Mozilla yesterday took the unusual step of yanking Firefox 16 from distribution just a day after its release.
The company said a critical vulnerability triggered the move.
The bug was apparently overlooked by Mozilla while it was developing Firefox 16, or introduced by the fixes baked into the upgrade that started reaching users early Tuesday.
"Mozilla is aware of a security vulnerability in the current release version of Firefox (version 16). Firefox version 15 is unaffected," said Michael Coates, Mozilla's director of security assurance, in a Wednesday post to the company's security blog.
On Tuesday, Mozilla rolled out Firefox 16, which featured patches for 24 vulnerabilities, 21 of which were judged "critical," the open-source developer's highest threat ranking.
Coates' comment that Firefox 15 did not harbor the bug indicated that the vulnerability was either an entirely new, and overlooked, flaw that affected only Firefox 16, or that it was introduced by the patching process.
The vulnerability has been assigned #799952 in Mozilla's Bugzilla change- and bug-tracking database. As is Mozilla's practice for unpatched flaws, details on that bug are not viewable by the general public. That Bugzilla number did not match any of the 24 vulnerabilities patched Tuesday.
Coates did not note when Mozilla became aware of the new vulnerability, or how it was discovered. Notes from a Mozilla meeting yesterday, however, show the company was aware of it by 11 a.m. PT Wednesday, when it told developers that a "chemspill" -- Mozilla's term for an emergency update -- was necessary.
As a precaution, Mozilla has pulled Firefox 16 from its primary download site and stopped serving it to current users as an upgrade, even though there was no sign that the bug was being exploited.
Users who do not want to wait for the patch but are afraid of running their already-obtained copies of Firefox 16 can downgrade to its precursor, Coates said.
Firefox will be re-released as version 16.0.1 some time Thursday, after which Mozilla will re-open the upgrade gates.
Mozilla has had to quickly re-release Firefox before. In December 2011 it did just that with Firefox 9 a day after that browser's release.
However, this was the first time Mozilla has pulled a version of Firefox from distribution because of a vulnerability.
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