CVE-2023-22640 - Out-of-Bounds Write in Fortinet FortiOS and FortiProxy — Exploit Breakdown and Technical Walkthrough

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In early 2023, Fortinet published an advisory for CVE-2023-22640, a critical security flaw impacting multiple versions of FortiOS and FortiProxy. This out-of-bounds write vulnerability can let a logged-in attacker execute unauthorized code on the device just by sending specially crafted requests. If your organization runs any of the affected FortiOS or FortiProxy versions, this vulnerability could let a threat actor take full control of your perimeter security.

⚡ What Is CVE-2023-22640?

CVE-2023-22640 is a vulnerability classified as an “out-of-bounds write.” In short, it means attackers can overwrite parts of memory they shouldn't have access to. If exploited, they can force the device to run malicious code, change settings, or even brick the device altogether.

All versions of 2., 1.2, 1.1, 1.

Fixed in:

FortiProxy: 7.2.2, 7..8

Source: Fortinet advisory

Attackers can log in (even with basic user credentials)

Note: This is *not* a remote, unauthenticated attack. The attacker must have at least low-level credentials. But if they do, they can escalate privileges and run code as root/system.

Out-of-Bounds Write Explained

Imagine a small box with compartments for storing keys. The compartments are labeled -9. Out-of-bounds write is like stuffing a key into compartment 20—it doesn’t exist! You’re actually putting it somewhere you’re not allowed, possibly breaking the box, or letting you access stuff in the next room.

In FortiOS/FortiProxy, there’s a logic bug in code handling incoming web requests, especially in certain admin panels or user API calls. If a request contains a too-long or corrupted parameter, the software writes outside its intended memory—usually crashing the service, or sometimes executing bits of data as code.

Example: Simulated Exploit Skeleton (Python)

Below is a conceptual code snippet illustrating how an attacker might send a crafted request to trigger the bug.

import requests

# Replace with actual device IP and mapped vulnerable endpoint
url = "https://fortigate.example.com/login";

# Attacker logs in with valid credentials
session = requests.Session()
login_payload = {"username": "attacker", "password": "password"}
r = session.post(url, data=login_payload, verify=False)

# Now send a crafted request to vulnerable backend API
exploit_url = "https://fortigate.example.com/api/v2/monitor/user";
# Out-of-bounds parameter (oversized input triggers overwrite)
exploit_payload = {
    "parameter": "A" * 10000   # Very long input overflows mem buffer
}

r = session.post(exploit_url, json=exploit_payload, verify=False)

print(r.text)

> In reality, an attacker would need to reverse-engineer the exact vulnerable endpoint and required data structure. But essentially, this shows the attack flow: *Login → Send crafted payload*.

Open a shell for the attacker

- Let attacker upload/execute arbitrary code

Technical Deep Dive

Root Cause:  
Improper bounds-checking in the code handling user-supplied parameters in certain dashboards or monitoring APIs.

The logic may look (simplified C pseudocode)

#define BUF_SIZE 256

void handle_request(char *user_input) {
    char buf[BUF_SIZE];
    strcpy(buf, user_input); // No length check!
    // ...
}

🔗 References

- Fortinet PSIRT Official Advisory
- NVD - CVE-2023-22640
- FortiOS Upgrade and Hardening Guide
- Exploit Discussion Thread (Packet Storm)

FortiProxy: 7.2.2, 7..8

2. Review device access logs for strange login requests or huge POSTs.

Final Words

CVE-2023-22640 isn't just another minor bug—it enables authenticated attackers to turn your firewalls and proxies *against you*, running their code outside the lines. That's why patching is paramount. Even if you consider your network locked down, never underestimate what someone with basic login creds and the right exploit can do.

Stay secure—always patch early, audit accounts, and reduce remote access to just what's needed.

*This article was written exclusively for this context and is not available elsewhere. If you need help upgrading your Fortinet products or analyzing possible exploit traces, contact your security vendor or Fortinet support.*


Related reading:  
- How to Audit Your Fortinet Firewall for Security  
- Firmware Lifecycle Best Practices

Timeline

Published on: 05/03/2023 22:15:00 UTC
Last modified on: 05/10/2023 21:10:00 UTC