CVE-2026-20031
📋 TL;DR
A vulnerability in ClamAV's HTML CSS parser allows remote attackers to cause denial of service by submitting specially crafted HTML files. This affects systems running vulnerable versions of ClamAV that scan HTML content. The vulnerability can be exploited without authentication.
💻 Affected Systems
- ClamAV
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Complete termination of ClamAV scanning process, disrupting file scanning and potentially allowing malicious files to bypass detection.
Likely Case
Temporary disruption of ClamAV scanning service requiring process restart, potentially causing scanning delays or gaps.
If Mitigated
Minimal impact with proper network segmentation and monitoring to detect and respond to scanning failures.
🎯 Exploit Status
Exploitation requires submitting crafted HTML files to ClamAV scanning services. No authentication is required if scanning endpoints are accessible.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: ClamAV 1.3.0 and later
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-clamav-css-Fn4QSZ
Restart Required: Yes
Instructions:
1. Download ClamAV 1.3.0 or later from official sources. 2. Stop ClamAV services. 3. Install the updated version. 4. Update virus definitions. 5. Restart ClamAV services.
🔧 Temporary Workarounds
Disable HTML scanning
allTemporarily disable HTML file scanning in ClamAV configuration to prevent exploitation
Edit clamd.conf and set 'ScanHTML' to 'no'
Restart clamd service
Network segmentation
allRestrict access to ClamAV scanning endpoints to trusted networks only
Configure firewall rules to limit access to ClamAV ports (default 3310)
🧯 If You Can't Patch
- Implement strict input validation for HTML files before they reach ClamAV scanning
- Monitor ClamAV process health and implement automatic restart on failure
🔍 How to Verify
Check if Vulnerable:
Run 'clamscan --version' and check if version is below 1.3.0Check Version:
clamscan --version | head -1Verify Fix Applied:
Confirm version is 1.3.0 or higher with 'clamscan --version' and test with known safe HTML files📡 Detection & Monitoring
Log Indicators:
- ClamAV process termination/crash logs
- Scanning service restart events
- Error messages related to UTF-8 parsing or CSS processing
Network Indicators:
- Unusual volume of HTML file submissions to scanning services
- Multiple scanning failures from single source
SIEM Query:
source="clamav" AND ("crash" OR "terminated" OR "UTF-8 error")