CVE-2021-27213
📋 TL;DR
CVE-2021-27213 is a critical remote code execution vulnerability in pystemon's config.py file that allows attackers to execute arbitrary code via YAML deserialization. The vulnerability affects all pystemon instances using unsafe YAML loading methods. Anyone running pystemon before the February 13, 2021 patch is vulnerable to complete system compromise.
💻 Affected Systems
- pystemon
📦 What is this software?
Pystemon by Pystemon Project
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with root/administrator privileges, allowing attackers to install persistent backdoors, exfiltrate sensitive data, and pivot to other systems in the network.
Likely Case
Remote code execution with the privileges of the pystemon process, enabling data theft, cryptocurrency mining, or use as a botnet node.
If Mitigated
No impact if proper YAML SafeLoader is used or if the system is patched to version after February 13, 2021.
🎯 Exploit Status
The vulnerability is in YAML deserialization which is trivial to exploit with publicly available payloads.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2021-02-13 or later
Vendor Advisory: https://github.com/cvandeplas/pystemon/commit/dbeb87afefdb63de2f4cff69b6f10c5965d14b54
Restart Required: Yes
Instructions:
1. Update pystemon to version 2021-02-13 or later. 2. Replace unsafe yaml.load() with yaml.safe_load() in config.py. 3. Restart the pystemon service.
🔧 Temporary Workarounds
Manual config.py patch
linuxManually edit config.py to use SafeLoader instead of unsafe YAML loading
sed -i "s/yaml.load(/yaml.safe_load(/g" /path/to/pystemon/config.py
🧯 If You Can't Patch
- Disable pystemon service immediately and remove from network
- Implement strict network segmentation and firewall rules to isolate vulnerable systems
🔍 How to Verify
Check if Vulnerable:
Check config.py for yaml.load() calls without SafeLoader parameterCheck Version:
Check pystemon version or commit date (should be >= 2021-02-13)Verify Fix Applied:
Verify config.py uses yaml.safe_load() or yaml.load(..., Loader=yaml.SafeLoader)📡 Detection & Monitoring
Log Indicators:
- Unusual process execution from pystemon
- Suspicious YAML parsing errors
- Unexpected system commands in pystemon logs
Network Indicators:
- Outbound connections from pystemon to unknown IPs
- Unusual traffic patterns from pystemon port
SIEM Query:
process_name:pystemon AND (cmdline:*yaml* OR cmdline:*python* -exec*)