Computer Associates caused some headaches last week after its antivirus software inadvertently flagged part of the Windows OS (operating system) as malware.

The Sans Internet Storm Center reported the problem on Friday, saying 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. The Sans note on this issue can be found here.

The problem was that eTrust Antivirus was mistakenly flagging the Windows Lsass.exe process, said Bob Gordon, a CA spokesman. "CA quickly discovered and fixed an issue that temporarily caused some customers to detect a problem in their Lsass.exe files," he said in an email.

According to Gordon, it took CA less than seven hours to fix the mix-up. CA's latest updates can be found here.

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's guidance for users who have been experiencing crashes as a result of the buggy update can be found here.

CA's product and similar antivirus products use 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 mix-up apparently did not disrupt a large number of users, but it still reflects poorly on vendors such as CA, Ullrich said. "It's another loss in trust toward the antivirus business," he said. "It tells you that the antivirus vendors don't do the testing."