CVE-2021-31627
📋 TL;DR
This CVE describes a buffer overflow vulnerability in Tenda AC9 routers that allows attackers to execute arbitrary code by manipulating the index parameter. The vulnerability affects Tenda AC9 V1.0 through V15.03.05.19(6318) and AC9 V3.0 V15.03.06.42_multi firmware versions. Attackers can potentially gain full control of affected routers.
💻 Affected Systems
- Tenda AC9
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise leading to network infiltration, credential theft, man-in-the-middle attacks, and persistent backdoor installation.
Likely Case
Router takeover allowing network traffic interception, DNS hijacking, and lateral movement into connected devices.
If Mitigated
Limited impact if routers are behind firewalls with strict inbound filtering and network segmentation.
🎯 Exploit Status
Public GitHub repository contains exploit details and proof-of-concept code. Buffer overflow via index parameter manipulation is straightforward.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after V15.03.05.19(6318) for V1.0 and after V15.03.06.42_multi for V3.0
Vendor Advisory: http://tenda.com
Restart Required: Yes
Instructions:
1. Log into router admin interface. 2. Navigate to firmware update section. 3. Download latest firmware from Tenda website. 4. Upload and apply firmware update. 5. Reboot router after update completes.
🔧 Temporary Workarounds
Network Segmentation
allIsolate router management interface from untrusted networks
Access Control Lists
linuxRestrict access to router management interface to trusted IPs only
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
🧯 If You Can't Patch
- Replace affected routers with patched models or different vendor products
- Deploy network firewall in front of router to block exploit attempts
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface under System Status or Firmware Update sectionCheck Version:
curl -s http://router-ip/status.cgi | grep firmwareVerify Fix Applied:
Confirm firmware version is newer than affected versions and test index parameter handling📡 Detection & Monitoring
Log Indicators:
- Unusual HTTP requests to router management interface with malformed index parameters
- Multiple failed exploit attempts
Network Indicators:
- HTTP traffic to router management port with suspicious parameter patterns
- Unexpected outbound connections from router
SIEM Query:
source="router-logs" AND (uri="*index=*" OR method="POST" AND uri="*/goform/*") AND status=200