F-Secure has issued a critical security patch for its antivirus engine after a flaw was discovered that could crash the protection software.

An advisory from the security firm warns that specially prepared ZIP files could be designed to bring down the antivirus engine and leave targeted systems vulnerable. Customers are being urged to patch systems immediately. 

"Our guidance here is the same as for patches from any other vendor: patch now before someone figures out how to exploit the vulnerability," said Mikko Hyppönen, chief research officer at F-Secure.

The flaw was discovered by penetration tester Thierry Zoller, who won praise from F-Secure for giving the firm time to devise a patch before releasing details of the problem.