Firefox Vulnerability Allows Tor User Fingerprinting

Researchers have discovered a vulnerability that could allow threat actors to fingerprint Firefox users, even in Private Browsing mode. The issue also affects the Tor anonymity browser, which is based on Firefox.

The vulnerability, tracked as CVE-2026-6770, is related to the IndexedDB browser API, which is used for storing structured data on the client side.

Firefox stores IndexedDB database names using internal UUID mappings, and when a website lists those databases, the order they come back in remains the same across different sites while the same browser process is running.

This enables unrelated sites to independently observe the same ordering and use it to link a user’s activity across domains without any cookies or shared storage. The fingerprint persists across reloads and new private sessions, until the browser is fully restarted.

[ Read: Claude Mythos Finds 271 Firefox Vulnerabilities ]

Threat actors could exploit this to fingerprint users in Firefox’s Private Browsing mode and even when Tor’s New Identity feature is used.

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The New Identity feature in Tor is specifically designed to prevent a user’s activity across different sites from being linked by clearing browsing history, cookies, and active connections.

“In Tor Browser, the stable identifier effectively defeats Tor Browser’s ‘New Identity’ isolation within a running browser process, allowing websites to link sessions that are expected to be fully isolated from one another,” the researchers explained.

Mozilla patched CVE-2026-6770 with the release of Firefox 150. The organization assigned the flaw a ‘medium severity’ rating and described it only as “other issue in the Storage: IndexedDB component”.

The Tor Project has also adopted the patch, rolling it out to users last week with the release of Tor Browser 15.0.10.

Originally published by SecurityWeek

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