CVE-2021-31909
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on JetBrains TeamCity servers by injecting malicious arguments. It affects all TeamCity installations before version 2020.2.3. Attackers can potentially gain full control of affected systems.
💻 Affected Systems
- JetBrains TeamCity
📦 What is this software?
Teamcity by Jetbrains
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining root/admin privileges, data exfiltration, lateral movement, and persistent backdoor installation.
Likely Case
Unauthorized code execution leading to service disruption, data theft, and potential ransomware deployment.
If Mitigated
Limited impact with proper network segmentation and access controls, potentially only affecting the TeamCity service.
🎯 Exploit Status
Exploitation is straightforward with publicly available proof-of-concept code. No authentication required in default configurations.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.2.3 or later
Vendor Advisory: https://blog.jetbrains.com/blog/2021/05/07/jetbrains-security-bulletin-q1-2021/
Restart Required: Yes
Instructions:
1. Backup TeamCity configuration and data. 2. Download TeamCity 2020.2.3 or later from JetBrains website. 3. Stop TeamCity service. 4. Install the updated version. 5. Restart TeamCity service. 6. Verify successful upgrade.
🔧 Temporary Workarounds
Network Isolation
allRestrict network access to TeamCity server to only trusted IP addresses
# Use firewall rules to restrict access
iptables -A INPUT -p tcp --dport 8111 -s trusted_ip -j ACCEPT
iptables -A INPUT -p tcp --dport 8111 -j DROP
Reverse Proxy with Input Validation
allDeploy a reverse proxy with strict input validation to filter malicious requests
# Example nginx configuration snippet
location / {
proxy_pass http://teamcity:8111;
proxy_set_header Host $host;
# Add input validation rules here
}
🧯 If You Can't Patch
- Immediately isolate the TeamCity server from internet access and restrict internal network access
- Implement strict monitoring and alerting for suspicious process execution and network connections
🔍 How to Verify
Check if Vulnerable:
Check TeamCity version in Administration → Server Administration → Server Health → VersionCheck Version:
On TeamCity server: cat /opt/teamcity/version.txt or check web interface at http://server:8111/admin/admin.html?item=diagnosticsVerify Fix Applied:
Verify version is 2020.2.3 or later in Administration → Server Administration → Server Health → Version📡 Detection & Monitoring
Log Indicators:
- Unusual process execution from TeamCity service
- Suspicious command-line arguments in TeamCity logs
- Failed authentication attempts followed by successful exploitation
Network Indicators:
- Unusual outbound connections from TeamCity server
- Suspicious payloads in HTTP requests to TeamCity
SIEM Query:
source="teamcity.log" AND ("cmd.exe" OR "powershell" OR "bash" OR "sh") AND NOT expected_process