CVE-2021-20134
📋 TL;DR
This vulnerability allows authenticated remote attackers to perform absolute path traversal in Quagga services on D-Link DIR-2640 routers, enabling them to write arbitrary files. By appending malicious content to shell scripts, attackers can achieve remote code execution and obtain root access. Only D-Link DIR-2640 routers running firmware version 1.11B02 or earlier are affected.
💻 Affected Systems
- D-Link DIR-2640
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote unauthenticated attacker gains full root shell access to the router, enabling complete device compromise, network traffic interception, lateral movement to internal networks, and persistent backdoor installation.
Likely Case
Authenticated attacker gains remote code execution with root privileges, allowing them to modify router configuration, intercept traffic, install malware, or use the router as a pivot point into internal networks.
If Mitigated
If proper network segmentation and access controls are implemented, the impact is limited to the router itself without allowing lateral movement to other systems.
🎯 Exploit Status
Exploitation requires authentication to Quagga services, but the advisory mentions remote unauthenticated root shell can be obtained, suggesting authentication bypass or default credentials may be exploitable.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware version > 1.11B02
Vendor Advisory: https://supportannouncement.us.dlink.com/announcement/publication.aspx?name=SAP10201
Restart Required: Yes
Instructions:
1. Download latest firmware from D-Link support site. 2. Log into router admin interface. 3. Navigate to System Tools > Firmware Update. 4. Upload and install the new firmware. 5. Reboot the router.
🔧 Temporary Workarounds
Disable Quagga Services
linuxDisable the vulnerable Quagga routing services if not required for network functionality
killall zebra
killall ripd
chmod -x /usr/sbin/zebra
chmod -x /usr/sbin/ripd
Restrict Network Access
linuxBlock external access to Quagga service ports using firewall rules
iptables -A INPUT -p tcp --dport 2601 -j DROP
iptables -A INPUT -p tcp --dport 2602 -j DROP
🧯 If You Can't Patch
- Isolate affected routers in separate VLAN with strict access controls
- Implement network monitoring for suspicious traffic to/from router management interfaces
🔍 How to Verify
Check if Vulnerable:
Check firmware version via router web interface or SSH: cat /etc/version or show versionCheck Version:
cat /etc/version || show version || grep -i version /proc/versionVerify Fix Applied:
Verify firmware version is > 1.11B02 and check that Quagga services no longer accept path traversal in log configuration📡 Detection & Monitoring
Log Indicators:
- Unusual log file modifications in /var/log or other directories
- Quagga service configuration changes
- Shell script modifications in system directories
Network Indicators:
- Unexpected connections to router management ports (2601, 2602)
- Traffic patterns suggesting router compromise
SIEM Query:
source="router_logs" AND (event="log_config_change" OR event="file_modification" AND file_path="*.sh")