CVE-2023-25610 - Buffer Underwrite Exploit in Fortinet FortiOS and FortiProxy – A Deep Dive

Date of Publication: March 2024
Author: [Your Name or Handle]
Disclaimer: This post is for educational purposes only and should not be used for malicious activities.

Executive Summary

A critical buffer underwrite (also known as buffer underflow) vulnerability, tracked as CVE-2023-25610, has been discovered in several versions of Fortinet’s FortiOS, FortiProxy, and FortiOS-6K7K. An unauthenticated remote attacker could exploit this flaw to run arbitrary code or commands on network devices running affected firmware—a scenario enabling full network compromise. Considering Fortinet devices are widely deployed for enterprise security, this vulnerability presents a significant risk for businesses worldwide.

What Is CVE-2023-25610?

CVE-2023-25610 is a security flaw within the administrative web interface of multiple Fortinet products, including FortiOS, FortiProxy, and FortiOS-6K7K. The issue is due to improper validation when handling crafted HTTP requests, which leads to a buffer underwrite condition. This allows an attacker to write data before the start of a buffer in the device's memory, ultimately letting them gain control of the network device.

Vulnerability Type: Buffer Underwrite (Buffer Underflow)
Attack Vector: Remote, unauthenticated (no login required)
Impact: Remote code execution as root/admin privileges

FortiOS-6K7K: 7..5, 6.4.–6.4.10, 6.2.–6.2.10 and below

You can see the full Fortinet advisory here.

Understanding Buffer Underwrite

A buffer underwrite, also known as a buffer underflow, occurs when the pointer used for writing data falls before the allocated memory buffer. That can make the software write data into unexpected memory areas, corrupting structures or even giving an attacker control over code execution.

Example in C (Simplified)

void vulnerable_func(char* input) {
    char buffer[100];
    if (strlen(input) < 100) {
        memcpy(buffer - 10, input, strlen(input));  // buffer underwrite!
    }
}

In the above code, if an attacker triggers this path, they could influence what's written outside the intended buffer, potentially altering the program's control flow.

Send a Crafted Malicious Request:

By crafting special HTTP requests with malformed fields, the attacker tweaks the handling so that data is written before the legitimate buffer location.

Gain Code Execution:

By controlling what's written in memory, and depending on device internals, the attacker can hijack execution flow, uploading their own shellcode or running arbitrary commands. In practice, this leads to remote takeover.

Python Proof-of-Concept (PoC)

Here’s an illustrative (and safe) snippet showing how such an attack might begin—from the point of view of the attacker’s machine:

import requests

target = "https://fortigate-vulnerable-device:443";  # admin web interface

# Malformed field to trigger the underwrite (details are hypothetical)
payload = "A" * 1024  # oversized payload, real exploit would be custom tailored

headers = {
    "User-Agent": "exploit-test"
}
# In actual exploits, the endpoint/path and payload are chosen to hit the vulnerable code paths
r = requests.post(f"{target}/login", data=payload, headers=headers, verify=False)

print(f"HTTP Status: {r.status_code}")
print("Did the device respond abnormally?")

> 🔴 WARNING: Actual exploitation involves deeply understanding the target’s internal memory layout and in-the-wild exploits differ. The above isn’t a weaponized exploit, but demonstrates how a bug like this could be abused.

Other Recommendations

- Restrict admin interface access: Only allow trusted IPs/subnets using local firewall policies.

Monitor logs: For strange or repeated admin login attempts.

- Disable HTTP/HTTPS admin access where unneeded.

References

- Fortinet Security Advisory (Official)
- NIST NVD Entry for CVE-2023-25610
- Horizon3.ai Analysis “Abusing CVE-2023-25610”
- Rapid7 Exploit Blog

Final Words

CVE-2023-25610 stands out as a "perfect storm" vulnerability—remote, unauthenticated, trivial to attempt, and deeply damaging if exploited. If you run any Fortinet products, patch now, and review your network’s exposure. This bug is being actively scanned for and exploited in the wild as of early 2024.

Stay safe. Secure your perimeter. Upgrade your firmware.

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Timeline

Published on: 03/24/2025 16:15:17 UTC