A vulnerability identified as CVE-2026-0866 allows malicious actors to exploit malformed ZIP headers to bypass antivirus and Endpoint Detection and Response (EDR) systems. This flaw occurs because most security solutions rely on metadata within ZIP archives to scan and process files. When the compression method field is altered, security scanners may fail to decompress the archive correctly, resulting in a false negative and leaving the malicious payload undetected. Attackers can then use a custom loader to access the embedded malicious data, circumventing standard extraction tools. Cisco has been confirmed as affected by this vulnerability, while nearly 30 other security vendors have not disclosed their status. The cybersecurity community is urged to evolve scanning methodologies, including not relying solely on declared metadata and implementing aggressive detection modes. Organizations should verify their antivirus and EDR solutions for vulnerability to CVE-2026-0866 and monitor for custom loaders.