Earlier this week, security researchers published reports that Firefox and Tor Browser were vulnerable to “man-in-the-middle” (MITM) attacks under special circumstances. Firefox automatically updates installed add-ons over an HTTPS connection. As a backup protection measure against mis-issued certificates, we also “pin” Mozilla’s web site certificates, so that even if an attacker manages to get an unauthorized certificate for our update site, they will not be able to tamper with add-on updates.

Due to flaws in the process we used to update “Preloaded Public Key Pinning” in our releases, the pinning for add-on updates became ineffective for Firefox release 48 starting September 10, 2016 and ESR 45.3.0 on September 3, 2016. As of those dates, an attacker who was able to get a mis-issued certificate for a Mozilla Web site could cause any user on a network they controlled to receive malicious updates for add-ons they had installed.

Users who have not installed any add-ons are not affected. However, Tor Browser contains add-ons and therefore all Tor Browser users are potentially vulnerable. We are not presently aware of any evidence that such malicious certificates exist in the wild and obtaining one would require hacking or compelling a Certificate Authority. However, this might still be a concern for Tor users who are trying to stay safe from state-sponsored attacks. The Tor Project released a security update to their browser early on Friday; Mozilla is releasing a fix for Firefox on Tuesday, September 20.

To help users who have not updated Firefox recently, we have also enabled Public Key Pinning Extension for HTTP (HPKP) on the add-on update servers. Firefox will refresh its pins during its daily add-on update check and users will be protected from attack after that point.