CVE-2021-33267
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on D-Link DIR-809 routers via a stack buffer overflow in the formStaticDHCP function. Attackers can exploit this by sending a specially crafted POST request to the device's web interface. All users of affected firmware versions are at risk.
💻 Affected Systems
- D-Link DIR-809
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise leading to persistent backdoor installation, credential theft, and use as a pivot point into internal networks.
Likely Case
Remote code execution allowing attackers to modify device configuration, intercept network traffic, or join botnets.
If Mitigated
Limited impact if device is behind firewall with restricted web interface access and proper network segmentation.
🎯 Exploit Status
Public proof-of-concept code exists, making exploitation straightforward for attackers with basic skills.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware newer than DIR-809Ax_FW1.12WWB03_20190410
Vendor Advisory: https://www.dlink.com/en/security-bulletin/
Restart Required: Yes
Instructions:
1. Visit D-Link support site. 2. Download latest firmware for DIR-809. 3. Log into router web interface. 4. Navigate to System Tools > Firmware Update. 5. Upload and apply new firmware. 6. Reboot router.
🔧 Temporary Workarounds
Disable Remote Management
allPrevent external access to router web interface
Log into router > Advanced > Remote Management > Disable
Restrict Web Interface Access
allLimit web interface access to specific IP addresses
Log into router > Advanced > Access Control > Add allowed IPs
🧯 If You Can't Patch
- Segment router on isolated network segment
- Implement network firewall rules to block access to router web interface from untrusted networks
🔍 How to Verify
Check if Vulnerable:
Check firmware version in router web interface under System Tools > FirmwareCheck Version:
curl -s http://router-ip/ | grep -i firmwareVerify Fix Applied:
Confirm firmware version is newer than DIR-809Ax_FW1.12WWB03_20190410📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to /formStaticDHCP
- Multiple failed login attempts followed by successful POST
Network Indicators:
- Unusual outbound connections from router
- POST requests with abnormally long parameters to router IP
SIEM Query:
source="router.log" AND (uri="/formStaticDHCP" OR method="POST") AND size>1000