CVE-2020-28899
📋 TL;DR
CVE-2020-28899 is an authentication bypass vulnerability in ZyXEL LTE4506-M606 routers that allows remote attackers to execute administrative functions without credentials. Attackers can change router passwords, retrieve Wi-Fi credentials, send SMS messages, and modify network settings. Organizations and individuals using affected ZyXEL routers are vulnerable.
💻 Affected Systems
- ZyXEL LTE4506-M606
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise allowing attackers to change all settings, steal credentials, intercept network traffic, send unauthorized SMS messages, and pivot to internal networks.
Likely Case
Attackers changing router passwords to lock out legitimate administrators, stealing Wi-Fi credentials, and potentially redirecting traffic for interception.
If Mitigated
Limited impact if router is behind firewall with restricted WAN access, though internal threats could still exploit if network access is gained.
🎯 Exploit Status
Exploitation requires sending crafted JSON to /cgi-bin/gui.cgi endpoint. No authentication needed, making it trivial for attackers with network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: V1.00(ABDO.3)C0 or later
Vendor Advisory: https://www.zyxel.com/support/Zyxel-security-advisory-for-CGI-vulnerability-of-LTE.shtml
Restart Required: Yes
Instructions:
1. Download latest firmware from ZyXEL support portal. 2. Log into router admin interface. 3. Navigate to Maintenance > Firmware Upgrade. 4. Upload and apply the firmware update. 5. Reboot router after update completes.
🔧 Temporary Workarounds
Restrict Web Interface Access
linuxBlock external access to router web interface using firewall rules
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP
Network Segmentation
allIsolate router management interface to separate VLAN
🧯 If You Can't Patch
- Disable remote management and ensure router web interface is only accessible from trusted internal networks
- Implement network monitoring for suspicious requests to /cgi-bin/gui.cgi endpoint
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface under Maintenance > System Info. If version is V1.00(ABDO.2)C0 or earlier, device is vulnerable.Check Version:
curl -s http://router-ip/cgi-bin/gui.cgi | grep -i versionVerify Fix Applied:
Verify firmware version shows V1.00(ABDO.3)C0 or later after update. Test that unauthenticated requests to /cgi-bin/gui.cgi return authentication errors.📡 Detection & Monitoring
Log Indicators:
- Unauthenticated POST requests to /cgi-bin/gui.cgi
- Multiple failed login attempts followed by successful administrative actions
- Unexpected configuration changes in router logs
Network Indicators:
- Unusual outbound traffic from router to external IPs
- SMS sending activity from router without administrator action
- Port scanning originating from router
SIEM Query:
source="router.log" AND (uri="/cgi-bin/gui.cgi" AND NOT user=*) OR (event="password_change" OR event="config_change")