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Beware of malformed Browser Elections

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Security researchers have warned of a new vulnerability afflicting older versions of Windows that could allow attackers to take complete control of machines running the operating systems.

The flaw in “BowserWriteErrorLogEntry()” function within the Windows mrxsmb.sys driver “could be exploited by remote attackers or malicious users to cause a denial of service or take complete control of a vulnerable system,” researchers from French security firm Vupen warned. The warning came after proof-of-concept code was posted on Monday to the Full-disclosure mailing list.

Attacks are triggered by sending vulnerable machines malformed Browser Election requests that cause a heap overflow in the mrxsmb.sys driver. The term “Browser” in this context doesn't refer to an application used for browsing websites, but rather, to networking technology used by older versions of Windows. The malformed Browser Election requests contain an “overly long Server Name string,” according to vulnerability tracking service Secunia, which has also weighed in on the the report.

Vupen, which rates the vulnerability as critical, has confirmed the bug in Windows Server 2003 SP2 and Windows XP SP3. Secunia rates it as moderately critical, the third most serious notch in its five-tier system.

A Microsoft spokesman said company researchers are investigating the reports and “will take appropriate action to help protect customers” when the inquiry is complete. ®

Update

Mark Wodrich of the Microsoft Security Response Center has blogged that the vulnerability doesn't allow attackers to remotely execute code on vulnerable 32-bit systems. Remote execution by be possible on vulnerable systems with 64 bits because they have 8GB of contiguous virtual address space mapped. The company is continuing to investigate.

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