February 15, 2011, 6:14 PM — A security researcher yesterday disclosed a new unpatched bug in Windows that some experts believe could be used to remotely hijack a PC.
Microsoft said it is investigating the flaw, but provided no information on any analysis it's conducted thus far.
"Microsoft is investigating public claims of a possible vulnerability in Windows SMB [Server Message Block]," said Jerry Bryant, a group manager with the Microsoft Security Response Center (MSRC), in an e-mail Tuesday. "Once we're done investigating, we will take appropriate action to help protect customers. This may include providing a security update through the monthly release process, an out-of-cycle update or additional guidance to help customers protect themselves."
The researcher, identified only as "Cupidon-3005," posted exploit code Monday for the vulnerability, which is reportedly in the "BowserWriteErrorLogEntry()" function within the "mrxsmb.sys" driver. The driver processes requests to the Server Message Block protocol that Windows uses for network communication.
SMB is mainly used to provide file- and printer-sharing to Windows machines.
According to French security company Vupen, which rated the bug as "critical," a successful exploit could "cause a denial of service or take complete control of a vulnerable system." The former would crash Windows and produce the notorious "Blue Screen of Death" that illustrates a serious collapse of the operating system.
Danish vulnerability tracker Secunia, which ranked the flaw as "moderately critical" -- the middle threat level in its five-step system -- also said that hackers could exploit the bug to compromise a PC.
"Successful exploitation may allow execution of arbitrary code," warned Secunia.
Secunia added that a buffer overflow could be triggered by sending a too-long Server Name string in a malformed Browser Election Request packet. In this context, "browser" does not mean a Web browser, but describes other Windows components which access the OS' browser service.
Vupen confirmed that Windows XP Service Pack 3 (SP3) and Windows Server 2003 SP2 are vulnerable to attack, while Secunia reported that other versions of Windows may also be affected.
Cupidon-3005 taunted Microsoft in a message posted to the Full Disclosure security mailing list. "Apologies if this puts a downer on the MSRC valentines day sausage fest," the message read.
Microsoft's next regularly-scheduled Patch Tuesday is March 8, but if the company keeps to its usual timeline, it's unlikely to issue a fix by then unless a large number of in-the-wild attacks exploiting the vulnerability appear in the next three weeks.