Virus writers have created a worm that exploits coding flaws in the infamous Sasser worm to spread.

Dabber uses a flaw in the FTP server component of the Sasser worm. The worm will only infect users already infected by Sasser, according to security services firm LURHQ. "Even though we have seen worms utilize backdoors left behind by other worms, this is the first time we have seen a worm using a vulnerability in another worm in order to propagate," it said.

Worms like Doomjuice and Deadhat exploited the back doors opened by the MyDoom virus to spread but using flaws in virus code to propagate other malicious code is a significant departure. Dabber, first spotted yesterday, is spreading, but only to a modest extent, possibly because the spreading mechanism is quite complex.

Dabber scans for Sasser-infected hosts on port 5554. When it finds infected PCs it uses code from a Sasser-FTP exploit developed by "mandragore" of the Romanian Security Research team to seize control of PCs. Dabber than installs itself and deletes the registry keys of Sasser and other viruses. It creates a backdoor on infected machines on TCP port 9898 allowing hackers to download additional code, which might be far more malicious than Dabber itself.

To remove Dabber LURHQ advises users to kill the package.exe process using the Windows Task Manager. Remove the "sassfix" registry key. Delete package.exe from the Windows system directory and all start-up folders. Anti-virus vendors are in the process of developing signature updates to automatically detect and remove the worm. ®

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