Mozilla has released Firefox 3.0.8 to fix a pair of code execution holes that put users of the browser at risk of drive-by download attacks.  It includes a fix for one of the flaws exploited during this year’s CanSecWest Pwn2Own hacker contest.

The update also fixes a separate zero-day flaw disclosed earlier this week on a public exploit site. Both issues are rated “critical,” Mozilla’s highest severity rating.

The raw details:

• MFSA 2009-13: Security researcher Nils reported via TippingPoint’s Zero Day Initiative that the XUL tree method _moveToEdgeShift was in some cases triggering garbage collection routines on objects which were still in use. In such cases, the browser would crash when attempting to access a previously destroyed object and this crash could be used by an attacker to run arbitrary code on a victim’s computer. This vulnerability does not affect Firefox 2, Thunderbird 2, or released versions of SeaMonkey.
• MFSA 2009-12: Security researcher Guido Landi discovered that a XSL stylesheet could be used to crash the browser during a XSL transformation. An attacker could potentially use this crash to run arbitrary code on a victim’s computer. This vulnerability was also previously reported as a stability problem by Ubuntu community member, Andre. Ubuntu community member Michael Rooney reported Andre’s findings to Mozilla, and Mozilla community member Martin helped reduce Andre’s original testcase and contributed a patch to fix the vulnerability.