Microsoft gave its digital imprimatur to a rootkit that decrypted encrypted communications and sent them to attacker-controlled servers, the company and outside researchers said. The blunder allowed the malware to be installed on Windows machines without users receiving a security warning or needing to take additional steps. For the past 13 years, Microsoft has required third-party drivers and other code that runs in the Windows kernel to be tested and digitally signed by the OS maker to ensure stability and security. Without a Microsoft certificate, these types of programs can't be installed by default.

Earlier this month, Karsten Hahn, a researcher at security firm G Data, found that his company's malware detection system flagged a driver named Netfilter. He initially thought the detection was a false positive because Microsoft had digitally signed Netfilter under the company's Windows Hardware Compatibility Program. After further testing, Hahn determined that the detection wasn't a false positive. He and fellow researchers decided to figure out precisely what the malware does. "The core functionality seems to be eavesdropping on SSL connections," reverse engineer Johann Aydinbas wrote on Twitter. "In addition to the IP redirecting component, it also installs (and protects) a root certificate to the registry."

Spent some more time analyzing the Chinese netfilter driver discovered by @struppigel:

The core functionality seems to be eavesdropping on SSL connections. In addition to the IP redirecting component, it also installs (and protects) a root certificate to the registry.

— Johann Aydinbas (@jaydinbas) June 19, 2021

A rootkit is a type of malware that is written in a way that prevents it from being viewed in file directories, task monitors, and other standard OS functions. A root certificate is used to authenticate traffic sent through connections protected by the Transport Layer Security protocol, which encrypts data in transit and ensures the server to which a user is connected is genuine and not an imposter. Normally, TLS certificates are issued by a Windows-trusted certificate authority (or CA). By installing a root certificate in Windows itself, hackers can bypass the CA requirement. Microsoft's digital signature, along with the root certificate the malware installed, gave the malware stealth and the ability to send decrypted TLS traffic to hxxp://110.42.4.180:2081/s.

In a brief post from Friday, Microsoft wrote, "Microsoft is investigating a malicious actor distributing malicious drivers within gaming environments. The actor submitted drivers for certification through the Windows Hardware Compatibility Program. The drivers were built by a third party. We have suspended the account and reviewed their submissions for additional signs of malware." The post said that Microsoft has found no evidence that either its signing certificate for the Windows Hardware Compatibility Program or its WHCP signing infrastructure had been compromised. The company has since added Netfilter detections to the Windows Defender AV engine built into Windows and provided the detections to other AV providers. The company also suspended the account that submitted Netfilter and reviewed previous submissions for signs of additional malware.