Executive Summary

CVE-2026-36799 is a buffer overflow vulnerability in the Tenda G0 router firmware version V15.11.0.5. It exists in the formPortalAuth function, which handles HTTP requests. The vulnerability occurs when an attacker sends a specially crafted HTTP request with an excessively long 'gotoUrl' parameter. This parameter is copied into a fixed-size buffer without proper length checking, causing a buffer overflow.

This overflow can lead to the process crashing or the device becoming unstable, resulting in a denial of service (DoS).

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Impact Analysis

This vulnerability can be exploited by attackers to cause a denial of service on the affected Tenda G0 router. By sending a crafted HTTP request with a long 'gotoUrl' parameter, the attacker can crash the process or destabilize the device, potentially disrupting network connectivity and availability.

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Detection Guidance

This vulnerability can be detected by monitoring HTTP requests sent to the Tenda G0 router's formPortalAuth CGI endpoint. Specifically, look for unusually long gotoUrl parameters in HTTP requests, such as those with 888 or more repeated characters.

A practical detection method is to capture network traffic and filter for HTTP requests targeting the formPortalAuth endpoint with long gotoUrl parameters.

• Use tools like tcpdump or Wireshark to capture HTTP traffic to the router.
• Example tcpdump command to capture HTTP requests to the router (replace <router_ip> with the router's IP):
• tcpdump -A -s 0 host <router_ip> and tcp port 80
• Then, search captured traffic for requests containing 'formPortalAuth' and check if the 'gotoUrl' parameter is excessively long.

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Mitigation Strategies

To mitigate this vulnerability immediately, avoid exposing the Tenda G0 router's formPortalAuth CGI endpoint to untrusted networks.

Restrict access to the router's management interface to trusted IP addresses only.

If possible, apply any available firmware updates from Shenzhen Tenda Technology Co., Ltd that address this buffer overflow issue.

As a temporary measure, monitor and block HTTP requests with suspiciously long gotoUrl parameters targeting the formPortalAuth endpoint to prevent exploitation.

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