Mozilla has issued the first patch for Firefox 3.5, fixing a flaw that went public. Firefox 3.5.1 patches a critical flaw in the new TraceMonkey JavaScript engine's Just-in-Time (JIT) compiler. "This could be exploited by an attacker to run arbitrary code such as installing malware," the accompanying security advisory warned.

Exploit code for the vulnerability was posted to the milw0rm malware site, four days after Mozilla developers had discovered the bug and began working on a fix.

Andreas Gal, a project scientist at the University of California - and a key contributor to the TraceMonkey engine that Mozilla added to Firefox with Version 3.5 - said that it appeared the hacker had created the attack code after spotting discussions and test cases on Bugzilla, Mozilla's bug- and change-tracking database. "Looking at the exploit code and our test cases, I think this is self-inflicted and we should have hidden the bug earlier," said Gal in one of several comments appended to the vulnerability's Bugzilla entry.

Although Mozilla had originally planned to release Firefox 3.5.1 later in the month, developers accelerated the schedule to plug the hole.

Yesterday's update also addressed several unspecified stability issues and fixed a long-loading problem for some Windows users, according to Firefox 3.5.1's release notes.

Firefox 3.5.1 can be downloaded in Windows, Mac and Linux editions from Mozilla's site; current users can update by choosing "Check for Updates" under the "Help" menu.

While Mozilla rushed out a fix, rival Microsoft has yet to patch a bug that was publicly disclosed the same day that the attack code exploiting Firefox 3.5 hit the web.

Hackers have been using a vulnerability in an ActiveX control used to publish Excel spreadsheets online and to display those in Internet Explorer, Microsoft's browser.

Microsoft has provided a tool that users can download, install and run that disables the ActiveX control - and has provided instructions and tools for enterprises to do the same on a massive scale - but it did not deliver a patch for the underlying problem, its regularly-scheduled monthly patch day.

Exploits of the bug have been reported by several security organisations, including SANS' Internet Storm Center (ISC), which Thursday said that it had spotted SQL injection attacks against several websites. The SQL-based attacks try to inject script code into the site; the code points to a known hacker domain, which in turn links to other sites that serve up malicious JavaScript that launches an exploit for the ActiveX bug.

"If you haven't set those kill bits yet, be sure that you do now because the number of sites exploiting this vulnerability will probably rise exponentially soon," said Bojan Zdrnja, an ISC analyst in a warning posted to the centre's site. The "kill bits" reference is to the downloadable tool that Microsoft had created, which disables the ActiveX control by modifying the Windows registry.