CVE-2021-31755
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on Tenda AC11 routers via a stack buffer overflow in the /goform/setmac endpoint. Attackers can exploit this by sending a specially crafted POST request, potentially gaining full control of the device. All users with affected firmware versions are at risk.
💻 Affected Systems
- Tenda AC11 router
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise leading to persistent backdoor installation, network traffic interception, lateral movement to connected devices, and botnet recruitment.
Likely Case
Remote code execution allowing attackers to modify router settings, intercept traffic, or use the device as a pivot point for further attacks.
If Mitigated
Limited impact if device is behind firewall with restricted WAN access, though internal network attacks remain possible.
🎯 Exploit Status
Multiple public proof-of-concept exploits exist. CISA has added this to their Known Exploited Vulnerabilities catalog.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware versions after 02.03.01.104_CN
Vendor Advisory: https://www.tendacn.com/en/
Restart Required: Yes
Instructions:
1. Visit Tenda official website. 2. Download latest firmware for AC11 model. 3. Log into router admin interface. 4. Navigate to System Tools > Firmware Upgrade. 5. Upload and install new firmware. 6. Reboot router.
🔧 Temporary Workarounds
Block web interface access
linuxRestrict access to router web interface from untrusted networks
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.0.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.0.0/24 -j DROP
Disable remote management
allTurn off remote management feature in router settings
🧯 If You Can't Patch
- Isolate affected routers in separate VLAN with strict firewall rules
- Implement network segmentation to limit lateral movement from compromised routers
🔍 How to Verify
Check if Vulnerable:
Check firmware version in router web interface under System Status or System Tools > Firmware UpgradeCheck Version:
curl -s http://router-ip/goform/getStatus | grep versionVerify Fix Applied:
Verify firmware version is newer than 02.03.01.104_CN and attempt to reproduce exploit with test payload📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to /goform/setmac endpoint
- Multiple failed authentication attempts followed by successful setmac request
- Unexpected firmware or configuration changes
Network Indicators:
- Unusual outbound connections from router
- Traffic patterns suggesting command and control communication
- Port scanning originating from router
SIEM Query:
source="router-logs" AND (uri="/goform/setmac" OR method="POST" AND uri CONTAINS "setmac")