A memory corruption flaw was discovered in Xorg X server's Render extension. Problem originally reported as Firefox crashing X server issue was tracked down to an incorrect calculation issue in mod() macro by Olivier Fourdan:

https://bugzilla.redhat.com/showbug.cgi?id=495733#c15

This issue could cause mod() to return value greater than its second argument (the divisor), resulting in excessive read and write in Render composite operation, causing heap or video memory corruption.