CVE-2021-39509
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary commands on D-Link DIR-816 routers through command injection in the web interface. Attackers can exploit this by sending specially crafted HTTP requests to the vulnerable endpoint, potentially gaining full control of affected devices. This affects D-Link DIR-816 and DIR-816A2 routers running specific vulnerable firmware versions.
💻 Affected Systems
- D-Link DIR-816
- D-Link DIR-816A2
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise allowing attackers to install persistent backdoors, intercept network traffic, pivot to internal networks, or use the device for botnet activities.
Likely Case
Remote code execution leading to device takeover, credential theft, DNS hijacking, or participation in DDoS attacks.
If Mitigated
Limited impact if devices are behind firewalls with restricted web interface access and proper network segmentation.
🎯 Exploit Status
Public proof-of-concept code exists showing exploitation via HTTP POST requests to /goform/form2userconfig.cgi with shell metacharacters in parameters.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check D-Link security bulletin for latest patched firmware
Vendor Advisory: https://www.dlink.com/en/security-bulletin/
Restart Required: Yes
Instructions:
1. Visit D-Link support website. 2. Download latest firmware for your model. 3. Log into router web interface. 4. Navigate to firmware update section. 5. Upload and apply new firmware. 6. Reboot router after update completes.
🔧 Temporary Workarounds
Disable Remote Management
allPrevent external access to router web interface
Network Segmentation
allIsolate router management interface to trusted network segment
🧯 If You Can't Patch
- Disable the router's web management interface entirely if not needed
- Implement strict firewall rules to block all external access to port 80/443 on the router
🔍 How to Verify
Check if Vulnerable:
Check firmware version in router web interface under System Status or Tools > Firmware. If version is 1.10CNB05_R1B011D88210 or earlier, device is vulnerable.Check Version:
curl -s http://router-ip/ | grep -i firmware or check web interface directlyVerify Fix Applied:
After updating firmware, verify version has changed to a newer release than the vulnerable version. Test the /goform/form2userconfig.cgi endpoint with safe test payloads.📡 Detection & Monitoring
Log Indicators:
- HTTP POST requests to /goform/form2userconfig.cgi containing shell metacharacters like ;, |, &, $, (, )
- Unusual command execution in router logs
- Failed authentication attempts followed by successful exploitation
Network Indicators:
- Unusual outbound connections from router to unknown IPs
- DNS queries to suspicious domains from router
- Unexpected traffic patterns from router
SIEM Query:
source="router_logs" AND uri="/goform/form2userconfig.cgi" AND (payload="*;*" OR payload="*|*" OR payload="*&*" OR payload="*$(*" OR payload="*`*")🔗 References
- https://github.com/doudoudedi/main-DIR-816_A2_Command-injection
- https://github.com/doudoudedi/main-DIR-816_A2_Command-injection/blob/main/injection.md
- https://www.dlink.com/en/security-bulletin/
- https://github.com/doudoudedi/main-DIR-816_A2_Command-injection
- https://github.com/doudoudedi/main-DIR-816_A2_Command-injection/blob/main/injection.md
- https://www.dlink.com/en/security-bulletin/
