CVE-2023-3229 - Business Logic Errors in fossbilling/fossbilling Prior to .5.

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In May 2023, a security vulnerability, officially designated CVE-2023-3229, was disclosed in the fossbilling/fossbilling project. This long-lasting open-source billing platform for web hosts had what’s known as “business logic errors” – a fancy term for problems in how the app handles rules, actions, or permissions. In simple terms: attackers could exploit these oversights for unauthorized access or actions.

In this post, I’ll lay out how the vulnerability happens, what it looks like in code, and what you can do about it. Everything here is written in plain, easy-to-read American English. If you run, maintain, or use FOSS Billing (or similar software), this is must-know info.

What Is CVE-2023-3229?

CVE-2023-3229 refers to a set of business logic errors that existed in FOSSBilling up to version .5.. Business logic flaws aren’t programming bugs. Instead, they're mistakes in how an app's workflow or processes are designed—like letting users do actions they shouldn’t or bypassing checks intended to protect sensitive data.

Example: A customer being able to refund their own invoices, access someone else’s client area, or escalate privileges are classic business logic fails.

Where Did the Flaw Hide?

The flaw centered around missing or insufficient authorization checks in key controller actions. Let’s say the app is supposed to check “Is this user allowed to perform this task?” – but, in some cases, FOSSBilling just didn’t. Attackers could exploit these oversights using crafted requests.

Impact: Attackers could perform unauthorized tasks or access restricted data

- Root cause: Missing logic check and validation at the controller/action level

> Reference:
> GitHub Security Advisory – CVE-2023-3229
> NVD Listing

Here’s a simplified version of the kind of code that allowed the bug

// A vulnerable controller action in FOSSBilling before .5.

function refundInvoice($invoice_id) {
    // LACKS: Check if the user is the invoice owner or has permission!
    $invoice = getInvoiceById($invoice_id);
    $invoice->refund();

    return "Invoice refunded!";
}

What’s wrong?
There’s no check to see if the current user owns this invoice or has admin rights. Anyone who knows or guesses an invoice ID could refund it.

How Could Attackers Exploit This?

Let’s walk through an exploit scenario step-by-step.

1. Reconnaissance: The attacker creates a normal account and checks invoice IDs (often predictable, like #1001, #1002, etc).
2. Crafted Request: They send a manual POST request to /admin/invoice/refund with another user’s invoice ID.
3. No Proper Check: The vulnerable controller lacks a permission check – so it proceeds with the refund.
4. What Happens: The targeted user's invoice is refunded, possibly causing financial losses or disrupting services.

Example HTTP request that could exploit the logic

POST /admin/invoice/refund
Content-Type: application/json

{
    "invoice_id": 1002
}

If the backend doesn’t check authorization, this gets processed, and the refund runs.

In version .5. and later, checks are added before critical actions

function refundInvoice($invoice_id)
{
    $invoice = getInvoiceById($invoice_id);
    if ($invoice->user_id !== $_SESSION['user_id'] && !isAdmin()) {
        throw new Exception("Unauthorized");
    }
    $invoice->refund();

    return "Invoice refunded!";
}

Takeaway:
Every sensitive action must check that the user is permitted before going forward.

Mitigation — What Should You Do?

If you run FOSSBilling:

Upgrade ASAP to at least v.5.

- Download release here

For developers:

Summary

CVE-2023-3229 reminds us that business logic errors are as dangerous, if not more so, than regular software bugs. They can grant attackers huge power while appearing innocuous. If you’re using FOSSBilling prior to v.5., update right now.

If you build software, triple-check permissions before every sensitive action. Don’t trust user input, especially for things like IDs and actions that control money, data, or security. Automation and code reviews are your friends.

Timeline

Published on: 06/14/2023 06:15:00 UTC
Last modified on: 06/17/2023 01:42:00 UTC

CVE-2023-3229 - Business Logic Errors in fossbilling/fossbilling Prior to .5. – Exploit Details and Insights