CVE-2021-27705
📋 TL;DR
This critical buffer overflow vulnerability in Tenda G1 and G3 routers allows remote attackers to execute arbitrary code by sending a specially crafted request to the 'formQOSRuleDel' function. Attackers can take complete control of affected routers without authentication. All users of Tenda G1 and G3 routers with vulnerable firmware are affected.
💻 Affected Systems
- Tenda G1 Router
- Tenda G3 Router
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise allowing attackers to intercept all network traffic, install persistent malware, pivot to internal networks, and use the router as a botnet node.
Likely Case
Router takeover leading to DNS hijacking, credential theft from network traffic, and installation of cryptocurrency miners or ransomware.
If Mitigated
Limited impact if routers are behind firewalls with strict inbound filtering and network segmentation prevents lateral movement.
🎯 Exploit Status
Simple buffer overflow with publicly available proof-of-concept. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Later firmware versions (check Tenda website for latest)
Vendor Advisory: https://www.tenda.com.cn/
Restart Required: Yes
Instructions:
1. Log into router admin panel. 2. Navigate to System Tools > Firmware Upgrade. 3. Download latest firmware from Tenda website. 4. Upload and install firmware. 5. Reboot router.
🔧 Temporary Workarounds
Disable Remote Management
allPrevent external access to router admin interface
Firewall Rules
linuxBlock inbound access to router web interface (typically port 80/443)
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP
🧯 If You Can't Patch
- Replace affected routers with different models/brands
- Place routers behind dedicated firewall with strict inbound filtering
🔍 How to Verify
Check if Vulnerable:
Check firmware version in router admin panel under System Status or System ToolsCheck Version:
curl -s http://router-ip/ | grep -i firmwareVerify Fix Applied:
Verify firmware version is newer than v15.11.0.17(9502)_CN📡 Detection & Monitoring
Log Indicators:
- Multiple POST requests to /goform/formQOSRuleDel
- Unusual process execution in router logs
- Failed buffer overflow attempts in system logs
Network Indicators:
- Unusual outbound connections from router
- DNS queries to suspicious domains
- Traffic patterns indicating command and control
SIEM Query:
source="router.log" AND (url="/goform/formQOSRuleDel" OR "qosIndex" OR "buffer overflow")