CVE-2024-39844 – This vulnerability allows remote code execution... (How to Fix)

📋 TL;DR

This vulnerability allows remote code execution in ZNC IRC bouncer servers via the modtcl module when processing KICK commands. Attackers can execute arbitrary code on affected ZNC servers by sending specially crafted KICK messages. All ZNC servers with modtcl enabled before version 1.9.1 are affected.

💻 Affected Systems

Products:

ZNC

Versions: All versions before 1.9.1

Operating Systems: All platforms running ZNC

Default Config Vulnerable: ✅ No

Notes: Only vulnerable if modtcl module is loaded and enabled. modtcl is not enabled by default in ZNC installations.

⚠️ 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.

Recommended Actions:

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 system compromise allowing attackers to execute arbitrary commands with ZNC process privileges, potentially leading to full server takeover, data theft, or lateral movement within the network.

🟠

Likely Case

Remote code execution leading to installation of backdoors, cryptocurrency miners, or participation in botnets, with potential data exfiltration from the ZNC server.

🟢

If Mitigated

Limited impact if modtcl is disabled or proper network segmentation isolates the ZNC server from critical systems.

🌐 Internet-Facing: HIGH

🏢 Internal Only: MEDIUM

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires sending a KICK command to a vulnerable ZNC server with modtcl enabled. No authentication is required if the attacker can reach the ZNC server.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.9.1

Vendor Advisory: https://wiki.znc.in/ChangeLog/1.9.1

Restart Required: Yes

Instructions:

1. Download ZNC 1.9.1 from https://github.com/znc/znc/releases/tag/znc-1.9.1
2. Stop ZNC service
3. Install/upgrade to version 1.9.1
4. Restart ZNC service

🔧 Temporary Workarounds

Disable modtcl module

all

Remove or disable the modtcl module to prevent exploitation

znc --makepass
Edit ~/.znc/configs/znc.conf and remove LoadModule = modtcl
Restart ZNC

Network isolation

linux

Restrict access to ZNC server to trusted networks only

iptables -A INPUT -p tcp --dport [ZNC_PORT] -s [TRUSTED_NETWORK] -j ACCEPT
iptables -A INPUT -p tcp --dport [ZNC_PORT] -j DROP

🧯 If You Can't Patch

Immediately disable modtcl module if not required
Implement strict network access controls to limit ZNC server exposure

🔍 How to Verify

Check if Vulnerable:

Check if ZNC version is below 1.9.1 and modtcl module is loaded

Check Version:

znc --version

Verify Fix Applied:

Verify ZNC version is 1.9.1 or higher and modtcl is either disabled or patched

📡 Detection & Monitoring

Log Indicators:

Unusual KICK commands in ZNC logs
modtcl module errors or crashes
Suspicious process execution from ZNC user

Network Indicators:

Unexpected connections from ZNC server
IRC KICK commands to ZNC from untrusted sources

SIEM Query:

source="znc.log" AND ("KICK" OR "modtcl")

📊 Metadata

CVE ID: CVE-2024-39844

CVSS v3 Score: 9.8 (CRITICAL)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-94

Published: July 3, 2024

Last Updated: November 21, 2024

🔗 References

📤 Share & Export

📄 Export Markdown 📋 Export JSON

🔗 Related Vulnerabilities

If you're affected by CVE-2024-39844, you might also want to check these: