Sasser attacks provide fodder for new worm

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 on Thursday.

The worm -- dubbed Dabber -- has started spreading to Microsoft Windows systems, but probably 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, Agobot as well."

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

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 there, based on information leaked to Microsoft by informants interested in reward money.