Executive brief
Firefox for Android allows browser extensions to create tabs that appear private to the user but are not actually private, exposing data leakage. Extensions can share cookies and access tab data that should be restricted, even when not granted private browsing permissions. This misleads users about their privacy protection and could expose sensitive browsing activity to untrusted extensions.
Technical details
A vulnerability in the WebExtensions component of Firefox for Android causes tabs created by extensions via the tabs.create() API to be displayed as private in the UI when they are not actually private. The root cause is that Fenix's logic for opening extension tabs in the current browsing mode predates support for the "private browsing access flag" and does not properly check whether an extension has permission to access private browsing mode. When an extension without private browsing access calls tabs.create(), GeckoView creates a non-private tab but Fenix displays it as private, allowing the extension to access cookies and tab information that should be restricted. The vulnerability requires an extension to be installed; no network interaction is needed. The fix was implemented in Firefox 155 to properly validate private browsing permissions before opening tabs.
Affected products
- Mozilla Firefox for Android before 155
Timeline
- 2026-09-01: disclosed
- 2026-09-01: patched: fixed in Firefox 155