CVE-2021-27708
📋 TL;DR
This CVE describes a critical command injection vulnerability in TOTOLINK X5000R and A720R routers that allows remote attackers to execute arbitrary operating system commands by sending specially crafted HTTP requests. The vulnerability occurs because the firmware passes untrusted user input directly to the glibc system() function without proper sanitization. Anyone using the affected router models with vulnerable firmware versions is at risk.
💻 Affected Systems
- TOTOLINK X5000R router
- TOTOLINK A720R router
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the router allowing attackers to install persistent backdoors, intercept all network traffic, pivot to internal networks, and use the router as part of a botnet.
Likely Case
Router takeover leading to DNS hijacking, credential theft from network traffic, and installation of malware on connected devices.
If Mitigated
Limited impact if network segmentation isolates routers and strict firewall rules prevent external access to management interfaces.
🎯 Exploit Status
Multiple public proof-of-concept exploits exist showing simple HTTP requests with command injection payloads in the 'command' parameter.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: No official vendor advisory found
Restart Required: Yes
Instructions:
1. Check TOTOLINK website for firmware updates
2. If available, download latest firmware
3. Access router admin interface
4. Navigate to firmware upgrade section
5. Upload new firmware file
6. Wait for router to reboot
🔧 Temporary Workarounds
Network Segmentation and Access Control
linuxIsolate routers from untrusted networks and restrict access to management interfaces
iptables -A INPUT -p tcp --dport 80 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
Disable Remote Management
allTurn off web-based remote management if not required
🧯 If You Can't Patch
- Replace affected routers with different models from vendors with better security track records
- Place routers behind dedicated firewalls with strict inbound/outbound filtering
🔍 How to Verify
Check if Vulnerable:
Check router firmware version via admin interface and compare with affected versions. Test with non-destructive command injection payload like 'command=id' if authorized.Check Version:
curl -s http://router-ip/status.cgi | grep version or check web interfaceVerify Fix Applied:
Verify firmware version has changed from vulnerable versions. Test that command injection payloads no longer execute.📡 Detection & Monitoring
Log Indicators:
- HTTP requests containing system commands in parameters
- Unusual process execution from web service
- Suspicious command strings like 'wget', 'curl', 'nc', 'bash' in web logs
Network Indicators:
- HTTP POST requests to router with command injection patterns
- Unusual outbound connections from router to external IPs
SIEM Query:
source="router_logs" AND ("command=" OR "system(" OR "bash -c" OR "wget http" OR "curl http")