CVE-2021-42090
📋 TL;DR
CVE-2021-42090 is a remote code execution vulnerability in Zammad's Form functionality due to unsafe deserialization. Attackers can execute arbitrary code on affected Zammad instances, potentially compromising the entire system. All Zammad installations before version 4.1.1 are vulnerable.
💻 Affected Systems
- Zammad
📦 What is this software?
Zammad by Zammad
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary commands, access sensitive data, install malware, pivot to other systems, and maintain persistent access.
Likely Case
Remote code execution leading to data theft, service disruption, and potential lateral movement within the network.
If Mitigated
Limited impact with proper network segmentation, but still significant risk to the Zammad application and its data.
🎯 Exploit Status
Exploitation is straightforward with publicly available proof-of-concept code. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.1.1 and later
Vendor Advisory: https://zammad.com/en/advisories/zaa-2021-14
Restart Required: Yes
Instructions:
1. Backup your Zammad instance and database. 2. Update to Zammad 4.1.1 or later using your deployment method (package manager, Docker, manual). 3. Restart the Zammad service. 4. Verify the update was successful.
🔧 Temporary Workarounds
Disable Form Functionality
allTemporarily disable the vulnerable Form functionality if immediate patching is not possible.
# Edit Zammad configuration to disable form functionality
# Consult Zammad documentation for specific configuration changes
Network Access Controls
linuxRestrict network access to Zammad instances using firewalls or network segmentation.
# Example iptables rule to restrict access
iptables -A INPUT -p tcp --dport 3000 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 3000 -j DROP
🧯 If You Can't Patch
- Isolate Zammad instances from the internet and restrict internal network access
- Implement web application firewall (WAF) rules to block deserialization attacks
🔍 How to Verify
Check if Vulnerable:
Check Zammad version via web interface or command line. Versions before 4.1.1 are vulnerable.Check Version:
zammad versionVerify Fix Applied:
Verify Zammad version is 4.1.1 or later and test form functionality works without security issues.📡 Detection & Monitoring
Log Indicators:
- Unusual deserialization errors in application logs
- Suspicious POST requests to form endpoints
- Unexpected process execution or system commands
Network Indicators:
- Unusual outbound connections from Zammad server
- Traffic patterns indicating command and control activity
SIEM Query:
source="zammad.log" AND ("deserialization" OR "form" AND "POST") AND status=500