Executive Summary

The CVE-2026-36821 vulnerability affects the Tenda W20E router, specifically in the formCropAndSetWewifiPic function. This function improperly handles the picCropName parameter from HTTP requests, leading to a buffer overflow when this parameter is excessively long.

The vulnerability occurs because the code uses sprintf to copy the picCropName value into a buffer without proper length checks, which can cause memory corruption.

An attacker can exploit this by sending a crafted HTTP request with a very long picCropName value (e.g., 888 or more characters), causing the device to crash or become unstable.

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Impact Analysis

This vulnerability can cause a Denial of Service (DoS) on the affected Tenda W20E router.

By sending a specially crafted HTTP request with an overly long picCropName parameter, an attacker can crash the router's process or cause device instability, potentially disrupting network connectivity.

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Detection Guidance

This vulnerability can be detected by monitoring for HTTP requests sent to the Tenda W20E router that contain an excessively long picCropName parameter in the formCropAndSetWewifiPic function.

Specifically, an attacker sends a crafted HTTP request with a picCropName parameter value of 888 or more characters, which triggers the buffer overflow.

To detect potential exploitation attempts, you can capture and inspect HTTP traffic to the router for unusually long picCropName parameters.

• Use a network packet capture tool like tcpdump or Wireshark to filter HTTP requests to the router's IP address.
• Example tcpdump command to capture HTTP requests to the router (replace <router_ip> with the actual IP):
• tcpdump -A -s 0 host <router_ip> and tcp port 80
• Then, search the captured traffic for HTTP requests containing the parameter picCropName with a very long value (e.g., over 800 characters).

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Mitigation Strategies

Immediate mitigation steps include updating the router firmware to a version that is not vulnerable, such as version V16.01.0.6 or later.

If an update is not immediately available, restrict access to the router's HTTP management interface to trusted networks or IP addresses to prevent exposure to crafted requests.

Additionally, monitor network traffic for suspicious HTTP requests with long picCropName parameters and block or alert on such traffic.

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