Categories: Firefox security

Firefox 85 Cracks Down on Supercookies


By Steven Englehardt and Arthur Edelstein

Trackers and adtech companies have long abused browser features to follow people around the web. Since 2018, we have been dedicated to reducing the number of ways our users can be tracked. For example, we’ve blocked cookies from known trackers and scripts from known fingerprinting companies.

In Firefox 85, we’re introducing a fundamental change in the browser’s network architecture to make all of our users safer: we now partition network connections and caches by the website being visited. Trackers can abuse caches to create supercookies and can use connection identifiers to track users. But by isolating caches and network connections to the website they were created on, we make them useless for cross-site tracking.

What are supercookies?

In short, supercookies can be used in place of ordinary cookies to store user identifiers, but  they are much more difficult to delete and block. This makes it nearly impossible for users to protect their privacy as they browse the web. Over the years, trackers have been found storing user identifiers as supercookies in increasingly obscure parts of the browser, including in Flash storage, ETags, and HSTS flags.

The changes we’re making in Firefox 85 greatly reduce the effectiveness of cache-based supercookies by eliminating a tracker’s ability to use them across websites.

How partitioning network state prevents cross-site tracking

Like all web browsers, Firefox shares some internal resources between websites to reduce overhead. Firefox’s image cache is a good example: if the same image is embedded on multiple websites, Firefox will load the image from the network during a visit to the first website and on subsequent websites would traditionally load the image from the browser’s local image cache (rather than reloading from the network). Similarly, Firefox would reuse a single network connection when loading resources from the same party embedded on multiple websites. These techniques are intended to save a user bandwidth and time.

Unfortunately, some trackers have found ways to abuse these shared resources to follow users around the web. In the case of Firefox’s image cache, a tracker can create a supercookie by  “encoding” an identifier for the user in a cached image on one website, and then “retrieving” that identifier on a different website by embedding the same image. To prevent this possibility, Firefox 85 uses a different image cache for every website a user visits. That means we still load cached images when a user revisits the same site, but we don’t share those caches across sites.

In fact, there are many different caches trackers can abuse to build supercookies. Firefox 85 partitions all of the following caches by the top-level site being visited: HTTP cache, image cache, favicon cache, HSTS cache, OCSP cache, style sheet cache, font cache, DNS cache, HTTP Authentication cache, Alt-Svc cache, and TLS certificate cache.

To further protect users from connection-based tracking, Firefox 85 also partitions pooled connections, prefetch connections, preconnect connections, speculative connections, and TLS session identifiers.

This partitioning applies to all third-party resources embedded on a website, regardless of whether Firefox considers that resource to have loaded from a tracking domain. Our own metrics show a very modest impact on page load time: between a 0.09% and 0.75% increase at the 80th percentile and below, and a maximum increase of 1.32% at the 85th percentile. These impacts are similar to those reported by the Chrome team for similar cache protections they are planning to roll out.

Systematic network partitioning makes it harder for trackers to circumvent Firefox’s anti-tracking features, but we still have more work to do to continue to strengthen our protections. Stay tuned for more privacy protections in the coming months!

Thank you

Re-architecting how Firefox handles network connections and caches was no small task, and would not have been possible without the tireless work of our engineering team: Andrea Marchesini, Tim Huang, Gary Chen, Johann Hofmann, Tanvi Vyas, Anne van Kesteren, Ethan Tseng, Prangya Basu, Wennie Leung, Ehsan Akhgari, and Dimi Lee.

We wish to express our gratitude to the many Mozillians who contributed to and supported this work, including: Selena Deckelmann, Mikal Lewis, Tom Ritter, Eric Rescorla, Olli Pettay, Kim Moir, Gregory Mierzwinski, Doug Thayer, and Vicky Chin.

We also want to acknowledge past and ongoing efforts carried out by colleagues in the Brave, Chrome, Safari and Tor Browser teams to combat supercookies in their own browsers.