Mozilla on Monday disclosed it blocked two malicious Firefox add-ons installed by 455,000 users that were found misusing the Proxy API to impede downloading updates to the browser.
The two extensions in question, named Bypass and Bypass XM, "interfered with Firefox in a way that prevented users who had installed them from downloading updates, accessing updated blocklists, and updating remotely configured content," Mozilla's Rachel Tublitz and Stuart Colville said.
Because Proxy API can be used to proxy web requests, an abuse of the API could enable a bad actor to control the manner Firefox browser connects to the internet effectively.
In addition to blocking the extensions to prevent installation by other users, Mozilla said it's pausing on approvals for new add-ons that use the proxy API until the fixes are broadly available. What's more, the California-based non-profit said it'd deployed a system add-on named "Proxy Failover" that ships with further mitigations to address the issue.
Users who have installed the problematic add-ons are highly advised to remove them by heading the Add-ons section and explicitly searching for "Bypass" (ID: 7c3a8b88-4dc9-4487-b7f9-736b5f38b957) or "Bypass XM" (ID: d61552ef-e2a6-4fb5-bf67-8990f0014957).
Developers of add-ons that require the use of the proxy API are also required to start including a "strict_min_version" key in their manifest.json files targeting Firefox browser versions 91.1 or above.