Mozilla re-released Firefox 16 today after pulling the browser from distribution Wednesday when one of its developers found a critical bug that could be used by attackers to hijack machines.
At around 2:30 p.m. ET, Mozilla turned on its upgrade servers and started pushing Firefox 16.0.1 to users who had earlier downloaded the flawed browser, or who were still running version 15 and earlier. About 30 minutes later, the open-source developer restored the patched program to its primary and secondary download pages.
Yesterday, Mozilla yanked Firefox 16 from its download websites and stopped serving it to existing users as an upgrade. The withdrawal was prompted by the discovery of a vulnerability, which the company then said "could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters."
On Wednesday, Mozilla promised to ship an emergency update -- it calls them "chemspills" in a nod to security toxicity -- today.
Mozilla has now provided more information about the bug, which it rated as critical.
"Mozilla security researcher 'moz_bug_r_a4' reported a regression where security wrappers are unwrapped without doing a security check in defaultValue()," an accompanying advisory noted. "This can allow for improper access to the Location object. In versions 15 and earlier of affected products, there was also the potential for arbitrary code execution."
The patch also addressed the issue reported by U.K.-based researcher Gareth Heyes early Wednesday that allowed access to user information, including Twitter usernames, through Firefox 16.
Firefox 16.0.1 fixed four flaws altogether, including two bugs that had caused a large number of crashes on users' systems.
Although Mozilla has had to quickly re-release Firefox before -- in December 2011 it pulled Firefox 9 a day after that edition's release -- this was the first time the firm pulled Firefox from distribution because of a vulnerability.
According to Web metrics company Net Applications, Firefox accounted for 20% of all browsers used worldwide last month, enough to keep Mozilla in second place -- behind only Microsoft and its Internet Explorer -- in the race for browser share.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is firstname.lastname@example.org.