CVE-2025-67445: Totolink X5000R Firmware DoS Vulnerability

CVE-2025-67445 Overview

CVE-2025-67445 is a denial-of-service vulnerability affecting the TOTOLINK X5000R router firmware version 9.1.0cu.2415_B20250515. The vulnerability exists in the /cgi-bin/cstecgi.cgi component, where improper handling of the CONTENT_LENGTH environment variable allows attackers to cause memory exhaustion or a segmentation fault. When the lighttpd web server's request size limit is not properly enforced, a crafted large POST request can crash the management CGI, resulting in complete loss of availability of the web administration interface.

Critical Impact

Unauthenticated remote attackers can render the router's web management interface unavailable by sending specially crafted POST requests, disrupting network administration capabilities.

Affected Products

• TOTOLINK X5000R Firmware version 9.1.0cu.2415_B20250515
• TOTOLINK X5000R Hardware

Discovery Timeline

• 2026-02-24 - CVE CVE-2025-67445 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2025-67445

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption), which manifests in the CGI handler's memory allocation routine. The core issue lies in how the CGI binary processes incoming HTTP POST requests without validating the size of the content being requested.

When the cstecgi.cgi handler receives an HTTP POST request, it reads the CONTENT_LENGTH environment variable provided by the lighttpd web server. The CGI then calls malloc(CONTENT_LENGTH + 1) to allocate memory for storing the incoming request body. This allocation occurs without sufficient bounds checking to ensure the requested size is within acceptable limits.

Under normal circumstances, the lighttpd web server would enforce request size limits that would prevent excessively large values from reaching the CGI. However, when these limits are not properly configured or enforced, an attacker can specify an arbitrarily large CONTENT_LENGTH value in their request headers. This can lead to either memory exhaustion when the system attempts to allocate an extremely large memory block, or a segmentation fault when the allocation fails and the CGI attempts to use the resulting null or invalid pointer.

Root Cause

The root cause of this vulnerability is the absence of proper input validation on the CONTENT_LENGTH value before memory allocation. The CGI handler trusts the value provided by the web server without implementing its own sanity checks to ensure the value falls within reasonable bounds. This represents a fundamental secure coding error where user-controlled input directly influences resource allocation without validation.

Attack Vector

The attack can be executed remotely over the network by any unauthenticated attacker who can reach the router's web management interface. The attacker needs to craft an HTTP POST request to /cgi-bin/cstecgi.cgi with an excessively large CONTENT_LENGTH header value. When the CGI processes this request, it attempts to allocate memory based on the malicious value, leading to either:

1. Memory exhaustion: The system runs out of available memory attempting to fulfill the allocation request
2. Segmentation fault: The malloc() call returns NULL due to insufficient memory, and subsequent code dereferences this null pointer

In either case, the CGI process crashes, making the web administration interface unavailable until the service recovers or the device is rebooted. Technical details and a proof-of-concept demonstrating this vulnerability are available in the GitHub PoC repository.

Detection Methods for CVE-2025-67445

Indicators of Compromise

• Abnormally large HTTP POST requests to /cgi-bin/cstecgi.cgi with CONTENT_LENGTH values exceeding typical thresholds
• Repeated crashes or restarts of the CGI process or lighttpd web server on affected TOTOLINK devices
• Web management interface becoming unresponsive following external network traffic
• Memory utilization spikes on the affected router coinciding with incoming web requests

Detection Strategies

• Implement network intrusion detection rules to flag HTTP POST requests with unusually large CONTENT_LENGTH headers targeting TOTOLINK web interfaces
• Monitor for repeated HTTP 500 errors or connection resets from the router's web management port
• Deploy SentinelOne Singularity to detect anomalous process behavior and resource consumption patterns on network-connected devices

Monitoring Recommendations

• Enable detailed logging on perimeter firewalls for traffic destined to router management interfaces
• Configure alerts for web server process crashes or unexpected terminations on network infrastructure devices
• Regularly review router system logs for evidence of memory allocation failures or segmentation faults

How to Mitigate CVE-2025-67445

Immediate Actions Required

• Restrict access to the router's web management interface to trusted administrative networks only using firewall rules or access control lists
• Disable remote web management if not required, limiting access to local LAN connections
• Implement network segmentation to isolate critical network infrastructure from untrusted networks
• Monitor for firmware updates from TOTOLINK and apply patches promptly when available

Patch Information

As of the last NVD update on 2026-02-25, no official patch has been documented from TOTOLINK for this vulnerability. Administrators should monitor the TOTOLINK official website for security advisories and firmware updates addressing this issue.

Workarounds

• Configure firewall rules to block external access to port 80/443 on affected TOTOLINK routers
• Implement rate limiting on the web server to restrict the number of requests from single sources
• Configure lighttpd to enforce strict max-request-size limits to prevent excessively large POST requests from reaching the CGI handler
• Consider placing affected devices behind a reverse proxy with request size validation capabilities

# Configuration example
# Example iptables rule to restrict management interface access to trusted network
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP

# Block external access to CGI endpoint
iptables -A INPUT -p tcp --dport 80 -m string --string "/cgi-bin/cstecgi.cgi" --algo bm -j DROP