Mozilla admitted today that its Bugzilla bug tracking system was breached by an attacker, who was then able to get access to information about unpatched zero-day bugs.

While Mozilla doesn't have finite timelines on when the breach occurred, it may well have happened as far back as September 2013. According to Mozilla, the attacker was able to breach a user's account that had privileged access to Bugzilla, including the non-public zero-day flaw information.

As far as Mozilla has been able to determine at this time, the attacker accessed approximately 185 bugs that were non-public. Of those bugs, Mozilla considered 53 to be severe vulnerabilities. That said, Mozilla claims that 43 of the severe flaws had already been patched in the Firefox browser by the time the attacker accessed the bug information.

That leaves 10 bugs that the attacker had access to before they were patched, and that's where the potential risk to Firefox users lies.

"One of the bugs [opened] less than 36 days was used for an attack using a vulnerability that was patched on August 6, 2015," Mozilla stated in an FAQ on the breach. "Other than that attack, however, we do not have any data indicating that other bugs were exploited."

The one bug that was exploited in the wild was used to collect private data from Firefox users who visited a Russian news site.

What is particularly interesting about the Mozilla Bugzilla breach is how it occurred in the first place. While the attacker was able to learn about new Firefox zero-day flaws, no zero-day flaw was required by the attacker to get access to Bugzilla.

"Information uncovered in our investigation suggests that the user re­used their Bugzilla password with another website, and the password was revealed through a data breach at that site," Mozilla's FAQ stated.

That means that a privileged Bugzilla user used the same password at multiple sites, which is generally considered to be a security faux pas and isn't something that someone who has been granted secure access should have done. Password reuse overall though is a big problem, which is why Facebook and Google both routinely look at password dumps in an attempt to protect their users from breaches.

In a blog post, Firefox security lead Richard Barnes detailed what Mozilla is now doing to improve Bugzilla's security.

"We are updating Bugzilla's security practices to reduce the risk of future attacks of this type," Barnes wrote. "As an immediate first step, all users with access to security-sensitive information have been required to change their passwords and use two-factor authentication."

It is somewhat surprising that Mozilla did not enforce two-factor authentication for its sensitive information in the past. Without two-factor authentication, all the attacker needed was one set of credentials to gain access.

The good news in this story is that there is no current threat to Firefox users as Mozilla has patched all the issues that the attacker may have had early access to. It's also reassuring that in the wake of this incident, Mozilla will now take its own security more seriously than ever before.

Sean Michael Kerner is a senior editor at eWEEK and InternetNews.com. Follow him on Twitter @TechJournalist.