As a result of the snafu McAfee users who applied the update were falsely warned that their systems were infected by the Exploit-BO JavaScript virus after visiting sites including ESPN and Friendster, the SANS Institute's Internet Storm Centre warns.
The dodgy update is DAT 5197 released on January 2. McAfee pulled the update and issued a replacement signature update (DAT 5198) shortly afterwards.
Faulty anti-virus signature updates are not uncommon across the industry. Spookily rival vendor CA experienced exactly the same type of problem, again involving legitimate JavaScript files been falsely identified as viruses only on Monday. This suggests a general difficulty in tuning heuristic (generic) detection of anti-signature tools to recognise the difference between legitimate JavaScript apps and malware.