CVE-2025-11845
📋 TL;DR
A null pointer dereference vulnerability in Zyxel networking devices allows authenticated administrators to trigger a denial-of-service condition by sending a crafted HTTP request to the certificate downloader CGI program. This affects Zyxel VMG3625-T50B and WX3100-T0 firmware through specific versions. The vulnerability requires administrator credentials to exploit.
💻 Affected Systems
- Zyxel VMG3625-T50B
- Zyxel WX3100-T0
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Device becomes completely unresponsive, requiring physical reboot or factory reset to restore functionality, disrupting network services for all connected users.
Likely Case
Temporary service interruption affecting the web management interface and potentially some network functions until device automatically restarts or is manually rebooted.
If Mitigated
Minimal impact with proper access controls preventing unauthorized administrators from exploiting the vulnerability.
🎯 Exploit Status
Exploitation requires authenticated administrator access. The crafted HTTP request triggers the null pointer dereference in the certificate downloader functionality.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor advisory for specific patched versions
Restart Required: Yes
Instructions:
1. Access device web interface with admin credentials. 2. Navigate to firmware update section. 3. Download latest firmware from Zyxel support portal. 4. Upload and apply firmware update. 5. Reboot device after update completes.
🔧 Temporary Workarounds
Restrict Admin Access
allLimit administrative access to trusted IP addresses and networks only
Configure firewall rules to restrict access to device management interface (typically ports 80/443) from specific IP ranges
Disable Unnecessary Services
allDisable remote management if not required
Access web interface > Administration > Remote Management > Disable
🧯 If You Can't Patch
- Implement strict access controls allowing only trusted administrators to access the management interface
- Monitor for unusual HTTP requests to certificate downloader endpoints and implement rate limiting
🔍 How to Verify
Check if Vulnerable:
Check firmware version in device web interface under System Information or via CLI command 'show version'Check Version:
show version (CLI) or check System Information in web interfaceVerify Fix Applied:
Verify firmware version is newer than affected versions listed in the advisory📡 Detection & Monitoring
Log Indicators:
- Multiple failed authentication attempts followed by successful admin login
- HTTP requests to certificate downloader CGI endpoints with unusual parameters
- Device reboot logs without normal shutdown sequence
Network Indicators:
- HTTP POST requests to /cgi-bin/certificate_downloader or similar endpoints with crafted parameters
- Sudden drop in device responsiveness after specific HTTP requests
SIEM Query:
source="device_logs" AND (uri="/cgi-bin/certificate*" AND method="POST") AND (status=500 OR device="unresponsive")