Executive Summary

CVE-2026-36778 is a stack-based buffer overflow vulnerability found in the Tenda O3 Wireless Router firmware version 1.0.0.5. It specifically affects the R7WebsSecurityHandler CGI handler, where the username parameter is not properly validated or sanitized. An attacker can send a crafted HTTP request with an excessively long username, causing the username to overflow a fixed-size buffer on the stack when copied using strcpy. This overflow can lead to a denial of service by crashing or rebooting the device, and potentially allows arbitrary code execution.

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Impact Analysis

This vulnerability can impact you by allowing an attacker to cause a denial of service (DoS) on your Tenda O3 Wireless Router. By exploiting the buffer overflow in the username parameter, the attacker can crash or reboot the device, disrupting network connectivity and availability. Additionally, there is a potential risk that the attacker could execute arbitrary code on the device, which could lead to further compromise or control over the router.

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Compliance Impact

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

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Detection Guidance

This vulnerability can be detected by monitoring for HTTP requests sent to the R7WebsSecurityHandler CGI endpoint that contain an unusually long username parameter combined with a non-empty password parameter.

A practical detection method involves capturing and inspecting HTTP traffic to identify requests targeting the R7WebsSecurityHandler with a username parameter exceeding normal length limits.

• Use network packet capture tools like tcpdump or Wireshark to filter HTTP requests to the vulnerable endpoint, for example: tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
• Use curl or similar tools to manually test the endpoint by sending crafted HTTP requests with long username parameters, e.g.: curl -X POST 'http://<router-ip>/R7WebsSecurityHandler' -d 'username=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&password=any'

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Mitigation Strategies

Immediate mitigation steps include restricting access to the R7WebsSecurityHandler CGI endpoint to trusted networks or devices only, such as by using firewall rules or access control lists.

Additionally, monitoring and blocking HTTP requests with abnormally long username parameters can help prevent exploitation attempts.

If available, updating the router firmware to a version that patches this vulnerability is the most effective mitigation.

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