CVE-2025-7673
📋 TL;DR
A buffer overflow vulnerability in the zhttpd URL parser of Zyxel VMG8825-T50K routers allows unauthenticated attackers to cause denial-of-service or potentially execute arbitrary code via specially crafted HTTP requests. This affects all systems running firmware versions before V5.50(ABOM.5)C0. The high CVSS score of 9.8 indicates critical severity.
💻 Affected Systems
- Zyxel VMG8825-T50K
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with full system compromise, allowing attacker to install malware, pivot to internal networks, or permanently damage the device.
Likely Case
Denial-of-service causing router reboot or crash, disrupting network connectivity for all connected devices.
If Mitigated
Limited to denial-of-service if exploit fails to achieve code execution, still causing service disruption.
🎯 Exploit Status
Buffer overflow vulnerabilities in URL parsers are commonly exploited, and the unauthenticated nature lowers the barrier.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: V5.50(ABOM.5)C0
Vendor Advisory: https://www.zyxel.com/service-provider/global/en/zyxel-security-advisory-remote-code-execution-and-denial-service-vulnerabilities-cpe
Restart Required: Yes
Instructions:
1. Log into router admin interface. 2. Navigate to Maintenance > Firmware Upgrade. 3. Download firmware V5.50(ABOM.5)C0 from Zyxel support site. 4. Upload and apply the firmware update. 5. Reboot the router after installation completes.
🔧 Temporary Workarounds
Disable Remote Management
allDisable web management interface from WAN/Internet to prevent external exploitation.
Network Segmentation
allPlace affected routers in isolated network segments with strict firewall rules limiting HTTP access.
🧯 If You Can't Patch
- Implement strict network access controls to limit HTTP traffic to the router's management interface
- Monitor for abnormal HTTP requests or router reboots and have incident response procedures ready
🔍 How to Verify
Check if Vulnerable:
Check firmware version via router web interface at System Info > Firmware Version or via SSH with 'cat /proc/version'Check Version:
ssh admin@router-ip 'cat /proc/version' or check web interfaceVerify Fix Applied:
Confirm firmware version shows V5.50(ABOM.5)C0 or later in System Info📡 Detection & Monitoring
Log Indicators:
- Multiple malformed HTTP requests to router IP
- Router reboot events in system logs
- zhttpd process crashes
Network Indicators:
- Unusual HTTP traffic patterns to router management port (typically 80/443)
- Requests with abnormally long URLs or special characters
SIEM Query:
source="router-logs" AND (http_uri_length>1000 OR http_status=500) OR event="system_reboot"