Anti-virus software companies are warning about a new e-mail worm that targets unpatched Windows machines with either of two recently disclosed software vulnerabilities.

The new worm, known as both "Plexus" and "Explet.A," was first detected in the middle of last week and spreads by exploiting Windows machines with vulnerabilities used by two recent worms, Sasser and Blaster, according to alerts. McAfee Anti-virus Emergency Response Team and Symantec have both said the new worm does 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, or 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 DCOM interface, which handles messages sent using the RPC protocol.

Plexus spreads in files attached to e-mail messages with fake 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 it runs each time Windows starts. It also scans the hard drive, harvesting e-mail addresses from a variety of files, including stored Web pages.

The worm then uses those e-mail addresses to target other users, sending out a flood of messages using a built-in 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.

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