Stack Overflow in Tenda O3v3 Router Firmware

Mitigation Strategies

Immediate mitigation steps include restricting access to the formSetCfm CGI endpoint to trusted users only, such as by implementing network-level access controls or firewall rules.

Additionally, monitoring and blocking HTTP requests with suspiciously long funcpara1 parameters targeting the formSetCfm endpoint can help prevent exploitation.

If possible, update the device firmware to a version that patches this vulnerability or contact the vendor for a security update.

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Executive Summary

CVE-2026-36791 is a stack-based buffer overflow vulnerability found in the Tenda O3v3 device version 1.0.0.5. It occurs in the formSetCfm CGI handler, specifically in the save_list_data parameter. The vulnerability arises because the function save_list_data uses the sprintf function with a user-controlled parameter without proper length checks, which can overflow the buffer.

An attacker can exploit this by sending a specially crafted HTTP request to the formSetCfm CGI endpoint with an overly long parameter, causing the device to overflow its stack buffer.

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Impact Analysis

Exploitation of this vulnerability can lead to a Denial of Service (DoS) condition, causing the device to crash or reboot unexpectedly.

Additionally, there is potential for arbitrary code execution, which could allow an attacker to take control of the affected device.

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Detection Guidance

This vulnerability can be detected by monitoring for crafted HTTP requests sent to the formSetCfm CGI endpoint of the Tenda O3v3 device, specifically those with the parameter funcname set to save_list_data and an unusually long funcpara1 parameter.

A practical detection method is to capture and inspect HTTP traffic targeting the device for requests matching this pattern.

• Use network packet capture tools like tcpdump or Wireshark to filter HTTP requests to the formSetCfm endpoint.
• Example tcpdump command to capture relevant HTTP requests: tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep 'formSetCfm'
• Search captured HTTP requests for the parameter funcname=save_list_data and check if funcpara1 parameter length is unusually long, indicating a potential exploit attempt.

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Compliance Impact

The provided information does not specify any direct impact of the CVE-2026-36791 vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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