Security researchers at Microsoft have warned of a significant climb in exploits of a Windows bug it patched with an emergency fix last month, confirming earlier reports by Symantec.
Microsoft again urged users to apply the MS08-067 patch if they have not already done so.
The new attacks, which Microsoft's Malware Protection Center said began over the weekend but spiked in the past two days, use the same worm Symantec first spotted last Friday.
Dubbed "Conficker.a" by Microsoft and "Downadup" by Symantec, the worm exploits the vulnerability in the Windows Server service, used by all versions of the operating system to connect to file and print servers on a network.
Microsoft patched the bug in an out-of-cycle update five weeks ago after it discovered a small number of infected PCs, most of them in Southeast Asia .
According to Ziv Mador, a researcher with the Malware Protection Center, the new wave of attacks has spread in corporations and hit "several hundred" home users. Most of the infection reports have come from U.S. users, said Mador in a post to the center's blog , but his team has received calls from users in several other countries too.
The worm avoids infecting Ukrainian computers, Mador said, which may indicate the malware was written by a Ukrainian; hackers often purposefully skip systems in the country where they live, hoping that will postpone or eliminate any reaction by local authorities.
"It is also interesting to note that the worm patches the vulnerable API in memory so the machine will not be vulnerable anymore," said Mador. "It is not that the malware authors care so much about the computer as they want to make sure that other malware will not take it over too."
The worm also resets the machine's system restore point, said Microsoft in its technical write-up , which may make it difficult or impossible to "roll back" Windows to a pre-infection state.
PCs that have been patched with the MS08-067 fix are protected, Mador stressed.
Last week, Symantec bumped up its ThreatCon security alert status from "1" to "2" in response to attacks it had tracked hitting its customers and honeypots. Others security vendors, however, disputed the uptick.