CVE-2021-40239
📋 TL;DR
A buffer overflow vulnerability in Miniftpd's do_retr function allows attackers to execute arbitrary code or crash the FTP server. This affects all systems running vulnerable versions of Miniftpd. Attackers can exploit this remotely without authentication.
💻 Affected Systems
- Miniftpd
📦 What is this software?
Miniftpd by Miniftpd Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Denial of service (FTP server crash) or limited code execution within the FTP service context.
If Mitigated
Contained impact if service runs with minimal privileges and network segmentation limits lateral movement.
🎯 Exploit Status
Buffer overflow in FTP RETR command handling makes exploitation straightforward. Public GitHub issue demonstrates the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check GitHub repository for patched version
Vendor Advisory: https://github.com/Gabe-commiter/Miniftpd/issues/2
Restart Required: Yes
Instructions:
1. Check Miniftpd GitHub repository for patched version. 2. Download and compile patched source code. 3. Stop current Miniftpd service. 4. Replace binary with patched version. 5. Restart Miniftpd service.
🔧 Temporary Workarounds
Disable FTP service
linuxCompletely disable Miniftpd if not required
sudo systemctl stop miniftpd
sudo systemctl disable miniftpd
Network segmentation
linuxRestrict FTP access to trusted networks only
iptables -A INPUT -p tcp --dport 21 -s TRUSTED_NETWORK -j ACCEPT
iptables -A INPUT -p tcp --dport 21 -j DROP
🧯 If You Can't Patch
- Implement strict network access controls to limit FTP service exposure
- Run Miniftpd with minimal privileges and in a containerized environment
🔍 How to Verify
Check if Vulnerable:
Check Miniftpd version against GitHub advisory. Test with proof-of-concept if available.Check Version:
miniftpd --version or check binary compilation dateVerify Fix Applied:
Verify patched version is running and test RETR command with malformed input.📡 Detection & Monitoring
Log Indicators:
- Multiple failed RETR commands
- FTP service crashes
- Unusual RETR command patterns
Network Indicators:
- Excessive RETR commands from single source
- FTP protocol anomalies
SIEM Query:
source="ftp.log" AND (command="RETR" AND size>normal_threshold) OR (process="miniftpd" AND event="crash")