Plexus worm targets Microsoft vulnerabilities

Antivirus software companies have warned of an e-mail worm that targets unpatched Microsoft Windows machines with either of two recently disclosed software vulnerabilities.

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

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

Plexus exploits the Windows component called Local Security Authority Subsystem Service, or LSASS, which Microsoft patched in April.  Plexus can also crawl through a hole in a Windows component called the DCOM (Distributed Component Object Model) 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 e-mail 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 e-mail addresses from a variety of files, including stored web pages written in HTML.

The worm then uses those e-mail addresses to target other users, sending out a flood of messages using a built-in SMTP (Simple Mail Transfer Protocol) 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.

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.

Paul Roberts writes for IDG News Service