CVE-2021-31757
📋 TL;DR
This critical vulnerability in Tenda AC11 routers allows remote attackers to execute arbitrary code via a stack buffer overflow in the setVLAN form handler. Attackers can exploit this by sending a specially crafted POST request to the vulnerable endpoint, potentially gaining full control of affected devices. All users of Tenda AC11 routers with firmware up to version 02.03.01.104_CN are affected.
💻 Affected Systems
- Tenda AC11
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise allowing attackers to install persistent malware, pivot to internal networks, intercept all network traffic, or brick the device.
Likely Case
Remote code execution leading to device takeover, creation of botnet nodes, credential theft, or network traffic interception.
If Mitigated
Limited impact if devices are behind firewalls with strict ingress filtering and network segmentation.
🎯 Exploit Status
Public exploit code exists in GitHub repositories, making exploitation straightforward for attackers with basic skills.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: Not available
Restart Required: No
Instructions:
No official patch available. Check Tenda website for firmware updates beyond version 02.03.01.104_CN.
🔧 Temporary Workarounds
Disable web management interface
allDisable the vulnerable web interface to prevent exploitation
Access router settings > Management > Disable web management
Block access to vulnerable endpoint
linuxUse firewall rules to block access to /goform/setVLAN
iptables -A INPUT -p tcp --dport 80 -m string --string "/goform/setVLAN" --algo bm -j DROP
🧯 If You Can't Patch
- Isolate affected devices in separate network segments with strict firewall rules
- Implement network monitoring for suspicious POST requests to /goform/setVLAN
🔍 How to Verify
Check if Vulnerable:
Check firmware version in router web interface or via SSH: cat /proc/versionCheck Version:
cat /proc/version or check web interface System Status pageVerify Fix Applied:
Verify firmware version is newer than 02.03.01.104_CN📡 Detection & Monitoring
Log Indicators:
- POST requests to /goform/setVLAN with unusual payload length
- System crash or reboot logs
Network Indicators:
- Unusual outbound connections from router
- POST requests to /goform/setVLAN from external IPs
SIEM Query:
source="router_logs" AND (uri="/goform/setVLAN" OR message="buffer overflow")