CVE-2026-20215: ClamAV 7z Parser Buffer Overflow DoS Vulnerability

Technical summary

The vulnerability stems from insufficient boundary checking (CWE-120: Buffer Copy without Checking Size of Input) in ClamAV's 7z file format parser. During file scanning operations, the parser does not adequately validate the size of content within 7z archives before copying data into a fixed-size buffer. This results in an out-of-bounds buffer write when processing a malformed 7z file. The memory corruption is triggered at scan time—the attacker's crafted file need only be submitted to a ClamAV instance via any normal file scanning interface (file upload, email gateway, filesystem monitor, etc.). CVSS v3.1 score of 7.5 (HIGH severity) reflects the high availability impact (process crash) with low attack complexity and no authentication requirement, though the vector indicates no confidentiality or integrity impact from the memory corruption itself.

Business impact

Organizations using ClamAV or Cisco Secure Endpoint for antivirus scanning face service disruption. A successful exploit causes the scanning process to terminate, creating a window where malware may bypass detection. In gateway or email scanning deployments, this could lead to unscanned messages reaching end users. In endpoint protection scenarios, scanning service degradation reduces real-time visibility. While not directly data-exfiltrating, the denial-of-service impact undermines the core security control, making it operationally critical for environments where ClamAV is the primary or sole antivirus layer.

Affected systems

ClamAV itself is affected. Cisco Secure Endpoint, which integrates ClamAV as its scanning engine, is also impacted. The vulnerability affects any deployment where ClamAV is installed and performing file scanning—including email gateways, web filtering appliances, file servers, and endpoints. Organizations should verify which versions of ClamAV and Secure Endpoint are in use; patch availability and version-specific impact should be confirmed against vendor advisories.

Exploitability

Exploitability is straightforward. The attack vector is network-based: an attacker simply crafts a malformed 7z file and submits it for scanning. No authentication is required, no user interaction is needed beyond the normal scanning process, and no special configuration is necessary. However, the attacker must have a way to get the file to the ClamAV scanner—this typically means upload endpoints, email delivery, or network-accessible file shares. The main limiting factor is not technical difficulty but organizational exposure (how many scanning endpoints can an attacker reach). The vulnerability is not currently listed on CISA's Known Exploited Vulnerabilities (KEV) catalog, suggesting active exploitation has not yet been widely reported, though the straightforward nature of the attack means this should not be assumed as assurance.

Remediation

Apply vendor patches immediately upon release. Cisco and the ClamAV maintainers will issue updates; consult vendor advisories for specific version numbers and affected product lines. Temporary mitigations may include disabling 7z file scanning if operationally feasible, restricting file upload endpoints, or increasing monitoring for ClamAV process crashes. However, these workarounds may reduce security posture and should be treated as short-term measures only.

Patch guidance

Obtain patches from Cisco (for Secure Endpoint) and the official ClamAV repository. Verify the specific version numbers against your deployed instances before patching. Test patches in a non-production environment first, particularly in air-gapped or mission-critical scanning deployments. Prioritize patches for internet-facing file upload services and email gateways, as these represent the highest attack surface. Establish a timeline for complete deployment across all ClamAV instances.

Detection guidance

Monitor for unexpected ClamAV process crashes or service restarts. Configure alerting on antivirus daemon failures. Review file upload logs and email gateway logs for 7z files submitted around the time of scanning failures—this may indicate exploitation attempts. Implement process monitoring to detect repeated crashes from specific file sources. After patching, verify that 7z scanning resumes normal operation without service interruptions.

Why prioritize this

This vulnerability merits urgent patching due to the combination of high severity (CVSS 7.5), ease of exploitation (network-based, no authentication), and broad impact on security infrastructure. Any organization using ClamAV or Secure Endpoint should treat this as a critical security control gap until patched. The denial-of-service impact, while not directly exposing data, creates an attack opportunity for follow-on malware delivery. The straightforward attack method and lack of current KEV listing do not diminish urgency—early patching reduces the window before active exploitation becomes prevalent.

Risk score, explained

The CVSS 7.5 (HIGH) score reflects: attack vector NETWORK (widely exploitable), attack complexity LOW (no special conditions required), privileges required NONE (unauthenticated), user interaction NONE (automatic scanning), scope UNCHANGED (impact confined to ClamAV process), confidentiality NONE, integrity NONE, but availability HIGH (process denial of service). This score appropriately captures the operational risk: not a direct data breach vector, but a high-confidence availability attack on a security-critical service. Organizations should not discount HIGH severity as "medium risk"—loss of antivirus scanning is a material control failure.

Frequently asked questions