Executive Summary

The CVE-2026-36823 vulnerability affects the Tenda W20E router in the formAddWebAuthUser function. This function processes user input from the webAuthUserInfo parameter without proper bounds checking. Specifically, the parameter is passed to a strncpy operation without verifying its length, which can lead to a buffer overflow.

An attacker can exploit this by sending a specially crafted HTTP request containing a very long webAuthUserInfo parameter. This causes the buffer overflow, which can crash the process or destabilize the device, resulting in a denial of service (DoS).

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Impact Analysis

This vulnerability can cause a denial of service (DoS) on the affected Tenda W20E router. By exploiting the buffer overflow, an attacker can crash the router's process or cause device instability, potentially disrupting network connectivity and availability.

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Detection Guidance

This vulnerability can be detected by monitoring for HTTP requests sent to the Tenda W20E router that invoke the addWebAuthUser action with an unusually long webAuthUserInfo parameter.

Specifically, an attacker exploits the vulnerability by sending a crafted HTTP request containing a webAuthUserInfo parameter with a very long string (e.g., 888 'a' characters followed by 50 or more newline characters).

To detect such attempts, you can capture and inspect HTTP traffic targeting the router's web interface for requests containing the addWebAuthUser action and analyze the length and content of the webAuthUserInfo parameter.

• Use a network packet capture tool like tcpdump or Wireshark to capture HTTP traffic on the router's management interface.
• Example tcpdump command to capture HTTP traffic on interface eth0: tcpdump -i eth0 -A 'tcp port 80'
• Search captured traffic for HTTP POST or GET requests containing 'addWebAuthUser' and inspect the webAuthUserInfo parameter length.
• Use command-line tools like grep or strings on captured data to find suspiciously long webAuthUserInfo parameters.

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Mitigation Strategies

Immediate mitigation steps include restricting access to the router's web management interface to trusted networks or IP addresses to prevent unauthorized HTTP requests.

Additionally, monitor the router for crashes or instability that may indicate exploitation attempts.

If possible, apply any available firmware updates or patches from the vendor that address this vulnerability.

As a temporary measure, consider disabling remote web management or using firewall rules to block suspicious HTTP requests containing the addWebAuthUser action.

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