A severe security vulnerability in Apache’s mod_auth_openidc module has been disclosed, potentially exposing sensitive web content to unauthorized users.
Security researchers have identified a flaw that allows unauthenticated users to access protected resources when specific configuration settings are in place.
Vulnerability Details
The vulnerability, tracked as CVE-2025-31492 and assigned a CVSS score of 8.2, affects mod_auth_openidc versions 2.4.16.10 and earlier.
This widely-used module implements OpenID Connect authentication functionality for Apache HTTP server and is deployed across numerous web applications requiring secure authentication.
OpenIDC published an advisory (GHSA-59jp-rwph-878r) on April 6, 2025, detailing how the bug in mod_auth_openidc results in disclosure of protected content to unauthenticated users.
Technical Analysis
The vulnerability manifests when the following specific conditions are met:
- OIDCProviderAuthRequestMethod is set to POST
- Authentication policy is configured with “Require valid-user”
- No application-level gateway (reverse proxy or load balancer) is protecting the server
Under these conditions, when a user attempts to access a protected resource, the server response inadvertently includes not only the authentication form but also the protected content itself.
This occurs because the module’s oidc_content_handler function fails to properly check this specific request pattern and returns DECLINED, causing Apache to append the protected content to the response.
As the security advisory explains: “When you request a protected resource, the response includes the HTTP status, the HTTP headers, the intended response (the self-submitting form), and the protected resource (with no headers)”.
Detection Challenges
What makes this vulnerability particularly concerning is that it can be difficult to detect through standard testing methods.
Most HTTP libraries automatically handle malformed responses by silently dropping invalid data and creating new connections, masking the problem from end users and security tools.
“It is hard to notice the error if you’re using an HTTP library to do requests – the invalid data will be silently dropped, a new connection will be created, and the end-user remains none the wiser,” notes the advisory.
Risk Assessment Table
| Aspect | Details |
|---|---|
| Vulnerability | OIDCProviderAuthRequestMethod POST leaks protected data |
| Severity | High |
| Affected Versions | = 2.4.16.11 |
| Disclosure Date | April 6, 2025 |
| Impact | Disclosure of protected content to unauthenticated users |
| Attack Vector | Network-based, requires specific configuration |
| Detection | Difficult (standard HTTP clients hide the issue) |
Remediation Options
Organizations using mod_auth_openidc are advised to take immediate action through one of the following approaches:
- Patch: Upgrade to mod_auth_openidc version 2.4.16.11 or later, which addresses the vulnerability.
- Configuration Change: Switch to OIDCProviderAuthRequestMethod GET (the default setting) instead of POST to avoid triggering the bug.
- Add Gateway Protection: Deploy an application-level gateway or reverse proxy that will prevent the leakage of unauthorized content.
Security experts recommend implementing these mitigations immediately, especially for systems handling sensitive information.
The developer Zandbelt implemented a solution to the problem by upgrading the oidc_content_handler component, preventing unwanted content from being automatically appended.
The relatively simple exploit conditions and potential for unauthorized access to protected resources make this a priority for any organization using Apache with mod_auth_openidc.
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