CVE-2021-20157
📋 TL;DR
This vulnerability allows unauthenticated attackers to force affected devices to reboot by exploiting a hidden administrative command. It affects DrayTek Vigor routers and modems, creating denial-of-service conditions. Network administrators using these devices are primarily affected.
💻 Affected Systems
- DrayTek Vigor routers
- DrayTek Vigor modems
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Sustained denial-of-service attacks could render critical network infrastructure unavailable for extended periods, disrupting business operations and connectivity.
Likely Case
Intermittent network outages and service disruptions as attackers reboot devices, causing temporary loss of connectivity for connected users and systems.
If Mitigated
Minimal impact with proper network segmentation and access controls limiting exposure to the vulnerable administrative interface.
🎯 Exploit Status
Simple HTTP request to specific endpoint triggers the reboot. No authentication or special conditions required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Varies by specific device model - check DrayTek security advisories
Vendor Advisory: https://www.draytek.com/about/security-advisory/
Restart Required: Yes
Instructions:
1. Identify your specific DrayTek device model. 2. Visit DrayTek support portal. 3. Download latest firmware for your model. 4. Backup current configuration. 5. Upload and apply firmware update through web interface. 6. Verify successful update and restore configuration if needed.
🔧 Temporary Workarounds
Disable WAN access to management interface
allPrevent external access to the vulnerable administrative interface
Access router web interface -> System Maintenance -> Management -> Set 'Allow management from WAN' to Disabled
Restrict management interface access
allLimit which IP addresses can access the management interface
Access router web interface -> System Maintenance -> Management -> Configure 'Trusted Hosts' with specific IP ranges
🧯 If You Can't Patch
- Implement network segmentation to isolate affected devices from untrusted networks
- Deploy network firewalls to block access to the device's management interface from unauthorized sources
🔍 How to Verify
Check if Vulnerable:
Check if device responds to reboot command via HTTP request to management interface without authenticationCheck Version:
Login to web interface -> System Maintenance -> Firmware Information -> Check firmware versionVerify Fix Applied:
Attempt the exploit after patching - device should not reboot from unauthenticated requests📡 Detection & Monitoring
Log Indicators:
- Multiple reboot events in system logs
- Unauthenticated access attempts to administrative endpoints
- HTTP requests to reboot-related URLs from unexpected sources
Network Indicators:
- HTTP POST requests to device management interface from external IPs
- Sudden increase in management interface traffic followed by device unavailability
SIEM Query:
source="router_logs" AND (event="reboot" OR url="*reboot*") AND src_ip NOT IN [trusted_management_ips]