Executive Summary

CVE-2026-11503 is a stack-based buffer overflow vulnerability found in the Tenda CX12L router firmware version 16.03.53.12. It exists in the function form_fast_setting_wifi_set within the /goform/fast_setting_wifi_set endpoint, which handles Wi-Fi configuration.

The vulnerability arises because the function processes the SSID parameter into a fixed-size 64-byte buffer without proper length validation. Specifically, it uses the unsafe sprintf function to append "_5G" to the SSID, which can overflow the buffer if the SSID is too long.

This overflow can corrupt the stack, potentially causing the httpd process to crash (Denial of Service) or allowing an attacker to execute arbitrary code with root privileges by overwriting the return address.

The root cause is the lack of bounds checking on the SSID length during 5GHz SSID generation.

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Impact Analysis

This vulnerability can have serious impacts including:

• Denial of Service (DoS) by crashing the router's httpd process, disrupting network connectivity.
• Remote Code Execution (RCE) with root privileges, allowing an attacker to take full control of the device remotely.
• Potential compromise of network security and privacy due to unauthorized access.

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Detection Guidance

This vulnerability can be detected by monitoring requests to the `/goform/fast_setting_wifi_set` endpoint on the Tenda CX12L router firmware version V16.03.53.12, specifically looking for unusually long SSID parameters that exceed the expected buffer size.

A practical detection method is to capture HTTP POST requests to this endpoint and analyze the SSID parameter length. An overly long SSID (longer than 61 characters before appending "_5G") may indicate an attempt to exploit the buffer overflow.

Commands to detect such attempts could include using network traffic capture tools like tcpdump or Wireshark to filter HTTP POST requests to `/goform/fast_setting_wifi_set` and then inspecting the SSID parameter length.

• Example tcpdump command to capture relevant traffic: tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
• Use grep or similar tools to extract and check the SSID parameter length from captured HTTP POST data.

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Mitigation Strategies

Immediate mitigation steps include restricting or blocking access to the vulnerable `/goform/fast_setting_wifi_set` endpoint from untrusted networks to prevent remote exploitation.

If possible, update the router firmware to a version where the vulnerability is fixed by replacing unsafe functions like `sprintf` with safer alternatives such as `snprintf` and implementing input validation to limit SSID length.

As a temporary workaround, monitor and filter incoming requests to ensure SSID parameters do not exceed safe lengths.

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Compliance Impact

The provided information does not specify how the CVE-2026-11503 vulnerability affects compliance with common standards and regulations such as GDPR or HIPAA.

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