CVE-2023-42790 - Understanding and Exploiting a Stack-Based Buffer Overflow in Fortinet FortiOS and FortiProxy

CVE-2023-42790 is a critical security vulnerability that affects various versions of Fortinet’s FortiOS and FortiProxy products. The flaw is a stack-based buffer overflow—one of the most dangerous types of bugs—that lets attackers execute unauthorized code or commands by sending specially-crafted HTTP requests. If you run one of the impacted Fortinet products, read on to understand how the bug works, how it can be exploited, and how to protect yourself.

2.. through 2..13

If you’re running one of these versions, you are at risk.

What Is a Stack-Based Buffer Overflow?

A stack-based buffer overflow happens when a program writes more data to a buffer located on the stack than it can hold. This extra data can overwrite crucial information like return addresses, letting an attacker redirect execution flow and run their own code.

In this case, the bug lies in how FortiOS and FortiProxy handle certain HTTP requests—specifically, malformed or specially crafted requests.

Fortinet Advisory:

https://www.fortiguard.com/psirt/FG-IR-23-301

NIST NVD Entry:

https://nvd.nist.gov/vuln/detail/CVE-2023-42790

Vulnerability Details

The exact function vulnerable to the overflow has not been publicly disclosed. Let’s summarize what is generally known:

Attackers can exploit the bug without authentication, remotely, over the network.

- The attack works by sending a specially crafted HTTP request—usually with an overflowing parameter in the URI, header, or POST body—to a vulnerable service.
- If successful, the attack lets the adversary execute arbitrary code with the privileges of the Fortinet process (which is typically highly privileged).

Example Exploit Code Snippet

While there’s no official proof-of-concept (PoC) code released by Fortinet for this bug, similar overflow exploits look like this (simplified for educational purposes):

import socket

VULN_HOST = "192..2.1"
VULN_PORT = 80

# Crafting an oversized HTTP header to trigger the overflow
overflow_payload = "A" * 500  # Adjust to trigger the overflow

http_request = (
    "GET / HTTP/1.1\r\n"
    f"Host: {VULN_HOST}\r\n"
    f"X-Custom-Header: {overflow_payload}\r\n"
    "Connection: close\r\n"
    "\r\n"
)

with socket.create_connection((VULN_HOST, VULN_PORT)) as sock:
    sock.sendall(http_request.encode())
    print("Overflow request sent!")

> Note: This is an illustrative example and may not directly exploit the CVE. The actual payload, vulnerable HTTP endpoint, and overflow point may vary. Real-world exploitation would require reverse engineering the Fortinet firmware or a detailed vulnerability writeup.

Exploit Impact

- Remote code execution: An attacker can run arbitrary code on the firewall, proxy, or VPN concentrator.
- Full device compromise: Since Fortinet processes run with high privileges, attackers could add accounts, exfiltrate data, capture traffic, or use your appliance to pivot to the rest of your network.
- Potential for worms: A remotely exploitable, pre-auth overflow could be weaponized to automatically spread between affected appliances.

Monitor Network Traffic.

Use IDS/IPS solutions to detect suspicious HTTP requests targeting your appliance.

Conclusion

CVE-2023-42790 is a serious risk to enterprises using Fortinet’s widely deployed firewalls, proxies, and VPN routers. Its remote, pre-auth nature makes it especially attractive to attackers. Patch now, restrict access, and keep an eye out for suspicious network activity.

For continuous updates, always check the FortiGuard PSIRT page and your vendor’s advisories.

Timeline

Published on: 03/12/2024 15:15:46 UTC
Last modified on: 03/19/2024 08:15:06 UTC