Antivirus software companies are warning customers about a new email worm that targets unpatched Microsoft Windows machines with either of two recently disclosed software vulnerabilities.

The new worm, known as both Plexus and Explet.A, was first detected last Wednesday and spreads by exploiting Windows machines with vulnerabilities used by two recent worms, Sasser and Blaster, according to alerts.

Network Associates' McAfee Antivirus Emergency Response Team and Symantec both said the new worm did not pose a serious threat, but issued software updates on Thursday to detect it.

Like Sasser, Plexus can exploit the recently disclosed hole in the Windows component called Local Security Authority Subsystem Service (LSASS) which Microsoft patched in April. Like the Blaster worm that appeared in August 2003, Plexus can also crawl through a hole in a Windows component called the Distributed Component Object Model (DCOM) interface, which handles messages sent using the RPC (remote procedure call) protocol. (See http://www.microsoft.com/technet/security/bulletin/MS03-026.mspx and http://www.microsoft.com/technet/security/bulletin/MS04-011.mspx.)

Plexus spreads in files attached to email messages with faked sender addresses and vague subjects such as "RE: order", "For you" and "Good offer". When users open the virus file, the worm is launched and alters the configuration of Windows so that the worm program runs each time Windows starts. It also scans the hard drive of infected computers, harvesting email addresses from a variety of files, including stored Web pages written in Hypertext Markup Language.

The worm then uses those email addresses to target other users, sending out a flood of messages using a built-in Simple Mail Transfer Protocol (SMTP) engine. It is also able to spread to other computers on a network using shared folders and the copies itself to the shared folder file on the KaZaa peer-to-peer network using a variety of file names, including Shrek_2.exe, playing on the popularity of the recently released animated film.

Antivirus companies recommended that Windows users who have not done so already apply software patches for the LSASS and DCOM and update their antivirus software to spot Plexus.