CVE-2026-20213: ClamAV PE Parser Memory Corruption & DoS Vulnerability

Technical summary

CVE-2026-20213 stems from insufficient boundary validation in ClamAV's PE file parser. Specifically, the parser fails to properly check buffer limits before writing data during PE content analysis, resulting in an out-of-bounds buffer write condition (CWE-120). A specially crafted PE file submitted for scanning can trigger this write, corrupting memory and crashing the ClamAV daemon or scanning process. The vulnerability requires network access but no authentication, and the parsing occurs automatically during normal scanning operations.

Business impact

Organizations relying on ClamAV for email gateway scanning, file server protection, or endpoint scanning will experience scanning service interruptions if exploited. This creates a window where malware can be uploaded or transmitted without detection. In environments where ClamAV is a critical layer of the security stack—particularly in containerized or cloud deployments—repeated crashes could degrade threat coverage and compliance posture. For vendors like Cisco who integrate ClamAV into Secure Endpoint, patch availability and deployment speed directly affect customer protection levels.

Affected systems

ClamAV installations across all supported versions prior to the fix are vulnerable. Cisco Secure Endpoint, which incorporates ClamAV, is also affected. The vulnerability applies regardless of deployment model—on-premises scanning services, cloud-based gateway integrations, and endpoint agents all process PE files as part of routine malware detection. Any system actively scanning files or email attachments is exposed.

Exploitability

Exploitation is straightforward: an attacker crafts a PE file with boundary-violating content and submits it for scanning—via email, web upload, or direct file system access depending on the target's architecture. No user interaction is required beyond the normal scanning process. The attack surface is broad because PE files are ubiquitous in Windows environments, and scanning is typically automatic. The CVSS score of 7.5 (HIGH) reflects network-accessible, low-complexity exploitation with no authentication barrier, though impact is currently limited to availability. However, memory corruption vulnerabilities carry residual risk of escalation.

Remediation

Patch ClamAV and any dependent products (such as Cisco Secure Endpoint) immediately upon vendor release. Most organizations should apply updates as part of their regular patch cycle, but given the ease of exploitation and DoS impact, expedited testing and deployment is warranted. Ensure scanning service monitoring is in place to detect unexpected restarts or crashes that may indicate exploitation attempts. Verify patching across all deployment points—mail gateways, endpoints, containerized scanners, and appliances.

Patch guidance

Consult the official ClamAV release notes and Cisco security advisories for specific patched versions and download locations. Test patches in a non-production environment before enterprise rollout. For Cisco Secure Endpoint customers, follow the standard update mechanism within your management console; patches are typically rolled out incrementally to managed devices. If manual intervention is required, prioritize internet-facing systems and high-traffic scanning nodes. Verify successful patching by checking ClamAV version post-deployment.

Detection guidance

Monitor ClamAV and scanning service logs for unexpected process termination or segmentation faults coinciding with file submissions. Implement alerting on scanning service crashes or restarts. Network-based detection is limited unless you inspect file submissions; however, behavioral monitoring of antivirus processes for abnormal memory usage or crashes can flag exploitation attempts. Endpoint Detection and Response (EDR) tools should flag unexpected scanner process terminations. Log file uploads or submissions to systems running vulnerable ClamAV instances and correlate with service availability events.

Why prioritize this

Although not yet on CISA's Known Exploited Vulnerabilities list, this vulnerability merits high priority due to its ease of exploitation, immediate impact on security infrastructure availability, and the broad attack surface presented by ubiquitous PE file handling. A successful exploit directly degrades an organization's ability to detect threats, creating operational risk. The memory corruption vector, while currently DoS-focused, suggests potential for future weaponization if deeper exploitation techniques emerge.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) reflects a network-accessible vulnerability requiring no authentication or user interaction, with a low complexity barrier to exploitation. The vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) emphasizes that the primary impact is availability—service disruption. The absence of confidentiality or integrity impact in the current baseline does not diminish the risk, as memory corruption vulnerabilities can be context-dependent; in certain configurations or with sophisticated payloads, impacts could exceed the baseline scoring. The vulnerability's position in security-critical infrastructure (antivirus/scanning) amplifies its business risk despite the moderate CVSS numeric value.

Frequently asked questions