Cybersecurity researchers at the CERT Coordination Center (CERT/CC) have raised an alarm about a newly identified evasion technique, designated as VU#976247. This technique has been increasingly adopted by threat actors who exploit malformed ZIP archives to circumvent Antivirus (AV) and Endpoint Detection and Response (EDR) scanning engines.
By manipulating the internal headers of these ZIP archives, attackers can effectively conceal malicious payloads, leading critical security tools to generate false negatives.
How the Evasion Technique Works
Standard ZIP archives are equipped with vital metadata, including version information, flags, and the specific compression method utilized to package the files. Antivirus engines and EDR solutions heavily depend on this declared metadata to preprocess and scrutinize files for potential threats.
When an attacker deliberately alters the compression method field within the header, the antivirus software encounters difficulties in properly decompressing the archive. As a result, the security tool is rendered incapable of analyzing the actual payload contained within the file.
While some security products may flag the modified file as corrupted or broken, they often fail to identify and neutralize the underlying malicious code. For the attack to successfully compromise a system, the hidden payload must still be extracted and executed.
Interestingly, standard extraction tools such as 7-Zip, unzip, bsdtar, and Python’s zipfile typically trust the tampered compression metadata. When these tools attempt decompression, they often return CRC or “unsupported method” errors, leaving the hidden payload undisclosed and unexecuted.
To navigate this obstacle, attackers resort to custom malware loaders. These specialized loaders are designed to disregard the tampered compression method field entirely. Instead, they directly extract and decompress the embedded malicious data, enabling the attacker to execute their code programmatically while evading detection from traditional AV engines.
This evasion tactic empowers attackers to successfully deliver malware beyond endpoint security defenses. CERT notes that the vulnerability, reported by security researcher Christopher Aziz, closely resembles previous archive manipulation techniques, such as CVE-2004-0935. Currently, networking giant Cisco has been confirmed as affected, while several other major security vendors, including AhnLab, Avast, Bitdefender, and Avira, remain listed with an “Unknown” status regarding their susceptibility to this specific technique.
Mitigation Strategies
To safeguard against this evasion tactic, organizations and security vendors must enhance their archive handling processes:
- Security vendors should not rely solely on declared archive metadata to dictate how file contents are scanned and managed.
- Antivirus and EDR scanners ought to adopt aggressive detection modes that validate the compression method fields against the actual characteristics of the content.
- Security tools should be configured to flag metadata inconsistencies and automatically route these files for more in-depth heuristic inspection.
- Organizations are encouraged to proactively reach out to their AV and EDR providers to ascertain whether their current software versions are vulnerable to malformed ZIP headers and to request necessary mitigation patches.
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