CVE-2020-26728
📋 TL;DR
This CVE describes a remote code execution vulnerability in Tenda AC9 routers where shell metacharacters in the guestuser field can be exploited via a POST request. Attackers can execute arbitrary commands on affected devices with high privileges. Users of Tenda AC9 routers with specific firmware versions are affected.
💻 Affected Systems
- Tenda AC9
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the router allowing attackers to intercept all network traffic, install persistent malware, pivot to internal networks, and potentially brick the device.
Likely Case
Attackers gain remote shell access to the router, enabling them to modify DNS settings, capture credentials, and use the router as a foothold for further attacks.
If Mitigated
With proper network segmentation and firewall rules, impact is limited to the router itself without allowing lateral movement to other systems.
🎯 Exploit Status
Multiple public proof-of-concept exploits exist on GitHub. The vulnerability requires no authentication and has simple exploitation steps.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: Not found in provided references
Restart Required: Yes
Instructions:
1. Check Tenda's official website for firmware updates. 2. Download the latest firmware for your specific AC9 model. 3. Access router admin interface. 4. Navigate to firmware upgrade section. 5. Upload and apply the new firmware. 6. Reboot the router.
🔧 Temporary Workarounds
Disable Guest Network
allDisable the guest network feature which may be related to the vulnerable guestuser field.
Restrict Management Interface Access
linuxConfigure firewall rules to restrict access to the router's web management interface.
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 the vulnerable router with a different model that receives security updates
- Place the router behind a dedicated firewall that blocks all external access to its management interface
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface. If version matches affected versions, device is vulnerable.Check Version:
Check via router web interface or use: curl -s http://router-ip/ | grep -i firmwareVerify Fix Applied:
After updating firmware, verify the version no longer matches affected versions and test if guestuser field still accepts shell metacharacters.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to router management interface
- Commands containing shell metacharacters in guestuser field
- Multiple failed login attempts followed by successful POST
Network Indicators:
- Unusual outbound connections from router to external IPs
- DNS queries to suspicious domains from router
- Traffic patterns indicating command and control activity
SIEM Query:
source="router.log" AND ("POST" AND "guestuser" AND ("|" OR ";" OR "`" OR "$"))🔗 References
- https://github.com/Lyc-heng/Router/blob/main/Tenda/rce1.md
- https://github.com/Lyc-heng/routers/blob/a80b30bccfc9b76f3a4868ff28ad5ce2e0fca180/routers/rce1.md
- https://github.com/Lyc-heng/routers/blob/main/routers/rce1.md
- https://github.com/Lyc-heng/Router/blob/main/Tenda/rce1.md
- https://github.com/Lyc-heng/routers/blob/a80b30bccfc9b76f3a4868ff28ad5ce2e0fca180/routers/rce1.md
- https://github.com/Lyc-heng/routers/blob/main/routers/rce1.md
