CVE-2023-27997 - Understanding the Fortinet SSL-VPN Heap Overflow and Its Real-World Exploitation

In the world of network security, even the best products can harbor dangerous vulnerabilities. In this long read, we'll dive into CVE-2023-27997—a heap-based buffer overflow affecting Fortinet's FortiOS and FortiProxy SSL-VPN components. We'll explain, step by step and in simple terms, what the vulnerability is, how it works, which products and versions are affected, and how attackers could exploit this security hole to take control of systems.

You’ll find exclusive insights, actual code snippets, official references, and a full picture for defending your network.

What Is CVE-2023-27997?

CVE-2023-27997 is a heap-based buffer overflow, categorized under CWE-122. Heap-based buffer overflows occur when a program writes more data to a buffer allocated on the heap than it can hold. This can cause adjacent memory to become overwritten, opening the door for attackers to inject malicious code or commands.

1.1 (all versions)

The vulnerability is present in the SSL-VPN component, which is enabled on many FortiGate firewalls by default.

Official Security Bulletin

Fortinet PSIRT Advisory FG-IR-23-097

How Does the Vulnerability Work?

The unsafe buffer handling exists within the way SSL-VPN parses inbound requests. When an attacker sends specially crafted data, the software incorrectly processes it and writes past the end of a heap buffer. This could allow the remote attacker to execute arbitrary code—meaning, they can potentially run any command on your firewall with the same privileges as the vulnerable process.

Here's a simple breakdown

1. Attacker Connects: They make a connection to the SSL-VPN web portal on the target FortiOS/FortiProxy device.
2. Sends Malicious Payload: The attacker sends crafted request data (for example, within a specific HTTP header or parameter).
3. Memory Overwritten: The vulnerable code allocates a buffer and fails to check the length of the data before copying it, so the memory next to the buffer is overwritten.
4. Code Execution: If the attacker designs their payload carefully, they can control the overwritten memory to redirect the program’s flow—making the device run the attacker's code.

Let's visualize a simplified vulnerable C code pattern (not Fortinet code, for illustration)

void vulnerable_function(char *input) {
    char *buf = malloc(256);
    strcpy(buf, input); // No length check!
    // ... process request
    free(buf);
}

If input is larger than 256 bytes, memory gets corrupted.

Scan for FortiOS or FortiProxy SSL-VPN web interfaces exposed to the internet.

- For each target, send a specially crafted HTTPS request, containing payloads designed to trigger the overflow.
- Each payload is tuned to the version, memory layout, and platform—the attacker needs to overcome ASLR, DEP, and possibly race conditions.
- If successful, they get control of execution flow and can, for example, spawn a remote shell, create an admin account, or pivot further into the protected network.

A generalized (pseudocode) Python exploit could look like

import requests

# Target FortiGate SSL-VPN
target = "https://vpn.example.com/remote/login";

# Payload triggers buffer overflow (real payloads will be binary, version-specific)
overflow_payload = "A" * 300 + "\xde\xad\xbe\xef"  # Overflows buffer, overwrites metadata

headers = {
    "User-Agent": "Mozilla/5.",
    # The bug may live in specific headers or POST arguments
}

data = {
    "username": overflow_payload,
    "password": "irrelevant"
}

requests.post(target, data=data, headers=headers, verify=False)

Disclaimer: This is a simplified illustration—a real attack is more complex, and publishing real weaponized exploits is unethical.

Public Exploitations

Rapid7, Shadowserver, and CISA have all reported mass scanning and exploitation in the wild. Breaches using this bug are known to have resulted in:

Unusual VPN sessions from foreign IP addresses

You can check your FortiGate logs for suspicious login attempts or anomalies.

How to Protect Your Network

1. Patch Immediately
Fortinet has released fixed FortiOS and FortiProxy versions. Download and apply updates from the Fortinet Support Portal.

2. Disable SSL-VPN if Unneeded

If you don’t use SSL-VPN, turn it off to remove the attack surface.

3. Block Access

Restrict SSL-VPN access to trusted IP addresses only.

4. Monitor for Threats

Use intrusion detection, logs, and network monitoring for signs of exploitation.

Conclusion

CVE-2023-27997 is a serious, internet-facing vulnerability with proof-of-concept code in the wild and active exploitation observed. If your organization uses Fortinet SSL-VPN, patch immediately. Don’t underestimate buffer overflows—they remain a weapon of choice for attackers even in modern software.

Remember: Always prioritize patching, monitor your perimeter, and stay alert for advisories on security-critical products.

Timeline

Published on: 06/13/2023 09:15:00 UTC
Last modified on: 06/20/2023 19:33:00 UTC