Security vulnerabilities can lurk in unexpected places, sometimes even in the control panels that protect our networks. CVE-2023-38999 is a recent and serious flaw found in OPNsense, a popular open-source firewall platform, that opens the door to a quick—and potentially disastrous—Denial of Service (DoS) attack. In this post, we’ll break down what went wrong, show code snippets demonstrating the vulnerability, and explain how attackers could take advantage of it.
What Is CVE-2023-38999?
In a nutshell, CVE-2023-38999 is a Cross-Site Request Forgery (CSRF) vulnerability affecting the System Halt API (/system/halt) of OPNsense before version 23.7. What does that mean for you? An attacker could trick an administrator into shutting down the firewall—remotely, instantly, and with just a *single link click*.
A Wildly Dangerous Oversight
In OPNsense, the System Halt API is supposed to allow secure shutdowns of the firewall hardware. But before version 23.7, this endpoint was not protected against CSRF attacks. That means a crafted request from a malicious website or email could trigger a shutdown without any confirmation or authentication beyond the admin’s active session.
Technical Summary
- Component affected: OPNsense System Halt API endpoint (/system/halt)
Exploit Details: How an Attacker Can Shut Down Your Firewall
Let’s step through how an attacker could exploit this flaw.
Step 1: Lure an Admin to a Malicious Website
If an OPNsense admin is logged in, visiting a compromised or malicious web page (even via an email link) could be enough to trigger the exploit.
Step 2: Crafted GET Request
The vulnerable endpoint processes simple GET requests. That means JavaScript or even a plain HTML tag could exploit it!
Code Snippet: HTML CSRF PoC
<!-- Malicious code to instantly halt the firewall -->
<img src="https://<opnsense-ip>/system/halt"; style="display:none" />
If an admin (already logged into OPNsense) loads this image (perhaps hidden in tracking pixels or banners), the browser will send the GET request, and the firewall halts.
JavaScript PoC
fetch("https://<opnsense-ip>/system/halt";)
.then(() => console.log("Firewall halted!"))
.catch(() => {});
Any method that causes the admin’s browser to request /system/halt can be used.
Why Is This a Big Deal?
The OPNsense firewall is often the only line of defense for many organizations. Shutting it down means all routing, filtering, and VPN connections are lost. If executed in a targeted attack, it could:
Has It Been Fixed?
Absolutely. OPNsense patched the vulnerability in version 23.7 by adding CSRF protection (usually via verifying unique tokens for POST requests).
Patch Reference:
Vulnerability Report:
What Should You Do?
If you’re running OPNsense:
Upgrade to version 23.7 or later as soon as possible. Avoid logging into your firewall from your regular web-browsing session. Use unique browsers or private/incognito mode for management.
Update Instructions:
OPNsense Upgrade Guide
And remember: If your firewall allows shutting down with a simple URL click, you’re one bad link away from disaster.
Summary
CVE-2023-38999 is a prime example of why even the most boring-sounding bugs—like CSRF flaws—can be a huge deal in network infrastructure. With a single GET request, an attacker could take down your OPNsense firewall, leaving you and your users vulnerable.
Stay patched. Stay vigilant. Don’t let an old-school web bug quietly cause a massive outage!
References
- NVD - CVE-2023-38999
- OPNsense 23.7 Changelog
- CSRF Basics - OWASP
Timeline
Published on: 08/09/2023 19:15:00 UTC
Last modified on: 08/15/2023 15:08:00 UTC