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:
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- OIDCProviderAuthRequestMethod is set to POST.
- Apache’s Require valid-user directive is active.
- 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/htmlMost 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
- Immediate Patching: Upgrade to mod_auth_openidc ≥2.4.16.11.
- Workarounds:
- Switch to OIDCProviderAuthRequestMethod GET (default configuration).
- Deploy a gateway or load balancer to sanitize responses.
- 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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