GuardBreaker: Derailing AI-assisted malware analysis with a code comment

Malware developers have long adapted their code and tactics to the defenses and scrutiny that are likely to stand in their way. Using various evasion and anti-analysis methods, they routinely attempt to hinder code analysis or prevent their malware from revealing its true behavior while under inspection. Other tools – notably, EDR killers, documented extensively by ESET researchers – go straight after security solutions themselves.

As LLM-based tools increasingly assist with various security tasks, including code triage and analysis, it was only a matter of time before threat actors began to look for practical ways to subvert them, too. Alongside conventional evasion techniques, some are taking a different tack: the adversarial input that’s intended to frustrate analysis is left in plain sight.

ESET researchers recently spotted one such attempt in a VBScript that the Russia-aligned group UAC-0099 used in the early stages of an attack against a target in Ukraine. By inserting a decoy request for guidance on building a nuclear weapon into the script’s comment, the bad actor aimed to trip the safety guardrails of an LLM-powered code scanner and cause it to stop inspecting the rest of the file – before ever reaching the malicious code. The script’s purpose was to download and install MATCHBOIL, a loader used exclusively by this group to deliver additional payloads.

This simple technique, which ESET has named GuardBreaker, relies on precisely the kind of ‘request’ that LLM models are known to decline:

Unlike many other tricks in attackers’ evasion playbooks, this decoy comment is there for ‘everyone’ – especially for the models analyzing the code – to see. In addition, it has no effect on the script’s behavior at runtime, of course. Nonetheless, its presence suggests that UAC-0099 was accounting for an AI system in the target’s defenses – just as in other recent attacks the group also checked for processes associated with established analysis tools such as IDA and Wireshark.