A new Internet worm is spreading by exploiting a flaw in the Sasser worm, according to an alert issued Thursday.

The new worm, tentatively named Dabber, takes advantage of a vulnerability in an FTP server component in the Sasser worm and may have infected thousands of computers infected with Sasser. Dabber is believed to be the first worm that spreads specifically by targeting a flaw in another worm’s code, according to an advisory published by LURHQ Corp., a Chicago managed security services company.

The worm uses code written to exploit the FTP flaw and was recently released on the Internet, scanning the Internet on port 5554 for computers running Microsoft Corp.’s Windows operating system and infected with Sasser, LURHQ said.

When it finds vulnerable hosts, it connects to the victim and uses a built-in FTP server to transfer the worm file, named “package.exe,” to the system. When run, the Dabber worm installs itself on Windows, shuts down the Sasser worm and other worm processes, then prevents them from running again. Dabber also opens TCP port 9898 as a backdoor, which can be used by a remote attacker to download other code or communicate with the infected host, LURHQ said.

Dabber did not appear to be spreading quickly Thursday, but the number of infections was escalating, LURHQ said.

Sasser appeared on May 1, and exploits a recently disclosed hole in a Windows component called the Local Security Authority Subsystem Service, or LSASS. Microsoft Corp. released a software patch, MS04-011, on April 13.

Dabber is similar to Sasser and earlier worms like Blaster because users do not need to receive an e-mail message or open a file to be infected. Instead, just having a Sasser-infected Windows machine connected to the Internet is enough to catch Dabber.

Last week, Microsoft acknowledged that more than 1.5 million copies of a Sasser cleanup tool were downloaded from its Web site within the first 48 hours after it was offered.

LURHQ posted instructions for shutting down and removing Dabber.

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