Computer Associates (CA) caused some headaches on Friday when its anti-virus software inadvertently flagged part of Windows as malware.

The SANS Internet Storm Center reported the problem Friday saying that an overnight update to CA's eTrust Antivirus signatures had caused the software to flag a security-related process in Windows as malicious. The faulty update caused some Windows 2003 servers to crash and become unusable, SANS said.

eTrust has mistakenly flagged the Windows Lsass.exe process, said a CA spokesman. "CA quickly discovered and fixed an issue which temporarily caused some customers to detect a problem in their Lsass.exe files," he said.

According to CA, it took less than seven hours to fix the mix-up. The problem was introduced in the 30.3.3054 update, released at 2:53 a.m, Eastern Standard Time (EST) and fixed in the 30.3.3056 update, issued at 9:34 a.m EST.

The Lsass.exe process is part of Windows' security mechanism. So users who had set their eTrust Antivirus to automatically remove malicious software may have found that their systems crashed and were unable to boot up Windows once Lsass.exe was removed.

CA has produced guidance for users who have been experiencing crashes as a result of the buggy update.

Anti-virus software like CA's uses digital fingerprints, called signatures, to identify malicious software. In this case, eTrust Antivirus apparently mistook Lsass.exe for the Win32/Lassrv.B virus.

It is not unheard of for signature files to mistakenly identify legitimate software as malware, but it is remarkable that CA's software made the mistake with a well-known Windows component, according to Johannes Ullrich, chief research officer at SANS. CA should have been able to detect the problem in its quality-assurance testing, he said.

The mixup apparently did not disrupt a large number of users, but it still reflects poorly on vendors like CA, Ullrich said. "It's another loss in trust toward the anti-virus business," he said. "It tells you that the anti-virus vendors don't do the testing."