CVE-2026-20338 Overview
CVE-2026-20338 is a denial-of-service vulnerability in the zip archive parser of ClamAV. An unauthenticated, remote attacker can trigger a memory double-free condition by submitting a crafted zip file for scanning. Successful exploitation terminates the ClamAV scanning process, disrupting antivirus operations on the affected host.
The flaw is classified under CWE-415 (Double Free) and impacts availability without exposing confidentiality or integrity. Because ClamAV is often integrated into mail gateways, file transfer inspection, and endpoint scanning workflows, a crash of the scanning engine can leave downstream traffic unprotected.
Critical Impact
Remote, unauthenticated attackers can crash the ClamAV scanner by delivering a malformed zip archive, disabling malware inspection on the affected system.
Affected Products
- ClamAV zip archive parser (see Cisco Security Advisory for exact versions)
- Cisco products bundling vulnerable ClamAV builds
- Third-party security appliances and mail gateways embedding ClamAV
Discovery Timeline
- 2026-08-07 - CVE-2026-20338 published to NVD
- 2026-08-11 - Last updated in NVD database
Technical Details for CVE-2026-20338
Vulnerability Analysis
The vulnerability resides in ClamAV's zip archive parsing routines. When ClamAV inspects the contents of a zip file, improper memory handling causes the same heap allocation to be released twice. This double-free corrupts heap metadata and forces the scanning process to abort.
Because the attack vector is network-based and requires no privileges or user interaction, any pipeline that feeds untrusted files into ClamAV is exposed. Mail servers, file upload scanners, and secure web gateways are typical exposure points. Repeated exploitation can be scripted to keep the scanning daemon offline, effectively bypassing malware inspection while the process restarts.
The impact is limited to availability. The advisory does not describe memory disclosure or code execution paths, and the CWE-415 classification aligns with a crash-oriented outcome rather than arbitrary write primitives.
Root Cause
The defect stems from improper tracking of heap pointers during zip content processing. A code path in the parser frees a buffer and later frees the same pointer again without nulling or ownership checks. Classic double-free conditions like this trip glibc heap consistency checks and abort the process.
Attack Vector
An attacker crafts a malformed zip archive and submits it through any channel that hands files to ClamAV. Delivery methods include email attachments, HTTP uploads, clamdscan submissions, and inspection queues on network security appliances. No authentication or user interaction is required to reach the vulnerable parser.
No public proof-of-concept or exploit code is currently available. Refer to the Cisco Security Advisory for vendor-supplied technical details.
Detection Methods for CVE-2026-20338
Indicators of Compromise
- Unexpected termination or repeated restarts of clamd, clamscan, or clamdscan processes.
- Segmentation faults or double free or corruption messages in system logs referencing ClamAV binaries.
- Spikes in zip file submissions from a single source immediately preceding scanner crashes.
Detection Strategies
- Monitor process exit codes and crash signals for ClamAV components across mail gateways and endpoints.
- Correlate ClamAV service failures with inbound file submissions to identify likely trigger samples.
- Retain crashing zip samples for offline analysis and sandbox replay against a patched scanner.
Monitoring Recommendations
- Alert on ClamAV daemon restart frequency exceeding baseline thresholds.
- Track scan queue backlog growth as a downstream signal of scanner instability.
- Ingest ClamAV logs and OS-level crash telemetry into a centralized SIEM for correlation.
How to Mitigate CVE-2026-20338
Immediate Actions Required
- Apply the ClamAV updates referenced in the Cisco Security Advisory to all systems running the ClamAV engine.
- Inventory third-party appliances that embed ClamAV and confirm vendor patches are installed.
- Enable automatic restart with backoff for clamd to reduce downtime while patches are deployed.
Patch Information
Cisco has published fixed ClamAV versions in the security advisory cisco-sa-clamav-WuuvVd26. Consult the vendor advisory for the exact fixed release numbers and platform-specific update procedures.
Workarounds
- Reject or quarantine zip archives at the perimeter until the scanning engine is patched.
- Limit the maximum archive size and recursion depth accepted by ClamAV to reduce parser exposure.
- Isolate ClamAV instances behind rate limits and per-source submission quotas to blunt repeated crash attempts.
# Example clamd.conf hardening to reduce zip parser exposure
MaxScanSize 100M
MaxFileSize 25M
MaxRecursion 10
MaxFiles 1000
ArchiveBlockEncrypted yes
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
