CVE-2023-42789 - How a Fortinet Out-of-Bounds Write Leads to Remote Code Execution

Fortinet devices are widely used to secure enterprise networks, making them attractive targets for attackers. In late 2023, a critical vulnerability (CVE-2023-42789) was disclosed that could allow attackers to execute unauthorized code remotely on several FortiOS and FortiProxy versions through crafted HTTP requests. In this exclusive long-read post, we’ll break down what this vulnerability is, how it works, and how attackers can exploit it—with code snippets and all the main facts.

What Is CVE-2023-42789?

CVE-2023-42789 is an out-of-bounds write vulnerability that impacts specific versions of Fortinet’s FortiOS and FortiProxy products. The flaw allows an attacker with network access to the device web interface to write data outside the bounds of allocated memory. This can corrupt memory structures and, in certain cases, lead to remote code execution (RCE).

2.. through 2..13

Official advisory:
https://www.fortiguard.com/psirt/FG-IR-23-300

In Simple Terms: What’s an Out-of-Bounds Write?

A program keeps pieces of data in memory. If it tries to write data past the end of one of these pieces (“out-of-bounds”), it can mess up other things stored just after it—sometimes even letting someone take full control of the program. In Fortinet’s case, the vulnerable code didn’t check the length of certain HTTP request parameters before copying them into fixed-length buffers.

1. The Weak Point—Handling HTTP Requests

Research suggests the vulnerability lies in how FortiOS and FortiProxy handle certain HTTP requests sent to their web interfaces. If an attacker sends a specially crafted request—using, for example, an oversized parameter—the program copies the malicious data into memory without checking if it fits.

Example (Pseudocode)

#define BUF_SIZE 128
char buffer[BUF_SIZE];

void handle_request(char *user_input) {
    // DANGEROUS: No length checking!
    strcpy(buffer, user_input);
}

If user_input longer than 128 bytes is sent, the buffer overflows, overwriting neighboring memory.

Attackers can use tools like Python’s requests or curl to send an evil HTTP POST request

import requests

target = "https://target-fortigate-ip/";
payload = "A" * 300  # Sending more data than the buffer can handle

# The actual parameters depend on the real vulnerable web path and parameter name.
response = requests.post(
    target + "some_vulnerable_path",
    data={"parameter": payload},
    verify=False
)

print(response.status_code)

*In a real-world exploit, the payload would not just be "A"*300, but carefully constructed shellcode or a return-oriented programming (ROP) chain to execute code on the device.*

3. From Memory Corruption to Code Execution

If the overwritten data includes pointers or control data, the attacker can control the execution flow. “Return addresses” or “function pointers” could be targeted, redirecting the device to execute code the attacker supplied as part of the overflow data.

Find a target

Locate a vulnerable FortiGate or FortiProxy instance with a web interface accessible over the Internet or LAN.

Send crafted HTTP

Dispatch specially crafted HTTP POST requests exploiting the out-of-bounds write, overwriting memory.

Achieve Execution

If the overflow is successful and code execution gained, launch further commands—like dropping web shells, opening reverse shells, or pivoting into the network.

Real-World Example: CVE-2023-42789 Proof-of-Concept Snippet

Here is how a basic test might look. (Warning: Educational purposes only! Don’t attempt this on systems you don’t control.)

import requests

url = "https://victim-firewall-ip/some_endpoint";
headers = {"Content-Type": "application/x-www-form-urlencoded"}

# Overly long input, may trigger overflow
data = {
    "vulnerable_param": "A" * 300  # Replace with the correct parameter and payload
}

# Ignore SSL warnings for demonstration (DO NOT use verify=False in production)
res = requests.post(url, headers=headers, data=data, verify=False)

if res.status_code == 500:
    print("Potentially vulnerable endpoint!")
else:
    print("Response code:", res.status_code)

Note:
A real working exploit would require precise knowledge of the target’s memory layout and state, so attackers would use debugging and fuzzing to fine-tune their attack.

Mitigation

If you use Fortinet gear, update ASAP!
- Patch FortiOS and FortiProxy to the fixed versions noted in the Fortinet advisory.

References

- Fortinet PSIRT Advisory: FG-IR-23-300
- NIST NVD CVE-2023-42789 entry
- Fortinet Out-of-Bounds Write vulnerabilities explained (community post)

Conclusion

CVE-2023-42789 shows how a simple programming mistake—a missing length check—can have huge consequences, turning a firewall into a launchpad for attacks. If you run affected Fortinet products, patch them now, and review access settings. Attackers know about this flaw, and so should you.

Stay safe!

*By [YourName], security educator and network defender. For the exclusive long-read content, follow this series for more practical breakdowns of real-world vulnerabilities.*

Timeline

Published on: 03/12/2024 15:15:46 UTC
Last modified on: 03/15/2024 15:22:17 UTC