Some copies of email attachments infected with the prolific SirCam worm also carry copies of other viruses, security experts have warned.
MessageLabs, a managed services firm that scans its users email for viruses, said it has intercepted more than 100 emails doubly infected emails which have the potential to trip up the disinfection process of antiviral scanners.
Incomplete or botched cleaning could, at least in theory, create modified versions of the SirCam virus that are hard to detect, it warned. This is noteworthy because the bandwidth-hogging, privacy-threatening SirCam worm is on its way to becoming the most common virus ever.
MessageLabs has reported intercepting more than 100 doubly infected files, such as Sircam infected with FunLove, which infects applications in the Windows and Program Files folders; or SirCam infected with Kriz, a polymorphic Windows executable virus.
Graham Cluley, senior technology consultant at antivirus vendor Sophos, said the possibility of a "cocktail" of different infections affecting the same executable has been known about for some time and does carry with it a small risk.
"Normally an antivirus scanner will be able to strip off different layers of viruses. But when viruses mate together in a single file they can corrupt themselves," said Cluley.
This is rare but if it happens vendors would need to update their antivirus signatures to detect the hybrid infection or, if appropriate, fix a bug in their own software.
Double infections occur because users have got themselves infected with two viruses, the PC equivalent of contracting both herpes and syphilis. Users can most effectively protect themselves against viruses by practising "safe computing" - that is by regularly updating their antiviral protection and by deleting emails containing suspicious attachments. ®
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