everyone else's, in order to disappear into the crowd.

Regardless, the Canvas API makes tracking easier. According to a discussion on the Firefox bug tracking forum, when Firefox 58 arrives as a beta release next month, it should have an option to "Manage canvas extraction permission."

The discussion thread indicates there's still some debate about the wording of the option and how it will be presented. Presently, it looks like the permission could be presented as a tooltip on a button that lets users open a panel to manage settings for websites accessing image data.

But Mozilla isn't currently planning to expose the feature to the public via its beta or stable channels.

"Mozilla is working with the Tor Project to add a number of privacy and security features to the shared codebase that both Mozilla and Tor use to produce Firefox and Tor browser respectively," said Selena Deckelmann, director of Firefox Runtime, in an email to The Register. "Canvas Fingerprinting is one such feature, however it is disabled by default and we have no current plans to ship Canvas Fingerprinting in Firefox beyond the Nightly channel."

Deckelmann said the Canvas privacy feature is part of Mozilla's Tor Uplift project to incorporate their patches into Firefox core code to make it easier for them to build the Tor browser.

There may be some consequences to disabling access to the Canvas, suggests Mozilla contributor Tim Nguyen, because Firefox does this by spoofing the User Agent so the browser appears to be an older version, Firefox 50, and spoofs timezones so they appear as UTC (which may affect calendar entries).

What's more, the beneficial effect of gagging the Canvas may be minimal, because so many other browser settings will rat users out. But it's a step in the right direction, for Tor users at least. ®

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