Like the original worm, the variation affects computers running Microsoft's Windows XP and Windows 2000 operating systems.
Blaster takes advantage of a known vulnerability in a Windows component called the DCOM (Distributed Component Object Model) interface, which handles messages sent using the RPC (Remote Procedure Call) protocol.
Windows XP users infected with Blaster report frequent system reboots and messages about "System Shutdown". Both Windows XP and Windows 2000 users may experience significant system slowdowns when using Windows or surfing the internet if their machine is infected, said Alfred Huger, director of engineering at Symantec Security Response.
The latest worm, called W32/Blaster-B, works like the original Blaster, but comes packaged as a file called teekids.exe as opposed to msblast.exe used by the original worm.
Blaster-B also replaces an internal message stating "LOVE YOU SAN" with some colourful suggestions for Microsoft chairman and chief software architect Bill Gates, according to security firm Sophos.
A second threat, the W32/RpcSpybot-A, exploits the same vulnerability as Blaster. It contains code that creates a backdoor on infected machines which could give an attacker control over that machine, Sophos said.
RpcSpybot-A uses the same exploit as the worm, but is an IRC (Internet Relay Chat) trojan which scans the internet for vulnerable systems. It exploits the RPC DCOM security hole, and then uses IRC to control the infected systems remotely for use in a denial-of -service attack, Symantec's Huger said.
Similar threats using the DCOM security hole have been circulating on the internet for weeks, predating the release of Blaster, he added.
Neither Sophos nor Symantec knew of any infections stemming from RpcSpybot-A.
The emergence of new versions of the Blaster worm complicates the job of blocking attacks for customers who have not patched their vulnerable Windows systems, said Chris Belthoff, senior security analyst at Sophos.
"We're starting to see the floodgates open. We were hopeful that Blaster would be the first and last, but it seems like that's not going to happen," he added.
Belthoff and others recommended that those using vulnerable versions of Windows XP and Windows 2000 apply a software patch released by Microsoft in July. ( http://www.microsoft.com/technet/security/bulletin/MS03-026.asp)
Users can also search their hard drive for the worm file, msblast.exe or the new teekids.exe. Symantec's Huger said users who find the file should delete it and immediately obtain and install the appropriate patch from Microsoft.Paul Roberts writes for IDG News Service