A new variant of the infamous Blaster worm which claims to fix infected systems is circulating on the Net.

The Welchia worm exploit the same Remote Procedure Call (RPC) DCOM vulnerability used by Blaster to load itself onto unpatched Windows machines.

Welchia (AKA MSblast-D, Nachi and LovSan-D) also exploits the WebDav vulnerability (see this MS advisory from March) using TCP port 80. The worm specifically targets machines running Microsoft IIS 5.0 Web server.

Once it loads itself onto machines, Welchia attempts to download the DCOM RPC patch from Microsoft's Windows Update Web site, install it, and then reboot the computer. The worm also attempts to remove Blaster from infected systems.

Welchia checks for active machines to infect by sending an ICMP echo, or PING, which will result in increased ICMP traffic.

Despite an apparent lack of malicious intent, Welchia is still creating a great deal of unwanted traffic as it scans for new computers to infect. And the worm attempts to alter machines without their user's permission.

Welchia provokes the same ethical issues as CodeGreen, the virus-like code that attempted to patch systems infected with the Code Red worm. CodeGreen only spread itself onto servers that scan a box onto which it has been installed - so it is far less aggressive than Welchia.

The idea of "fighting fire with fire" in this way was used before by the Cheese worm, which fixed a flaw exploited by the Li0n worm on vulnerable Linux boxes.

Meanwhile, the original Blaster worm (and its variants) continue to spread. According to Network Associates' Hackerwatch.org operation, Blaster variants have infected more than 1.4 million PCs and continues to find fresh victims at a rate of between 15,000 to 35,000 per hour. After a marked decrease in the spread of the worm last Tuesday, rates of infection from Blaster variants have stayed consistently high. The data comes from information collected from users of McAfee's Personal Firewall application and does not distinguish between variants of the worm. ®

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