Weafer said the update is arriving through the worm's peer-to-peer connectivity. It looks for the old .A variant and updates it with the improvements seen in version .C, which include better HTTP and P2P code, stronger defense mechanisms, and advanced anti-forensic techniques.
It also drops a binary that's part of the Waledac spam malware. "Waledac is about stealing your confidential information and putting back doors on your system," said Weafer.
Weafer said that because P2P updating is slow compared with other methods, it may be several days before the impact of Conficker's changes become apparent.
As computer security firms assess the risk posed by the Conficker/Downadup worm, the Department of Homeland Security has released a DHS-developed detection tool to help organizations scan for computers infected by the worm.
The DHS US-CERT team created worm-scanning software for federal and state government agencies, commercial vendors, and critical infrastructure owners. It's being made available through the Government Forum of Incident Response and Security Teams Portal and to private-sector partners through various Information Sharing and Analysis Centers.
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This story was edited on April 9 to clarify statements made by Symantec.