Windows URI Handling Flaw Leads to Drive-by Code Execution

A pair of German security researchers have publicly documented the discovery of a drive-by code execution vulnerability in Windows 10 and criticized Microsoft for botching its response to the still-unfixed security problem.

The security defect is an argument injection in the Windows 10/11 default handler for ‘ms-officecmd:’ URIs and was successfully chained with mitigation bypasses to launch single-click browser-based exploits on Microsoft’s flagship operating system.

The research team — Fabian Braunlein and Kukas Euler of Positive Security — published a demo exploit showing a remote code execution drive-by exploit and upbraided Microsoft’s security response center for initially misdiagnosing the severity of the flaw and denying full bug-bounty disclosure payments.

According to a technical paper published this week, Microsoft silently patched the issue after five months of receiving the vulnerability data (the company has not issued a CVE identifier) but Redmond’s patch “ failed to properly address the underlying argument injection.”

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The researchers claim the underlying argument injection weakness is currently still present on Windows 11, the newest version of the operating system.

From the research paper:

“Code execution is triggered by a malicious website which performs a Javascript redirect to a crafted ms-officecmd: URI (a scheme used by the Microsoft Office UWP app to launch other Office desktop applications). We exploit an argument injection vulnerability in the URI handler and bypass a security measure in Electron to inject an arbitrary OS command via the –gpu-launcher parameter of the Microsoft Teams Electron app.”

In addition to exploitation via booby-trapped websites, the researchers warn that hackers could launch these code execution attacks via desktop applications that perform unsafe URL handling.

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The research pair published the original advisory it sent to Microsoft and expressed frustration with the company’s responsiveness and communications transparency.

After initially dismissing the vulnerability report, the researchers say Microsoft acknowledged the severity with a “Critical, RCE” classification but paid out only a fraction of the bug bounty advertised for high-severity bugs.

The researchers had to file an appeal just to get the vulnerability acknowledged and then found itself haggling with the software giant over a $5,000 bug bounty payout.  

“We decided to find a code execution vulnerability in a default Windows 10 URI handler, and succeeded within two weeks. Considering the amount of URI handlers Windows ships with, it seems very likely that others are vulnerable too,” the researchers warned.

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