Executive Summary

The CVE-2026-5021 vulnerability affects the Tenda F453 router, version V1.0.0.3, and is a stack-based buffer overflow in the httpd service.

Specifically, the overflow occurs in the fromPPTPUserSetting function, which processes a user-supplied parameter named "delno." When "delno" is not empty, its value is assigned to a variable s_1, which is then passed to another variable s_3.

The variable s_3 is subsequently used as an argument in a call to the sprintf function without any bounds checking. This lack of length validation allows an attacker to overflow the stack buffer s_1.

Exploitation can be achieved by sending a crafted HTTP POST request to the endpoint /goform/PPTPUserSetting with an excessively long "delno" parameter, causing the overflow.

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Impact Analysis

Successful exploitation of this vulnerability can lead to denial of service (DoS) or remote code execution (RCE) on the affected device.

This means an attacker could crash the router or potentially take control of it remotely, which could disrupt network connectivity or allow unauthorized access.

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Detection Guidance

This vulnerability can be detected by monitoring for unusual or malicious HTTP POST requests sent to the endpoint /goform/PPTPUserSetting on Tenda F453 routers running version 1.0.0.3.

Specifically, detection involves looking for POST requests containing an excessively long or malformed "delno" parameter, which is used to trigger the stack-based buffer overflow.

A practical approach is to capture and inspect HTTP traffic targeting the router, focusing on requests to /goform/PPTPUserSetting.

• Use network packet capture tools like tcpdump or Wireshark to filter HTTP POST requests to /goform/PPTPUserSetting.
• Example tcpdump command to capture relevant traffic: tcpdump -i <interface> -A 'tcp port 80 and (((ip dst <router_ip>) and (tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x504f5354)))'
• Use grep or similar tools to search captured HTTP POST data for the "delno" parameter with unusually long values.
• Example command to test the vulnerability manually: send a crafted HTTP POST request with a long "delno" parameter to /goform/PPTPUserSetting using curl or a similar tool.
• Example curl command: curl -X POST http://<router_ip>/goform/PPTPUserSetting -d "delno=$(python3 -c 'print("a"*1000)')"

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Mitigation Strategies

Immediate mitigation steps include restricting access to the vulnerable endpoint and preventing exploitation attempts.

• Block or filter incoming HTTP POST requests to /goform/PPTPUserSetting on the affected Tenda F453 router from untrusted networks.
• Limit network exposure of the router’s management interface by placing it behind a firewall or restricting access to trusted IP addresses only.
• Monitor logs and network traffic for suspicious requests containing long or malformed "delno" parameters.
• If possible, update the router firmware to a version that patches this vulnerability once available from the vendor.

Until a patch is applied, avoid exposing the router’s management interface to the internet and consider disabling PPTP user settings if not needed.

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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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