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Mutant son of MyDoom plans three-pronged attack

By John Leyden, The Register Jun 3 2004 7:55AM

Virus writers have used code from the infamous Mydoom worm to create a potentially dangerous new Internet worm which uses multiple methods to spread.

Plexus-A spreads using three different methods: infected email attachments, file-sharing networks and Windows vulnerabilities (the LSASS vulnerability used by Sasser and the RPC DCOM flaw used by Blaster). The as yet unknown virus authors used MyDoom source code as the basis for creating Plexus, according to an analysis of the worm by Russian AV firm Kaspersky Labs.

David Emm, senior technology consultant at Kaspersky Labs, said that the multiple spreading methods is helping Plexus to infect more machines. No worm since Nimda has used as many methods to spread, according to Emm. Kaspersky rates Plexus as a moderate risk. It is spreading - but nothing like as fast as Sasser or Blaster - and the main concern about the worm stems from the fact it creates a backdoor for hackers on infected machines. These compromised machines could be used for spam runs or as a platform for DDoS attacks. However the motives of the virus authors behind the worm remain unclear.

Plexus-A chooses from five different email message headers in an attempt to bamboozle users. Each message has a different header, body and attachment name. The only characteristic which does not change is the file size: 16208 bytes when compressed with FSG and 57856 when uncompressed. Mac and Linux users are - as usual - immune but Plexus is a menace for Windows users.

Upon execution Plexus-A copies itself to the Windows system registry under the name upu.exe, which runs every time a machine is rebooted after infection. Plexus sends copies of itself to email addresses harvested from the hard drives of infected machines.

The worm is among the first to specifically target users of Kaspersky Labs' AV software. Plexus' payload includes attempts to prevent downloads of Kaspersky Anti-Virus database updates. Plexus also scans the Net for systems vulnerable to the flaws it exploits. The worm opens a backdoor onto infected machines on port 1250, making it possible for files to be remotely uploaded to and from the victim machine. The open port leaves the victim machine vulnerable to further attacks, Kaspersky Labs warns.

Users are advised patch Windows boxes, update anti-virus signature files and use firewalls to shelter against Plexus and similar irritants that are doubtless just around the corner. Is there no end to this viral madness? ®

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