CVE-2026-20243 Overview
CVE-2026-20243 is an out-of-bounds write vulnerability in the ALZ file format parser of ClamAV. An unauthenticated, remote attacker can trigger memory corruption by submitting a crafted ALZ file for scanning. The flaw stems from improper boundary checks on ALZ file content during scanning operations. Successful exploitation terminates the ClamAV scanning process, causing a denial-of-service (DoS) condition. The vulnerability may also permit additional impacts arising from memory corruption on the affected device. The weakness is classified under [CWE-120] Buffer Copy without Checking Size of Input.
Critical Impact
A remote, unauthenticated attacker can crash the ClamAV scanning engine by supplying a crafted ALZ archive, disrupting antivirus protection on affected systems.
Affected Products
- ClamAV (versions with the vulnerable ALZ file format parser)
- Cisco products bundling the affected ClamAV component
- Third-party security appliances and gateways integrating ClamAV scanning
Discovery Timeline
- 2026-07-01 - CVE-2026-20243 published to NVD
- 2026-07-01 - Last updated in NVD database
Technical Details for CVE-2026-20243
Vulnerability Analysis
The vulnerability resides in the ALZ archive format parser inside ClamAV. During scanning, the parser processes fields from the ALZ container without adequately validating length or offset values against the destination buffer size. This allows a crafted ALZ file to trigger an out-of-bounds buffer write. The write corrupts adjacent memory in the scanner process and typically causes an abnormal termination of the scanning daemon. Because ClamAV runs as a scanning service for mail gateways, file servers, and endpoints, a crash removes malware inspection from the data path. The advisory notes potential expanded impacts beyond DoS, consistent with memory corruption primitives.
Root Cause
The parser performs a buffer copy without validating the size of input against the target buffer, matching [CWE-120]. Length or offset fields inside the ALZ structure are trusted during decompression or metadata handling, so a manipulated header steers writes past allocated bounds.
Attack Vector
An attacker delivers a malicious ALZ file to any pathway that invokes ClamAV, including email attachments, web uploads, file shares, or storage buckets protected by on-access scanning. No authentication or user interaction is required beyond the file reaching the scanner. When ClamAV opens the file, the parser processes the malicious content and triggers the out-of-bounds write.
No verified public exploit code is available. The vulnerability mechanism is documented in the Cisco Security Advisory.
Detection Methods for CVE-2026-20243
Indicators of Compromise
- Unexpected termination or repeated restarts of the clamd scanning daemon on affected hosts.
- ClamAV log entries showing scanner crashes, segmentation faults, or aborted scans immediately after processing files with the .alz extension.
- Inbound files containing ALZ archive magic bytes from untrusted senders or unusual sources.
Detection Strategies
- Monitor process exit codes and crash telemetry for clamd and clamscan to identify anomalous terminations correlated with ALZ file scans.
- Inspect mail gateway and web proxy logs for ALZ archive submissions, which are uncommon in most enterprise environments outside Korean-language file exchange.
- Correlate ClamAV service restarts with the arrival of archive files to identify targeted DoS attempts.
Monitoring Recommendations
- Enable verbose ClamAV logging and forward events to a central log platform for correlation.
- Track ClamAV version strings across the fleet to confirm patched builds are deployed.
- Alert on repeated scanner failures affecting the same mail flow, upload endpoint, or file share.
How to Mitigate CVE-2026-20243
Immediate Actions Required
- Apply the ClamAV update referenced in the Cisco Security Advisory to all systems running the scanning engine.
- Inventory embedded uses of ClamAV in security appliances, mail gateways, and third-party products, and apply vendor updates once available.
- Restart ClamAV services after patching to ensure the fixed parser is loaded.
Patch Information
Refer to the Cisco Security Advisory for fixed ClamAV versions and product-specific updates. Deploy fixed releases through standard package management or vendor update channels.
Workarounds
- Disable ALZ archive scanning in ClamAV configuration until patched builds are deployed, accepting the tradeoff that ALZ archives will not be inspected for malware.
- Block or quarantine ALZ file attachments at the mail gateway and web proxy for sources that do not have a business need to transmit them.
- Configure watchdog supervision to auto-restart clamd after abnormal termination to reduce protection gaps during exposure.
# Configuration example: disable ALZ scanning in clamd.conf
# Edit /etc/clamav/clamd.conf and add:
ScanArchive yes
ArchiveBlockEncrypted no
# Restrict archive types processed (block ALZ at the gateway)
# Example Postfix header_checks rule to reject ALZ attachments:
# /name=[^>]*\.alz/ REJECT ALZ attachments not permitted
sudo systemctl restart clamav-daemon
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
