Researchers claim to have found exploit code for a new flaw that puts firms using Microsoft Vista and Windows 7 at great risk.
The news comes as Microsoft prepares to release patches on Patch Tuesday.
The flaw lies in the SMB (Server Message Block) 2 software in Windows, said Bojan Zdrnja, a handler for the SANS Internet Storm Center. Exploit code was released around 11 p.m. U.S. Eastern time, he said.
Zdrnja said he tested the exploit code and it works on fully patched Vista machines running Service Pack 1 or 2 as well as Windows 7. It may also affect Windows Server 2008. When successfully attacked, the exploit will cause the targeted machine to crash.
"You get the blue screen of death," Zdrnja said.
Researchers don't know yet if the flaw is remotely exploitable, he said. Just one malicious packet is needed to crash a machine. Most PCs on internal networks keep port 445 open, which is used for file sharing.
That's dangerous, since if a hacker already has access to a compromised computer within the network, it would be possible to crash all the other machines, Zdrnja said. Administrators should disable access to the port.
Home users usually have that port open, too, Zdrnja said. But for users who join a public Wi-Fi network, Windows will ask if it is a public network and, if it is, then block port 445.
A module for the exploit has already been created for Metasploit, a hacker toolkit used to attack PCs, Zdrnja said.
Microsoft is due to release its five patches on Tuesday, all for "critical" flaws, the company's most severe threat rating. Zdrnja said it's not known if this latest flaw will be addressed.
If it isn't patched on Tuesday, Zdrnja said the flaw is so potentially harmful that he would not be surprised if Microsoft did an off-schedule patch release.
"This is really serious," Zdrnja said. "It can potentially affect a huge number of machines."
The SANS Internet Storm Center has published a short diary entry about the flaw. Microsoft officials did not have an immediate comment but said they were investigating.