CVE-2025-31492: mod_auth_openidc Info Disclosure Bug

CVE-2025-31492 Overview

CVE-2025-31492 is a high-severity information disclosure vulnerability in mod_auth_openidc, an OpenID Certified authentication and authorization module for the Apache 2.x HTTP server. This vulnerability allows unauthenticated users to access protected content under specific configuration conditions, bypassing the intended authentication mechanisms implemented by the OpenID Connect Relying Party functionality.

The vulnerability stems from improper handling of authentication responses when the module is configured to use OIDCProviderAuthRequestMethod POST. When these conditions are met, the Apache HTTP server inadvertently appends protected content to authentication responses, exposing sensitive data to unauthorized users.

Critical Impact

Unauthenticated attackers can access protected resources without valid credentials, potentially exposing sensitive application data, user information, or internal system details to unauthorized parties.

Affected Products

• mod_auth_openidc versions prior to 2.4.16.11
• Apache 2.x HTTP servers using vulnerable mod_auth_openidc configurations
• Systems configured with OIDCProviderAuthRequestMethod POST without application-level gateway protection

Discovery Timeline

• 2025-04-06 - CVE-2025-31492 published to NVD
• 2025-04-17 - Last updated in NVD database

Technical Details for CVE-2025-31492

Vulnerability Analysis

This vulnerability represents an information exposure flaw (CWE-200) that occurs due to a logic error in how mod_auth_openidc handles authentication form responses. The flaw specifically manifests when three conditions are met: the OIDCProviderAuthRequestMethod is set to POST, a valid account exists in the system, and no application-level gateway or load balancer is protecting the server.

Under normal operation, when a user requests a protected resource without authentication, mod_auth_openidc should return a self-submitting HTML form that redirects the user to the OpenID Connect provider for authentication. However, due to the vulnerability, the response includes not only the authentication form but also the protected content itself, completely bypassing the authentication requirement.

Root Cause

The root cause lies in the interaction between mod_auth_openidc and the Apache HTTP server's request handling pipeline. When returning an authentication form, the module must return OK from the check_userid function to avoid triggering Apache's error handling path. However, this OK status signals to Apache that the request is authorized to proceed.

Subsequently, when oidc_content_handler is invoked, it performs several checks to determine whether it should intervene and prevent normal content output. The handler lacks a specific check for the authentication form response scenario, causing it to return DECLINED. This signals Apache to continue processing, resulting in the protected content being appended to the authentication response.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by simply requesting protected resources on an affected server. The attack conditions require:

1. The target Apache server runs a vulnerable version of mod_auth_openidc (prior to 2.4.16.11)
2. The server is configured with OIDCProviderAuthRequestMethod POST
3. At least one valid user account exists in the system
4. No application-level gateway, WAF, or load balancer filters the responses

When an unauthenticated attacker requests a protected resource, the server response contains the HTTP status, headers, the self-submitting authentication form, and critically, the protected resource content without proper header separation. This allows the attacker to extract the protected content directly from the response body.

Detection Methods for CVE-2025-31492

Indicators of Compromise

• HTTP responses from Apache servers containing both OpenID Connect authentication forms and additional content appended after the form
• Unusual response sizes for authentication redirect pages that exceed expected form payload sizes
• Log entries showing access to protected resources without corresponding successful authentication events
• Client requests to protected endpoints that receive content without subsequent authentication flow completion

Detection Strategies

• Implement response body analysis rules to detect authentication forms followed by unexpected content
• Monitor for HTTP responses where content-length exceeds typical OpenID Connect form sizes
• Deploy web application firewall rules to inspect responses for authentication form patterns combined with sensitive data patterns
• Enable detailed Apache access logging and correlate protected resource requests with authentication events

Monitoring Recommendations

• Enable verbose logging for mod_auth_openidc to capture authentication flow anomalies
• Configure SIEM rules to alert on protected resource access without preceding authentication success events
• Implement network traffic analysis to detect responses with mixed authentication and content payloads
• Regularly audit Apache server configurations for vulnerable OIDCProviderAuthRequestMethod POST settings

How to Mitigate CVE-2025-31492

Immediate Actions Required

• Upgrade mod_auth_openidc to version 2.4.16.11 or later immediately
• Temporarily change OIDCProviderAuthRequestMethod from POST to GET as an interim workaround if patching is not immediately possible
• Deploy an application-level gateway or WAF in front of affected servers to filter malformed responses
• Audit access logs for evidence of potential exploitation prior to patching

Patch Information

The vulnerability has been addressed in mod_auth_openidc versions 2.4.16.11 and later. The fix is available through the official GitHub commit and detailed in the GitHub Security Advisory GHSA-59jp-rwph-878r. Debian users should also reference the Debian LTS Announcement for distribution-specific patching guidance.

Organizations should prioritize updating the module through their package manager or by compiling from the latest source. The patch modifies oidc_content_handler to properly detect and handle the authentication form response scenario, preventing the protected content from being appended to the response.

Workarounds

• Change the authentication request method configuration from POST to GET by modifying OIDCProviderAuthRequestMethod GET in the Apache configuration
• Deploy a reverse proxy or application-level gateway that inspects and filters response bodies before they reach clients
• Implement network segmentation to limit exposure of affected Apache servers to trusted networks only
• Disable access to protected resources until the patch can be applied if the risk is unacceptable

# Configuration workaround - change to GET method
# In Apache configuration file (e.g., /etc/apache2/mods-enabled/auth_openidc.conf)
# Modify or add the following directive:
OIDCProviderAuthRequestMethod GET

# Restart Apache to apply changes
sudo systemctl restart apache2