Microsoft patch fails to fix Installer zero-day affecting every version of Windows

A hand holding a magnifying glass reveals a red lock, unlocked among several blue locked locks

Cyber criminals are testing out a proof-of-concept malware that targets a zero-day escalation of privilege exploit in the Microsoft Windows Installer.

The flaw, which enables hackers with a limited user account to elevate their privileges to become an administrator, affects every version of Microsoft Windows, including fully patched Windows 11 and Server 2022.

Malware samples have already been detected in the wild that are attempting to take advantage of this vulnerability, according to a blog post by security researchers at Cisco Talos.

It was security researcher Abdelhamid Naceri who initially discovered this elevation of privilege vulnerability and worked with Microsoft to address it. Microsoft then released an update that was intended to fix CVE-2021-41379 on 9 November as part of its monthly security update.

However, the patch failed to fix the vulnerability, and Naceri published a proof-of-concept exploit code on GitHub on 22 Nov that still works despite the fixes implemented by Microsoft.

“The code Naceri released leverages the discretionary access control list (DACL) for Microsoft Edge Elevation Service to replace any executable file on the system with an MSI file, allowing an attacker to run code as an administrator,” said Jaeson Schultz, technical leader for Cisco's Talos Security Intelligence & Research Group.


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According to a posting by Naceri on GitHub, the technique may not work on every installation, because windows installations, such as server 2016 and 2019, may not have the elevation service.

“I deliberately left the code which take over file open, so any file specified in the first argument will be taken over with the condition that SYSTEM account must have access to it and the file mustn't be in use. So you can elevate your privileges yourself,” he said.

Naceri added that the best workaround available at the time of writing this is to wait for Microsoft to release a security patch, due to the complexity of this vulnerability.

“Any attempt to patch the binary directly will break windows installer. So you better wait and see how Microsoft will screw the patch again,” he said.

Rene Millman

Rene Millman is a freelance writer and broadcaster who covers cybersecurity, AI, IoT, and the cloud. He also works as a contributing analyst at GigaOm and has previously worked as an analyst for Gartner covering the infrastructure market. He has made numerous television appearances to give his views and expertise on technology trends and companies that affect and shape our lives. You can follow Rene Millman on Twitter.