CVE-2021-43711
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to execute arbitrary commands on TOTOLINK EX200 routers by injecting malicious parameters into the downloadFlile.cgi binary. Attackers can gain full control of affected devices, potentially compromising network security. All users running vulnerable firmware versions are affected.
💻 Affected Systems
- TOTOLINK EX200
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device takeover leading to network compromise, data exfiltration, lateral movement to other devices, and persistent backdoor installation.
Likely Case
Router compromise allowing traffic interception, DNS hijacking, credential theft, and use as a botnet node.
If Mitigated
Limited impact with proper network segmentation and firewall rules preventing external access to the vulnerable endpoint.
🎯 Exploit Status
Public proof-of-concept code exists showing command injection via GET parameters. The exploit is straightforward and requires minimal technical skill.
🛠️ 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. Download latest firmware. 3. Access router admin interface. 4. Navigate to firmware upgrade section. 5. Upload and apply new firmware. 6. Reboot router.
🔧 Temporary Workarounds
Block CGI endpoint access
linuxUse firewall rules to block external access to the downloadFlile.cgi endpoint
iptables -A INPUT -p tcp --dport 80 -m string --string "downloadFlile.cgi" --algo bm -j DROP
iptables -A INPUT -p tcp --dport 443 -m string --string "downloadFlile.cgi" --algo bm -j DROP
Disable web interface external access
allConfigure router to only allow web interface access from internal network
🧯 If You Can't Patch
- Segment affected routers in isolated network zones with strict firewall rules
- Implement network monitoring for suspicious CGI requests and command execution patterns
🔍 How to Verify
Check if Vulnerable:
Test by accessing http://[router-ip]/cgi-bin/downloadFlile.cgi with command injection payloads (use only in authorized test environments)Check Version:
Check firmware version in router web interface under System Status or similar sectionVerify Fix Applied:
Verify the CGI endpoint no longer executes injected commands and returns appropriate error responses📡 Detection & Monitoring
Log Indicators:
- Unusual CGI requests to downloadFlile.cgi
- GET parameters containing shell metacharacters like ;, |, &, $()
- Multiple failed command injection attempts
Network Indicators:
- HTTP requests to /cgi-bin/downloadFlile.cgi with suspicious parameters
- Outbound connections from router to unexpected destinations
SIEM Query:
source="router_logs" AND (uri="*downloadFlile.cgi*" AND (param="*;*" OR param="*|*" OR param="*$(*" OR param="*`*"))