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Many organizations believe applying security patches fully resolves vulnerabilities. However, attackers often maintain persistent access even after patches are deployed. A recent Fortinet report revealed that threat actors retained read-only access to FortiGate devices by planting a hidden backdoor using symbolic links (symlinks) in systems with SSL-VPN enabled.
How Attackers Exploit Symbolic Links
Symbolic links (symlinks) are files that act as references to other files or directories. Attackers manipulate these to maintain persistence:
1. Create a Symlink Backdoor:
ln -s /etc/shadow /var/www/html/malicious_link
This links a critical system file to a web-accessible directory, allowing attackers to retain access.
2. Check for Suspicious Symlinks:
find / -type l -exec ls -la {} \; | grep -i "suspicious_path"
3. Remove Malicious Symlinks:
unlink /var/www/html/malicious_link
You Should Know: Steps to Fully Mitigate the Risk
1. Update FortiOS Immediately:
Ensure devices run:
- FortiOS 7.6.2, 7.4.7, 7.2.11, 7.0.17, or 6.4.16
2. Scan for Hidden Symlinks:
find / -xtype l -perm -o=w
3. Reset All Credentials:
passwd admin
4. Audit SSL-VPN Configurations:
grep -r "sslvpn" /etc/fortigate/
5. Factory Reset + Firmware Reinstall:
- Reflash firmware to eliminate firmware-level backdoors.
6. Monitor for Anomalies:
auditd -l | grep "symlink"
What Undercode Say
Patching alone is insufficient. Attackers leverage file system tricks, credential theft, and configuration manipulation to maintain access. Assume compromise, verify trust, and enforce:
– Linux:
chattr +i /etc/passwd Immutable critical files
– Windows:
fsutil behavior query symlinkEvaluation Check symlink policy
– Network Monitoring:
tcpdump -i eth0 'port 443' -w vpn_traffic.pcap
Expected Output:
A fully hardened system with no residual attacker pathways, verified through logs, file integrity checks, and credential rotation.
Relevant URLs:
References:
Reported By: Albertwhale Patching – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
