The security world never stops spinning, and sometimes, a simple coding mistake can shake up router safety for millions. In this article, we’ll break down CVE-2023-39239—a critical vulnerability discovered in the popular ASUS RT-AX56U V2 router series. We’ll examine how it happens, the code involved, how attackers can exploit it, and what you can do to stay safe. If you own or manage one of these routers, keep reading.

What is CVE-2023-39239?

CVE-2023-39239 is a format string vulnerability in the General function API used by ASUS RT-AX56U V2 routers. If you don’t validate user input properly, attackers may insert malicious “format string” characters (like %s or %x)—causing unexpected behavior. In the worst case, this lets hackers run any code they want on your device without even needing a password.

The problem centers around the apply.cgi module, a component used to handle user requests—for configuration changes, for example.

In short:
> An unauthenticated attacker can remotely take over or disrupt an ASUS RT-AX56U V2 router by sending a specially crafted request.

Where’s the Problem in the Code?

ASUS routers often use web UIs built with CGI scripts. apply.cgi processes lots of admin tasks. Let’s focus on the area where things go wrong:

Suppose there's a snippet (simplified for clarity)

// Vulnerable function in apply.cgi
void General_function(char *input_value) {
    char buffer[256];
    // No input validation!
    sprintf(buffer, input_value); // Format string vulnerability!
    // ...send buffer to syslog, UI, or system()
}

If input_value comes straight from the network (say, from an HTTP POST) and isn't checked, attackers can send something malicious. For example:
POST /apply.cgi?function=General_function&input_value=%x%x%x%x

Crash or reboot the router at will

And since this is in a public API, authentication is NOT required.

The attacker finds your router is exposed to the internet (default ports, UPnP, etc.).

2. They send a crafted HTTP POST or GET request to /apply.cgi with a function parameter calling General_function, and insert dangerous format string characters in their input_value.

Example (proof of concept)

curl "http://router-ip/apply.cgi?function=General_function&input_value=%x%x%x%x";

If output includes internal memory data, the vulnerability exists. Using more advanced format strings (%n), skilled hackers can even write specific bytes to specific addresses and hijack execution.

Original CVE Listing:

NVD - CVE-2023-39239

Firmware Vulnerability Note:

ASUS Security Advisory: CVE-2023-39239

Format String Attack Primer:

OWASP: Format string attack

What Should I Do?

- IF YOU OWN THIS ROUTER: Update firmware immediately. Asus frequently pushes security updates—check for updates here.

Conclusion

CVE-2023-39239 is a classic reminder: Simple coding mistakes like unvalidated format strings are still dangerous—even in 2024. If you manage an ASUS RT-AX56U V2, patch and harden now. An open port is all some attackers need.

If you’re curious about the underlying issue or want more technical details, be sure to check the references above. And don’t hesitate to nudge your friends, family, or IT admin to read this post—a secure network is a shared responsibility.

Timeline

Published on: 09/07/2023 08:15:00 UTC
Last modified on: 09/12/2023 20:47:00 UTC