malformed ZIP

Cybersecurity researchers at the CERT Coordination Center (CERT/CC) have identified a new evasion technique, VU#976247, used by threat actors to exploit malformed ZIP archives and bypass Antivirus (AV) and Endpoint Detection and Response (EDR) systems. Attackers manipulate the internal headers of ZIP files, particularly altering the compression method field, which causes security tools to fail in analyzing the malicious payload. While some security products may flag the modified files as corrupted, they often do not detect the underlying malicious code. Standard extraction tools typically trust the tampered metadata and may return errors, leaving the hidden payload undisclosed. Attackers use custom malware loaders to extract and execute the malicious data, circumventing traditional AV detection. Cisco has been confirmed as affected, while other vendors, including AhnLab, Avast, Bitdefender, and Avira, have an “Unknown” status regarding their vulnerability. To mitigate this threat, organizations should enhance archive handling processes, avoid relying solely on declared metadata, adopt aggressive detection modes, flag metadata inconsistencies, and consult with AV and EDR providers about vulnerabilities.