CVE-2024-21762 - Exploiting Out-of-Bounds Write in Fortinet FortiOS and FortiProxy (A Deep Dive)

Fortinet is a major player in network security, with their FortiOS and FortiProxy products protecting networks all over the world. In February 2024, one of the most severe and actively exploited vulnerabilities in the series, CVE-2024-21762, shook the cyber world. If you’re running Fortinet gear, this isn’t some far-off risk: it could mean real trouble.

This writeup will break down CVE-2024-21762, explain how the vulnerability works, show you what exploitation might look like, cover what you can do about it, and link to all the official sources.

What is CVE-2024-21762?

CVE-2024-21762 is an out-of-bounds write vulnerability in both FortiOS and FortiProxy. Basically, it lets an attacker write data to parts of memory they’re not supposed to. This trick can enable attackers to execute their own code, totally compromise the firewall, or pivot further into your network.

1.. through 1..7

Reference:
- Fortinet Advisory (FG-IR-24-015)
- NVD - CVE-2024-21762

How Does an Out-of-Bounds Write Work?

In technical terms, an out-of-bounds write happens when a program writes data outside of the bounds of a buffer — an allocated area in memory. When this happens, attackers can overwrite important parts of the application’s memory, often placing *malicious code* in just the right spot for the CPU to run it. That’s Remote Code Execution (RCE).

Let’s look at a simplified C code example to understand

#include <stdio.h>
#include <string.h>

void vulnerable_function(char *input) {
    char buffer[128];
    // No size check!
    strcpy(buffer, input);
    printf("Buffer: %s\n", buffer);
}

If an attacker sends more than 128 bytes, they can overwrite the memory next to buffer. In the real exploit, by crafting the “right” kind of request, attackers hijack FortiOS or FortiProxy.

Exploitation Details

The real-world exploit for CVE-2024-21762 happens through the FortiOS and FortiProxy web interfaces, especially the SSL VPN interface. Attackers can send malicious HTTP or HTTPS requests to the devices, trigger the out-of-bounds write, and inject code.

No authentication is needed.

- Attackers just need the web interface open to the internet. Shodan showed over 100,000 Fortinet devices exposed when this hit the news!

Example Snippet of a Hypothetical Exploit Request

*Note: For responsible disclosure, this is a simplified, non-working example for illustration only.*

import requests

target = "https://victim-firewall.example.com";
malicious_payload = "A" * 300  # 300 bytes, overflowing the buffer

# The vulnerable endpoint (example)
url = f"{target}/remote/login"

data = {
    "username": malicious_payload,  # oversize input triggers the bug
    "password": "irrelevant"
}

r = requests.post(url, data=data, verify=False)
print(r.status_code)

If the target is vulnerable, the attacker could get the firewall to execute malicious code, crash, or let the attacker get a shell.

NOTE: Real exploit code was quickly posted on dark web and eventually in public research, so patching is critical.

Immediately

- Update to the latest FortiOS / FortiProxy versions. Fortinet released patches in early February 2024 that fix this bug.
- Fortinet's Patch Instructions
- If patching takes time, disable remote management interfaces (especially SSL VPN) from the internet.

Conclusion & References

CVE-2024-21762 is a critical vulnerability that needs your attention if you use Fortinet firewalls or proxies. Exploitation is easy, public, and widespread. Don't delay — patch and get ahead of attackers.

Timeline

Published on: 02/09/2024 09:15:08 UTC
Last modified on: 02/10/2024 02:00:01 UTC