ClamAV Denial of Service (DoS) Vulnerability: Permanent Fix Guide for Ubuntu

 

ClamAV HTML crash DoS? Get Ubuntu commands to check vuln, a bash script to auto-patch, iptables mitigation, and a book to master any future CVE. No expiry.

What Happened

In April 2026, a security issue (CVE-2026-20031) was disclosed: ClamAV could crash when scanning a specially crafted HTML file. A remote attacker could use this to cause a denial of service (DoS). This guide gives you a permanent, reusable playbook to fix this – and any similar – vulnerability.

How to Check if You Are Vulnerable (Ubuntu Commands)

Run these commands today and any time a new ClamAV update is announced.

bash

# 1. Check your ClamAV version
clamscan --version

# 2. Compare with the fixed versions (from April 2026):
# Fixed: 1.4.4+dfsg-0ubuntu0.22.04.1 (Ubuntu 22.04)
#        1.4.4+dfsg-0ubuntu0.24.04.1 (Ubuntu 24.04)
#        1.4.4+dfsg-0ubuntu0.25.10.1 (Ubuntu 25.10)

# 3. Automatic vuln check script
if [[ $(clamscan --version | grep -oP '1\.4\.4\+dfsg-0ubuntu0\.(22\.04|24\.04|25\.10)\.1') ]]; then
    echo "✅ Not vulnerable (patched version)"
else
    echo "❌ VULNERABLE - HTML crash DoS risk"
fi

---

Automation Script to Apply the Fix (Bash – Ubuntu/Derivatives)

Save as fix-clamav-cve.sh and run with sudo bash fix-clamav-cve.sh.

bash

#!/bin/bash
# FIX for CVE-2026-20031 (ClamAV HTML crash DoS)
# Works on Ubuntu 22.04, 24.04, 25.10, and derivatives (Linux Mint, Pop!_OS, etc.)

set -e

echo "[*] Checking current ClamAV version..."
OLD_VER=$(clamscan --version 2>/dev/null | head -n1)

echo "[*] Updating package lists..."
apt update

echo "[*] Installing patched ClamAV..."
apt install -y clamav clamav-daemon

NEW_VER=$(clamscan --version | head -n1)
echo "[*] Updated from $OLD_VER to $NEW_VER"

echo "[*] Restarting ClamAV daemon..."
systemctl restart clamav-daemon
systemctl enable clamav-daemon

echo "[✓] CVE-2026-20031 fixed. Verify with: clamscan --version"

Why this script solves THIS CVE.

To learn how to create your own scripts for any future CVE, you need the book:

📘 Practical Binary Analysis: Build Your Own Linux Tools for Binary Instrumentation, Analysis, and Disassembly

This script solves a CVE. This book solves ALL the CVEs you’ve never seen.

Affiliate disclosure: As an Amazon Associate, I earn from qualifying purchases. This helps me keep writing in-depth security guides – at no extra cost to you.).

Alternative Mitigation (If You Can’t Update Now)

If you cannot restart services or apply the patch immediately, use iptables to block remote HTML attacks (only works if you know the attacker’s IP range or want to restrict access):

bash

# Block all external HTTP/HTTPS that might deliver crafted HTML to ClamAV
# (Assumes ClamAV listens on port 3310 for TCP)
iptables -A INPUT -p tcp --dport 3310 -j DROP

# Or rate-limit connections to reduce DoS impact
iptables -A INPUT -p tcp --dport 3310 -m limit --limit 10/min -j ACCEPT
iptables -A INPUT -p tcp --dport 3310 -j DROP

# Save rules (Ubuntu)
apt install iptables-persistent
netfilter-persistent save

AppArmor alternative (restrict ClamAV’s HTML parser capabilities):

bash

# Enforce a stricter AppArmor profile for clamd
aa-enforce /etc/apparmor.d/usr.bin.clamd
systemctl restart clamav-daemon

These are temporary – update as soon as possible.

Why This Book Solves the Root Problem

Practical Binary Analysis (No Starch Press) teaches you to:

• Build your own Linux fuzzing tools to find HTML parsing bugs before attackers do

• Instrument ClamAV binaries to understand exactly where the crash happens

• Write one-click patching scripts for undisclosed CVEs

 Conclusion

You now have a permanent workflow:

   1. Check version with the command snippet

   2. Run the script to patch

   3. Use iptables/AppArmor only as a short-term hold