CVE-2021-35324
📋 TL;DR
This vulnerability allows attackers to bypass authentication on TOTOLINK A720R routers by exploiting a flaw in the Form_Login function. Attackers can gain unauthorized access to the router's administrative interface without valid credentials. This affects users running the vulnerable firmware version on these specific router models.
💻 Affected Systems
- TOTOLINK A720R
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of router with ability to modify network settings, intercept traffic, install malware, or use as pivot point into internal network.
Likely Case
Unauthorized access to router admin panel leading to network configuration changes, DNS hijacking, or credential theft.
If Mitigated
Limited impact if router is behind firewall with restricted WAN access and strong internal network segmentation.
🎯 Exploit Status
Public GitHub repository contains proof-of-concept. Authentication bypass typically requires minimal technical skill to exploit.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: No official vendor advisory found
Restart Required: Yes
Instructions:
1. Check TOTOLINK official website for firmware updates
2. If update available, download and verify checksum
3. Access router admin panel (if possible)
4. Navigate to firmware update section
5. Upload new firmware file
6. Wait for router to reboot
7. Verify new firmware version
🔧 Temporary Workarounds
Disable WAN administration
allPrevent external access to router admin interface
Implement network segmentation
allIsolate router management interface to separate VLAN
🧯 If You Can't Patch
- Place router behind firewall with strict inbound rules blocking all WAN access to admin ports
- Change default admin credentials and implement strong password policies
🔍 How to Verify
Check if Vulnerable:
Check router firmware version via admin interface. If version is exactly V4.1.5cu.470_B20200911, device is vulnerable.Check Version:
Login to router admin panel and check System Status or Firmware Information pageVerify Fix Applied:
After firmware update, verify version has changed from vulnerable version. Test authentication bypass attempt fails.📡 Detection & Monitoring
Log Indicators:
- Multiple failed login attempts followed by successful login from same IP
- Admin access from unexpected IP addresses
- Configuration changes without authorized user login
Network Indicators:
- HTTP requests to /cgi-bin/login.cgi with unusual parameters
- Admin interface access from external IPs
SIEM Query:
source="router.logs" AND (event="admin_login" AND result="success" AND NOT user="authorized_user")