Executive Summary

CVE-2026-36818 is a buffer overflow vulnerability in the Tenda W20E router version 15.11.0.6. It occurs in the formAddWewifiWhiteUser function, specifically when processing the wewifiWhiteUserInfo HTTP parameter.

An attacker can exploit this vulnerability by sending a crafted HTTP request with an overly long wewifiWhiteUserInfo parameter. The function uses strncpy with an incorrect length calculation, which leads to a buffer overflow when the parameter exceeds expected limits.

This buffer overflow can cause the router process to crash or destabilize the device, resulting in a denial of service (DoS).

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Impact Analysis

This vulnerability can impact you by causing a denial of service (DoS) on the affected Tenda W20E router. An attacker exploiting this flaw can crash the router process or destabilize the device, leading to loss of network connectivity or interruption of services relying on the router.

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Detection Guidance

This vulnerability can be detected by monitoring for HTTP requests containing an overly long wewifiWhiteUserInfo parameter targeting the formAddWewifiWhiteUser function on the Tenda W20E router.

A practical detection method is to capture and analyze HTTP traffic to the router's web interface, looking for requests where the wewifiWhiteUserInfo parameter exceeds normal length (e.g., around 888 'a' characters followed by a newline).

• Use a network packet capture tool like tcpdump or Wireshark to filter HTTP POST requests to the router's management interface.
• Example tcpdump command to capture HTTP traffic on interface eth0: tcpdump -i eth0 -A 'tcp port 80 and (((ip dst <router_ip>) and (tcp[32:4] = 0x706f7374)) or ((ip src <router_ip>) and (tcp[32:4] = 0x706f7374)))'
• Inspect captured HTTP POST data for the wewifiWhiteUserInfo parameter with suspiciously long values.

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Mitigation Strategies

To mitigate this vulnerability immediately, avoid exposing the router's web management interface to untrusted networks.

Restrict access to the router's HTTP interface by using firewall rules or network segmentation.

Monitor for unusual HTTP requests with abnormally long wewifiWhiteUserInfo parameters and block or alert on such traffic.

Check for and apply any available firmware updates from Tenda that address this buffer overflow vulnerability.

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