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The CVE-2025-6599 vulnerability is an uncontrolled resource consumption flaw in the HTTP web server of the Zyxel DX3301-T0 routers. It enables a Slowloris-style application-layer denial-of-service (DoS) attack. The attack exploits the server’s connection handling mechanism by deliberately opening multiple concurrent HTTP connections and keeping them open for as long as possible. This is achieved by transmitting partial HTTP request headers at very slow, periodic intervals. The targeted web server allocates a thread or process to handle each incoming connection while waiting for the complete request. By sending these slow, incomplete requests, the attacker exhausts the finite pool of available connection slots or worker threads. Once all available threads are occupied waiting for the malicious requests to complete, the server can no longer accept new, legitimate connection attempts. This effectively blocks access to the device’s web-based management interface for genuine administrators. The attack operates at the application layer, targeting the HTTP service on its default port (typically 80 or 443). It requires minimal bandwidth from the attacker, as the attack traffic consists of small, slow packets. The vulnerability specifically affects firmware version 5.50(ABVY.6.3)C0 and all earlier releases. The core weakness is the server’s failure to enforce adequate timeouts or implement connection limits to protect its resource pool. Consequently, the management interface becomes temporarily unavailable, while other router services like routing, firewalling, and DHCP continue to function normally. Mitigation involves applying a firmware patch that fixes the resource management logic.
Platform: Zyxel DX3301-T0
Version: Up to 5.50(ABVY.6.3)C0
Vulnerability : Slowloris DoS
Severity: Medium
date: 2025-11-17
Prediction: 2026-01-31
What Undercode Say:
Analytics
Check current firmware version
show version
Check active HTTP connections (Linux-based systems)
netstat -tan | grep :80 | wc -l
Monitor system process/thread count for the web service
ps -eLf | grep http | wc -l
Basic curl test to web interface
curl -I http://
Simulate a slow header send for testing (educational)
printf “GET / HTTP/1.1\r\nHost: target\r\n” | nc -q 10 -i 1000
How Exploit:
- Attacker identifies target Zyxel router with vulnerable firmware.
- Using a tool like
slowhttptest,slowloris.py, or a custom script. - Script opens multiple sockets to the router’s port 80 (HTTP).
- Sends an incomplete “GET / HTTP/1.1\r\nHost: …” request on each socket.
- Periodically sends subsequent headers like “X-a: b\r\n” every 10-15 seconds.
- This keeps connections open, consuming all available server threads.
- Legitimate user requests are queued or rejected, causing a DoS.
- Attacker maintains the attack by sending minimal traffic.
Protection from this CVE
Apply Zyxel firmware update.
Configure firewall rate-limiting.
Implement connection timeouts.
Use external DoS protection.
Impact:
Web management interface unavailable.
Partial service disruption.
No data compromise.
Other services unaffected.
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Sources:
Reported By: nvd.nist.gov
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