The worm, also known as W32.Downadup, also is resuming its scan-and-infect activity, searching for unpatched systems that can be exploited.
"With the discovery of a new variant, it is even more important for users to remain vigilant in detecting the Conficker worm and systematically cleaning systems of these infections to prevent potential, future cyber events," US-CERT warned.
Although Conficker/Downadup has infected upwards of an estimated 10 million computers, it so far does not appear to have been engaged in overt malicious activity. Because the malicious code can be detected and removed, the number of currently infected computers is estimated at several million.
The most recent variant appears to download additional malicious code onto compromised systems, possibly including copies of the Waledac Trojan, a spam tool. This could indicate an interest in using a Conficker botnet for spamming. Waledac has previously spread via e-mail messages that contain malicious links.
The original W32.Downadup.A exploited only the MS08-067 vulnerability in Windows XP SP2 and Windows 2003 SP1 operating systems, for which Microsoft issued an unusual patch outside of its regular monthly patching cycle. The more recent .B variant has added password-guessing and the ability to copy itself to USB drives to its capabilities, giving it a wider dissemination throughout a network once it is inside. The authors of the malware appear to be trying to gather low-hanging fruit in a network.
On April 1 a .C variant was scheduled to become active that would provide additional protection for the worm's command and control network. The worm uses an algorithm to generate a pseudo-random list of domains for its command and control network, which its infected clients check daily for instructions. Symantec analysts who examined the new code said that the variant would use a new algorithm to determine what domains to contact. It went from generating 500 domains a day to 50,000 domains with the new algorithm. Because a command and control server can be a weak spot whose elimination can disable a botnet, this could make Conficker/Downadup more difficult to attack.
One of Conficker's defenses is blocking access to sites providing detection and cleanup tools. This also makes it relatively easy to detect a possible infection. US-CERT advises that a simple test for the presence of Conficker/Downadup infection is to visit security solution Web sites. Detection and removal tools are available for download free from Symantec, Microsoft and McAfee.
"If a user is unable to reach any of these Web sites, it may indicate a Conficker/Downadup infection," US-CERT said. "The most recent variant of Conficker/Downadup interferes with queries for these sites, preventing a user from visiting them. If a Conficker/Downadup infection is suspected, the system or computer should be removed from the network or unplugged from the Internet in the case for home users."
Instructions and information on how to manually remove a Conficker/Downadup infection from a system have been published by several security vendors -- including include Symantec and Microsoft -- which offer free tools to verify the presence of a Conficker/Downadup infection and remove the worm.
You also can call the Microsoft PC Safety hotline at 1-866-PCSAFETY for assistance.
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