CVE-2021-3304
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on Sagemcom F@ST 3686 v2 routers by sending an overly long sessionKey parameter to the goform/login endpoint. Attackers can gain full control of affected devices without authentication. Only users of specific Sagemcom router models are affected.
💻 Affected Systems
- Sagemcom F@ST 3686 v2
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of router with persistent backdoor installation, credential theft, network traffic interception, and lateral movement to internal devices.
Likely Case
Router takeover leading to DNS hijacking, credential harvesting, and denial of service for connected users.
If Mitigated
Limited impact if devices are behind firewalls with strict inbound filtering and network segmentation.
🎯 Exploit Status
Simple buffer overflow with public proof-of-concept available via Twitter references. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: Not publicly available
Restart Required: No
Instructions:
1. Check Sagemcom support for firmware updates
2. If update available, download from official vendor site
3. Upload firmware via router web interface
4. Apply update and verify version change
🔧 Temporary Workarounds
Disable Remote Management
allPrevent external access to router web interface
Access router admin interface -> Security -> Remote Management -> Disable
Restrict Management Access
allLimit web interface access to specific IP addresses
Access router admin interface -> Firewall -> Access Control -> Add allowed IPs only
🧯 If You Can't Patch
- Place router behind dedicated firewall with strict inbound rules blocking port 80/443
- Implement network segmentation to isolate router management interface from user networks
🔍 How to Verify
Check if Vulnerable:
Check router firmware version via web interface (Status -> Device Info) for version 3.495Check Version:
curl -s http://router-ip/status.html | grep -i 'firmware version'Verify Fix Applied:
Verify firmware version has changed from 3.495 to a newer version📡 Detection & Monitoring
Log Indicators:
- Unusually long sessionKey parameters in web logs
- Multiple failed login attempts with varying sessionKey lengths
Network Indicators:
- HTTP POST requests to /goform/login with payloads > 1000 bytes
- Traffic patterns suggesting buffer overflow exploitation
SIEM Query:
source="router_logs" AND uri="/goform/login" AND (sessionKey.length>1000 OR contains(sessionKey,"AAAAAAAA"))