CVE-2021-35327
📋 TL;DR
This vulnerability allows attackers to enable Telnet service on TOTOLINK A720R routers via a crafted POST request, then gain access using default credentials. Attackers can achieve remote code execution and full device compromise. Only TOTOLINK A720R routers running vulnerable firmware versions are affected.
💻 Affected Systems
- TOTOLINK A720R
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router takeover allowing traffic interception, credential theft, network pivoting, and persistent backdoor installation.
Likely Case
Router compromise leading to DNS hijacking, credential harvesting, and botnet recruitment.
If Mitigated
Limited impact if Telnet is disabled, strong credentials are set, and network segmentation isolates the router.
🎯 Exploit Status
Exploit requires sending a single crafted POST request. Default credentials (admin:admin) are well-known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check TOTOLINK for updated firmware
Vendor Advisory: No official vendor advisory found
Restart Required: Yes
Instructions:
1. Visit TOTOLINK support website. 2. Download latest firmware for A720R. 3. Access router admin panel. 4. Upload and apply firmware update. 5. Reboot router.
🔧 Temporary Workarounds
Disable Telnet service
allManually disable Telnet service in router configuration
Change default credentials
allChange admin password from default to strong unique password
🧯 If You Can't Patch
- Isolate router on separate VLAN with strict firewall rules
- Disable remote management and restrict admin interface to internal network only
🔍 How to Verify
Check if Vulnerable:
Check if router responds to crafted POST request enabling Telnet, then test Telnet login with default credentialsCheck Version:
Login to router admin panel and check firmware version in system statusVerify Fix Applied:
Attempt exploitation after patch - should fail. Verify Telnet service cannot be enabled via POST request.📡 Detection & Monitoring
Log Indicators:
- POST requests to unusual endpoints
- Telnet service startup logs
- Failed/successful Telnet authentication attempts
Network Indicators:
- Unexpected Telnet port (23) traffic
- POST requests to router management interface from external sources
SIEM Query:
source_ip=external AND dest_ip=router_ip AND (http_method=POST AND uri_contains="cgi-bin" OR port=23)