Apache mod_auth_openidc Flaw Lets Unauthenticated Users Access Protected Data

A critical flaw in Apache mod_auth_openidc (versions ≤2.4.16.10) allows unauthenticated attackers to bypass authentication and access protected resources.

The bug, CVE-2025-31492, patched in version 2.4.16.11, affects systems using OIDCProviderAuthRequestMethod POST without an application-level gateway or load balancer.

Technical Breakdown

The vulnerability stems from improper handling of authentication requests when the POST method is configured. Under specific conditions:

1. OIDCProviderAuthRequestMethod is set to POST.
2. Apache’s Require valid-user directive is active.
3. No intermediary gateway filters responses.

Attackers triggering a request to a protected resource receive a dual response:

• A self-submitting OpenID Connect (OIDC) form (intended for authentication).
• The protected content itself, appended after the form.

For example, a GET /foo/ request returns a 200 OK response containing both the OIDC login form and the restricted page (e.g., <h1>Protected page</h1>).

Sample Request and Response

Request:

textGET /foo/ HTTP/1.1
Accept: */*
Host: xxxxxxxxxxxxxxxxxxxxxxxx

Response:

HTTP/1.1 200 OK
Date: Wed, 09 Apr 2025 14:54:43 GMT
Server: Apache/2.4.63 (Unix) OpenSSL/3.0.2
Set-Cookie: mod_auth_openidc_state_Zjv-eHqSy08Do6CPJXYD-j_BJFk=eyJhbGciOiAiZGlyIiwgImVuYyI6ICJBMjU2R0NNIn0..DBQVvz1XSoTv7Pw0.d-DFmTTyBeu9nfGm0xaiJLBhsLSZLU4_PgpMwZi0-YmzzARn8sxjxuQc1yPiWMJ8Y0nCkyRP-VIn6VeOFNoHeKzIror1AMW5h1Wop0yky72x-o49Pc4SVKsF1T6p2jw8mZHow9VEC-HIaQilyzEBz5xoXp890KS5ih88NDj2nTulNOmQ56g_51osYx5N0sx-_i-EUsLNlxNgKXax37OckWtCzXCHT-TqYS5PJDoAQ6RAPGvgVnF48Nz9a0EN5aDhZfHQjIH46tjhca748A-Ft1LyMx3m3hkk3fU.fWYAzT6ukboFUu1EBUlKCg; Path=/; Secure; HttpOnly; SameSite=Lax
Content-Length: 1139
Content-Type: text/html

Most HTTP libraries discard malformed multipart data, but raw requests reveal the leaked content.

• Unauthorized Data Exposure: Sensitive pages or API endpoints become accessible without authentication.
• Stealthy Exploitation: The flaw is hard to detect in standard workflows, as many tools automatically discard malformed responses.
• Widespread Risk: Apache mod_auth_openidc is widely used for OAuth2/OpenID integration in enterprise environments.

Mitigation Steps

1. Immediate Patching: Upgrade to mod_auth_openidc ≥2.4.16.11.
2. Workarounds:
   • Switch to OIDCProviderAuthRequestMethod GET (default configuration).
   • Deploy a gateway or load balancer to sanitize responses.
3. Monitor Logs: Check for anomalous requests to protected endpoints.

Maintainer zandbelt addressed the issue by enhancing the oidc_content_handler to block unintended content appends.

Organizations are urged to act swiftly, as proof-of-concept exploits could emerge rapidly.

This vulnerability highlights the risks of misconfigured authentication workflows in critical infrastructure.

Administrators should prioritize patches and review OIDC configurations to prevent similar exposures.

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