Security Experts:

Google Says Threat Actors Using New Code Signing Tricks to Evade Detection

Financially motivated threat actors have started using new code signing tricks to increase the chances of their software evading detection on Windows systems, Google’s Threat Analysis Group reported on Thursday.

The new technique has been used by the operators of OpenSUpdater, which cybersecurity vendors have classified as adware, potentially unwanted program (PUP), or potentially unwanted application (PUA). These types of pieces of software can ruin the user experience and they may attempt to download and install other shady programs.

The operation observed by Google has impacted many users in the United States, particularly people who download game cracks and what the tech giant has described as “grey-area software.”

OpenSUpdater operators have been signing their files using code-signing certificates from a legitimate certificate authority. In mid-August, Google noticed that some samples had an invalid signature, and further analysis revealed that this was actually done as part of an attempt to evade detection.

“In these new samples, the signature was edited such that an End of Content (EOC) marker replaced a NULL tag for the 'parameters' element of the SignatureAlgorithm signing the leaf X.509 certificate,” Google researchers explained in a blog post. “EOC markers terminate indefinite-length encodings, but in this case an EOC is used within a definite-length encoding.”

This type of signature is detected as invalid by security products that leverage OpenSSL, but Google researchers noticed that the Windows operating system treats the signature as valid.

Windows says OpenSUpdated signature is valid

Google noticed that OpenSUpdater authors have been experimenting with invalid encodings — they identified other variations as well — in an effort to find ways of evading detection.

Google said it reported its findings to Microsoft. SecurityWeek has reached out to Microsoft for comment and will update this article if the company responds.

UPDATE: A Microsoft spokesperson told SecurityWeek, “This technique is not a security vulnerability and we do not plan to address it with a security update.”

The company also noted that an attacker would not be able to use the technique to tamper with digitally signed files or download malicious packages, and pointed out that Microsoft Defender Antivirus detects and removes OpenSUpdater.

Related: Sectigo Revokes Certificates Used to Sign Malware Following Recent Report

Related: Use of Fake Code Signing Certificates in Malware Surges

Related: Code Signing Flaw Affects all Mac OS Versions Since 2005

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.