CVE-2025-6599
📋 TL;DR
An uncontrolled resource consumption vulnerability in Zyxel DX3301-T0 firmware allows attackers to perform Slowloris-style DoS attacks. This can temporarily block legitimate HTTP requests and disrupt access to the web management interface. Only devices running firmware version 5.50(ABVY.6.3)C0 and earlier are affected.
💻 Affected Systems
- Zyxel DX3301-T0
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service for the web management interface, preventing administrative access until attack stops or device restarts.
Likely Case
Temporary disruption of web management interface access while attack is active, with other networking services continuing to function.
If Mitigated
Minimal impact if web interface is not internet-facing and network segmentation limits attack surface.
🎯 Exploit Status
Slowloris attacks are well-known and easy to implement; no authentication required to exploit.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor advisory for latest patched version
Restart Required: Yes
Instructions:
1. Check vendor advisory for latest firmware version. 2. Download firmware from Zyxel support portal. 3. Log into web interface. 4. Navigate to Maintenance > Firmware Upgrade. 5. Upload and apply new firmware. 6. Device will restart automatically.
🔧 Temporary Workarounds
Restrict Web Interface Access
allLimit access to web management interface to trusted IP addresses only
Configure firewall rules to restrict access to port 80/443 to specific management IPs
Disable Internet-Facing Web Interface
allRemove web interface from internet exposure
Configure WAN firewall to block incoming HTTP/HTTPS traffic to device
🧯 If You Can't Patch
- Implement network segmentation to isolate device from untrusted networks
- Deploy rate limiting or DoS protection at network perimeter
🔍 How to Verify
Check if Vulnerable:
Check firmware version in web interface under Maintenance > System InfoCheck Version:
Log into web interface and navigate to Maintenance > System InfoVerify Fix Applied:
Verify firmware version is newer than 5.50(ABVY.6.3)C0 after patching📡 Detection & Monitoring
Log Indicators:
- Multiple incomplete HTTP connections from single source
- Web interface becoming unresponsive
Network Indicators:
- Unusually high number of HTTP connections to port 80/443 with incomplete requests
- Slowloris attack patterns in network traffic
SIEM Query:
source_ip=* dest_port=80 OR dest_port=443 | stats count by source_ip | where count > threshold