Windows flaw could thwart user privilege limitations

Microsoft is investigating the bug

Microsoft has discovered a bug affecting Windows systems that could allow an intruder to gain nearly unlimited control of a computer by masquerading as a user with the highest level of system privileges. Microsoft said its security team is looking into the exploit.

The company released very little information, except for a brief notice on Nov. 24 on its Twitter security response page. The flaw was disclosed after someone posted proof-of-concept code on a "programming education site," according to Chester Wisniewski, senior security advisor at Sophos Canada, in a blog post. The code was subsequently removed, he noted.

The flaw enables elevation of privilege from a local user account level to the system account level. It also bypasses the user account control protection found in Windows Vista and Windows 7, Wisniewski explained. He described it as a Win32k.sys bug.

"The flaw is related to the way in which a certain registry key is interpreted and enables an attacker to impersonate the system account, which has nearly unlimited access to all components of the Windows system," he wrote in the blog.

In addition to Vista and Windows 7, other Windows operating systems (both 32-bit and 64-bit) are subject to the flaw, including Windows XP, Windows Server 2008 and Windows Server 2003, according to a post by Prevx blogger Marco Giuliani. He explained that the flaw cannot be exploited via remote code execution.

"It is a local privilege escalation exploit," Giuliani wrote. "This means that the potential malware must be already in the target machine to exploit this flaw." However, he described it as a critical flaw because it enables the local user to gain administrative privileges.

Microsoft hasn't rated the exploit nor said when, or if, it would provide a fix. Both antimalware vendors offered some advice in their blogs to avoid the zero-day flaw. The advice includes altering the registry for standard users or downloading security software.

About the Author

Kurt Mackie is the online news editor for the 1105 Enterprise Computing Group sites, including Redmondmag.com, RCPmag.com and MCPmag.com.

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