CVE-2024-20506 is a vulnerability identified in the ClamD service module of Cisco's Clam AntiVirus (ClamAV) affecting numerous legacy versions including 1.4.0, 1.3.2, 1.0.6 and earlier, as well as all 1.2.x, 0.105.x, 0.104.x, and 0.103.x versions. The root cause is that ClamD, running with elevated privileges, writes to its log file without verifying whether the log file has been replaced by a symbolic link. An authenticated local attacker can exploit this by substituting the ClamD log file with a symlink pointing to a critical system file. Upon restarting the ClamD process, the service appends log messages to the target file, effectively corrupting it. This attack requires local access and privileges sufficient to replace the log file and restart ClamD but does not require user interaction. The vulnerability impacts system integrity and availability by enabling unauthorized modification of critical files, potentially leading to system instability or denial of service. Confidentiality is not affected. The CVSS v3.1 score is 6.1 (medium severity), reflecting the local attack vector, low complexity, required privileges, and no user interaction. No public exploits have been reported yet, but the vulnerability poses a risk especially in environments where ClamAV is deployed on critical systems. The CWE-754 classification indicates improper check for symbolic links leading to security bypass. Since ClamAV is widely used as an open-source antivirus solution, especially in Linux-based environments, this vulnerability could affect many organizations that have not updated to patched versions.

For European organizations, the vulnerability could lead to corruption of critical system files on servers running vulnerable ClamAV versions, potentially causing system crashes, denial of service, or operational disruptions. This is particularly concerning for sectors relying on ClamAV for malware scanning on critical infrastructure, such as finance, healthcare, government, and telecommunications. The requirement for local authenticated access limits remote exploitation but insider threats or compromised accounts could leverage this flaw. Corruption of system files could also facilitate further attacks or complicate incident response efforts. The impact on availability and integrity could disrupt business continuity and lead to regulatory compliance issues under GDPR if service outages affect personal data processing. Organizations using legacy or unpatched ClamAV versions are at higher risk, especially if they lack strict process and file integrity monitoring controls.

1. Upgrade ClamAV to the latest available version where this vulnerability is patched. Since no patch links are provided, monitor Cisco and ClamAV official channels for updates. 2. Restrict local user privileges to prevent unauthorized modification of ClamD log files and control who can restart the ClamD service. 3. Implement file integrity monitoring on ClamD log files and critical system files to detect unauthorized symbolic link creation or file tampering. 4. Harden system permissions to prevent symbolic link attacks, including disabling symlink following in sensitive directories if possible. 5. Use process supervision tools that prevent unauthorized restarts of ClamD or require administrative approval. 6. Conduct regular audits of local user accounts and privileges to minimize insider threat risks. 7. Employ endpoint detection and response (EDR) solutions to detect suspicious local activity related to ClamD. 8. Develop incident response plans that include recovery procedures for corrupted system files to minimize downtime.