A recently fixed security vulnerability that affected both the Firefox and Tor browsers had a highly unusual characteristic that caused it to threaten users only during temporary windows of time that could last anywhere from two days to more than a month.

As a result, the cross-platform, malicious code-execution risk most recently visited users of browsers based on the Firefox Extended Release on September 3 and lasted until Tuesday, or a total of 17 days. The same Firefox version was vulnerable for an even longer window last year, starting on July 4 and lasting until August 11. The bug was scheduled to reappear for a few days in November and for five weeks in December and January. Both the Tor Browser and the production version of Firefox were vulnerable during similarly irregular windows of time.

While the windows were open, the browsers failed to enforce a security measure known as certificate pinning when automatically installing NoScript and certain other browser extensions. That meant an attacker who had a man-in-the-middle position and a forged certificate impersonating a Mozilla server could surreptitiously install malware on a user’s machine. While it can be challenging to hack a certificate authority or trick one into issuing the necessary certificate for addons.mozilla.org, such a capability is well within the means of nation-sponsored attackers, who are precisely the sort of adversaries included in the Tor threat model. Such an attack, however, was only viable at certain periods when Mozilla-supplied “pins” expired.

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Bug that hit Firefox and Tor browsers was hard to spot—now we know why