Worm feeds on Sasser-infected computers

A security researcher warns that computers hit by the Sasser worm remain vulnerable.

By Robert Lemos

Computers compromised by the Sasser worm may be vulnerable to a scavenging program that exploits a flaw in the software left behind by the worm, a security researcher said Thursday.

The worm—dubbed Dabber—has started spreading to Microsoft Windows systems, but likely won't have a large impact, said Joe Stewart, senior security researcher with network protection firm Lurhq.

"It is not going to be a big problem for anyone that is paying any attention at all to computer security," he said. "If somebody does get it, they probably already have Sasser and, most likely, ."

Dabber is not the first worm to exploit back doors into compromised systems left behind by previous attackers. Two worms, Doomjuice and Deadhat, .

However, Dabber may be the first worm to attack systems using a flaw in a previous malicious program. In this case, the file transfer protocol (FTP) server installed by Sasser to enable the worm to transfer itself to new hosts has a buffer-overflow vulnerability. Dabber uses that security flaw to spread to the new machine.

Once it copies itself to a new host, the worm will change the system settings so that operating system runs the malicious program every time it starts up. Dabber will also attempt to block other worms, which may have infected the machine, from running.

Finally, the worm will establish a back door into the software to allow knowledgeable attackers to take control of the system.

The scavenging worm arrives as German police are investigating more leads in the Sasser case. Already, the suspected author has been arrested in that country, based on information leaked to Microsoft by .

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