Symantec has acknowleged that a recent anti-virus update caused some Windows PCs to crash repeatedly.

"The affected products included Symantec Endpoint Protection 12.1 and its small-business and cloud-based variants, all of which are centrally managed products for corporate IT departments, and the consumer products Norton 360, versions 4 through 6, and Norton 2010, 2011 and 2012," writes SecurityNewsDaily's Paul Wagenseil. "All those products use SONAR, Symantec's behavioral-based malware detection software. The faulty update was a malware-definition update, which adds to the library of known malware, not a software update."

"After the problem manifested itself on 11 July, Symantec rolled back the rev11 signature set -- it was only being pushed out by the company's LiveUpdate servers for just over eight hours," writes ZDNet's David Meyer. "Soon afterwards, Symantec posted updated -- and less crash-prone -- 'r12' signatures to the public LiveUpdate production servers."

"Some users reported substantial numbers of affected Windows XP machines," writes Computerworld's Gregg Keizer. "Someone identified as Mark Daeth said more than 1,000 systems at his workplace had blue-screened. 'We have pushed out R12 to as many PCs as we can, but over 30% of our PC environment still will not boot,' said Daeth on Thursday, referring to the revised SONAR update. Daeth is the IT manager at Charlotte-based AAA Carolinas, the American Automobile Association group responsible for North Carolina and South Carolina members."

"For systems that refuse to run following the installation of this update Symantec has published a workaround to help get people’s XP machines back up and running," writes Forbes' Adrian Kingsley-Hughes.