Vulnerability in Apache mod_auth_openidc Module: Could Allow Unauthenticated Users to Access Protected Content
A serious security vulnerability has been discovered in the mod_auth_openidc module for OpenID Connect authentication and authorization in the Apache HTTP server. This issue could allow unauthenticated users to gain access to protected web resources.
The vulnerability, named CVE-2025-31492, has a CVSSv4 score of 8.2 and affects widely used OpenID Connect authentication systems, requiring immediate action from system administrators.
The issue in the Apache mod_auth_openidc module allows unauthenticated users to access content hidden behind authentication protocols. The vulnerability exists in versions prior to 2.4.16.11, particularly in systems configured with the OIDCProviderAuthRequestMethod POST authentication policy and Require valid-user. In this case, if an application-level gateway (such as a reverse proxy or load balancer) is not installed on the server, unauthenticated users could gain access to protected web resources.
According to the official security advisory from OpenIDC, this bug occurs when a request is sent to the server without requiring user authentication. As a result, the Apache server provides not only the authentication form but also the actual protected content, effectively nullifying the authentication protection.
The vulnerability arises from an error in the content processing system of the mod_auth_openidc module. In improperly configured systems, when an unauthenticated user requests a protected resource, the server sends a multipart response, which includes not only the authentication form but also the actual protected content. In this case, the oidc_content_handler function malfunctions, and the server presents protected content to the user.
During the system’s operation, when the check_userid function returns OK, the Apache server attempts to present the protected resource, leading to security issues since the user can view the content without any problems.
Several measures are recommended to mitigate the vulnerability in the Apache mod_auth_openidc module:
- Install the update: The issue was fixed in version 2.4.16.11 of the mod_auth_openidc module. Systems should be updated to this version.
- Change the authentication method: Changing the OIDCProviderAuthRequestMethod parameter to GET will prevent the vulnerability from being triggered and enhance security.
- Use a gateway: Installing an application-level gateway (such as a reverse proxy or load balancer) will help hide the protected content from unauthenticated users.
Security experts emphasize that installing an application-level gateway or reverse proxy is an effective way to address this issue.
The CVE-2025-31492 vulnerability in the Apache mod_auth_openidc module could allow unauthenticated users to access protected web resources, which poses a serious threat. This issue can affect all Apache servers, so system administrators need to immediately install updates, verify authentication methods, and implement security measures.
