CVE-2020-6835
📋 TL;DR
CVE-2020-6835 is a heap-based off-by-one buffer overflow vulnerability in Bftpd FTP server that occurs during file transfer error checking. This allows remote attackers to potentially execute arbitrary code or crash the service. Any system running vulnerable versions of Bftpd with FTP services enabled is affected.
💻 Affected Systems
- Bftpd FTP Server
📦 What is this software?
Bftpd by Bftpd Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, allowing attackers to install malware, steal data, or pivot to other systems.
Likely Case
Service crash causing denial of service and potential information disclosure through memory leaks.
If Mitigated
Limited impact if proper network segmentation and least privilege are implemented, potentially just service disruption.
🎯 Exploit Status
Heap-based exploitation requires specific conditions but public PoCs exist. Attackers can trigger the vulnerability by causing file transfer errors.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.4 and later
Vendor Advisory: http://bftpd.sourceforge.net/news.html#302460
Restart Required: Yes
Instructions:
1. Download Bftpd 5.4 or later from official source. 2. Stop Bftpd service. 3. Install new version following distribution package manager or compile from source. 4. Restart Bftpd service.
🔧 Temporary Workarounds
Disable Bftpd Service
linuxTemporarily disable Bftpd FTP server until patching can be completed
sudo systemctl stop bftpd
sudo systemctl disable bftpd
Network Access Control
linuxRestrict FTP access to trusted IP addresses only using firewall rules
sudo iptables -A INPUT -p tcp --dport 21 -s TRUSTED_IP -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 21 -j DROP
🧯 If You Can't Patch
- Implement strict network segmentation to isolate FTP servers from critical systems
- Deploy intrusion prevention systems (IPS) with rules to detect and block exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check Bftpd version with 'bftpd -v' or examine package version. Versions below 5.4 are vulnerable.Check Version:
bftpd -vVerify Fix Applied:
Verify version is 5.4 or higher with 'bftpd -v' and test file transfers work without crashes.📡 Detection & Monitoring
Log Indicators:
- Multiple file transfer errors followed by service crashes
- Abnormal memory usage patterns in Bftpd process
Network Indicators:
- Multiple FTP connections causing transfer errors
- Unusual FTP traffic patterns from untrusted sources
SIEM Query:
source="bftpd.log" AND ("segmentation fault" OR "heap corruption" OR "transfer error")