Windows 7 UAC flaw silently elevates malware access

Weak links in chain of trust

Researchers have uncovered yet another flaw in Microsoft's Windows 7 beta that could allow attackers to gain full administrative privileges by bypassing the operating system's UAC, or user access control.

Researcher Rafael Rivera Jr. has released proof-of-concept code that demonstrates how unauthorized third-party software can elevate its privileges and install a potentially malicious payload on the latest version of Windows, which is still in beta. Researchers warn that anyone using the OS is vulnerable.

"Unfortunately this flaw is not just a single point of failure," writes security blogger Long Zheng. "The breadth of Windows executables is just too many and too diverse and many are exploitable."

The vulnerability stems from Microsoft's attempts to make UAC more palatable by allowing certain applications to make changes to the OS without first prompting the user for permission. Executables that are digitally signed are essentially given fast-track permission under UAC's default configuration. And it turns out many of these third-party executables are in turn able to invoke still more third-party code.

This gives rise to what Zheng calls "piggybacking," in which a proxy executable launches an elevated instance of rundll32.exe, a file that has existed in one form or another since before the days of Windows 95. Rundll32.exe can then point to a malicious payload, and because the payload has inherited the administrative privileges from its authorized parent process, UAC never prompts the user.

Rivera's proof-of-concept uses a one-line file called Catapult.exe as the proxy application that launches the rundll file. That in turn is able to execute a multi-line C++ program called Cake.dll, which reads the current process token and shows an on-screen message box. Of course, an attacker could do much more nefarious things.

A Microsoft spokesman said: "We are not aware of anyone impacted by this issue at this time, but it has already been addressed in a later internal beta build." In the meantime, Windows 7 users may want to set UAC to "high." ®