D-Link is a well-known brand in the SOHO (Small Office/Home Office) networking equipment market, and their DIR-868L router has seen widespread use. However, in June 2023, a critical buffer overflow vulnerability—CVE-2023-39668—was discovered in its firmware (revA_1-12_eu_multi_20170316). This vulnerability is especially important to understand for anyone still running this device, as D-Link has marked it "UNSUPPORTED WHEN ASSIGNED." This means it is *not* getting patched, and end users should be aware of the risks.

This post aims to explain CVE-2023-39668 in simple terms, show how it works, and talk about how an attacker might use it.

Vulnerability Details

The vulnerability exists in the firmware's internal networking code. Specifically, the bug affects the usage of the inet_ntoa() function, which turns an IP address into a readable string. The root cause is that a developer did not check if data going into a buffer via the param_2 parameter would fit, leading to a stack-based buffer overflow.

Vulnerability: Stack buffer overflow in inet_ntoa() through param_2

- CVE: CVE-2023-39668

The function looks roughly like this

struct in_addr {
    unsigned long s_addr;
};

char *inet_ntoa(struct in_addr in) {
    // buffer not properly sized
    static char buffer[16];
    int param_2 = ... // <<== input comes from user/process
    
    // Overflow risk: param_2 might be too large for buffer
    snprintf(buffer, sizeof(buffer), "%d.%d.%d.%d",
        (param_2      ) & xFF,
        (param_2 >>  8) & xFF,
        (param_2 >> 16) & xFF,
        (param_2 >> 24) & xFF
    );
    return buffer;
}

If param_2 comes from user input (directly or indirectly), a skilled attacker can input data that's too large and overwrite memory nearby—the classic stack buffer overflow.

Hijack router settings or traffic

Since D-Link marked the product as unsupported, attackers might target this bug for years—there will be no firmware fix.

Proof of Concept (PoC): Example Attack

Disclaimer: This code is for educational purposes only.

Let's say the param_2 variable can be influenced by a user—for instance, via an HTTP request parameter or a device configuration.

Here’s a minimal Python example of what an attacker might do if the vulnerable firmware exposed param_2 through a web interface (URL for illustration only):

import requests

# Attacker crafts a GET request with a large value for param_2
evil_param_2 = "4294967295" # That's xFFFFFFFF, as large as it goes!
url = f"http://192.168..1/vulnerable_endpoint?param_2={evil_param_2}";

response = requests.get(url)
print(response.text)

If the firmware does not check this value, an attacker could send this request and overflow the buffer. A more advanced attacker could use specially crafted data to overwrite saved return addresses on the stack, allowing them to run any code they want.

The router’s vulnerable process crashes and restarts (*denial of service*), OR

- If the attacker is skilled/persistent, uses payload to get a shell on the device.

🛑 Once an attacker has code execution, your entire network could be at risk!

References (Read More)

- CVE-2023-39668 NVD Details
- Exploit Database – Search for D-Link DIR-868L
- Security Research on Buffer Overflows
- D-Link Security Advisory Archive

Conclusion

CVE-2023-39668 is a textbook example of how a small coding mistake—forgetting to check buffer sizes—can lead to large vulnerabilities in consumer devices. Because D-Link will not patch this router, best practice is to immediately replace or quarantine affected devices. If your network uses a DIR-868L with the vulnerable firmware, act now to avoid potential attacks.

*Stay safe—if you have questions about replacing SOHO gear, leave a reply below!*

Timeline

Published on: 08/18/2023 03:15:00 UTC
Last modified on: 08/23/2023 19:10:00 UTC