CVE-2025-31691
📋 TL;DR
This vulnerability allows attackers to bypass authorization checks in Drupal OAuth2 Server through forceful browsing, potentially accessing restricted functionality. It affects all Drupal sites using the OAuth2 Server module. The vulnerability exists in all versions before 2.1.0.
💻 Affected Systems
- Drupal OAuth2 Server
📦 What is this software?
Oauth2 Server by Oauth2 Server Project
⚠️ Risk & Real-World Impact
Worst Case
Attackers could gain unauthorized access to protected OAuth2 endpoints, potentially compromising user authentication flows, stealing tokens, or accessing administrative functionality.
Likely Case
Unauthorized access to OAuth2 server endpoints leading to token manipulation, privilege escalation, or data exposure.
If Mitigated
With proper network segmentation and access controls, impact would be limited to the OAuth2 server component only.
🎯 Exploit Status
Forceful browsing attacks typically require minimal technical skill once endpoint URLs are discovered.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.1.0
Vendor Advisory: https://www.drupal.org/sa-contrib-2025-020
Restart Required: No
Instructions:
1. Update the OAuth2 Server module to version 2.1.0 via Drupal's update manager or Composer. 2. Clear Drupal caches. 3. Verify the update was successful.
🔧 Temporary Workarounds
Disable OAuth2 Server Module
allTemporarily disable the vulnerable module until patching is possible
drush pm:disable oauth2_server
Web Server Access Controls
allImplement access restrictions at the web server level for OAuth2 endpoints
🧯 If You Can't Patch
- Implement strict network access controls to limit access to OAuth2 endpoints
- Enable detailed logging for all OAuth2 Server module activity and monitor for unauthorized access attempts
🔍 How to Verify
Check if Vulnerable:
Check the installed version of the OAuth2 Server module in Drupal's Extend page or via drush: drush pm:list | grep oauth2_serverCheck Version:
drush pm:list --fields=name,version | grep oauth2_serverVerify Fix Applied:
Confirm OAuth2 Server module version is 2.1.0 or higher and test authorization controls on OAuth2 endpoints📡 Detection & Monitoring
Log Indicators:
- Unauthorized access attempts to /oauth2/* endpoints
- Failed authorization checks in Drupal logs
- Unusual OAuth2 token generation patterns
Network Indicators:
- Direct requests to OAuth2 endpoints without proper authentication headers
- Unusual traffic patterns to authorization endpoints
SIEM Query:
source="drupal" AND (uri_path="/oauth2/*" AND (response_status="403" OR response_status="200" with missing auth headers))