CVE-2020-13663: Drupal Core Form API CSRF Vulnerability

CVE-2020-13663 Overview

A Cross-Site Request Forgery (CSRF) vulnerability exists in Drupal Core's Form API that does not properly handle certain form input from cross-site requests. This security flaw can be exploited by attackers to trick authenticated users into unknowingly submitting malicious requests to the Drupal application, potentially leading to other downstream vulnerabilities including unauthorized actions being performed on behalf of legitimate users.

Critical Impact

Attackers can exploit this CSRF vulnerability to perform unauthorized actions on behalf of authenticated users, potentially leading to account compromise, data manipulation, and administrative access takeover without proper authorization.

Affected Products

• Drupal Core (multiple versions)
• Drupal CMS installations using the Form API
• Web applications built on vulnerable Drupal Core versions

Discovery Timeline

• 2021-06-11 - CVE-2020-13663 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-13663

Vulnerability Analysis

This vulnerability resides in Drupal Core's Form API, a critical component responsible for handling form submissions, validation, and processing throughout Drupal-based websites. The Form API fails to properly validate and sanitize certain form inputs when they originate from cross-site requests, creating a security gap that can be exploited through CSRF attacks.

The exploitation chain typically involves an attacker crafting a malicious webpage or email containing specially constructed form submissions. When an authenticated Drupal user visits the attacker-controlled page or clicks a malicious link, their browser automatically sends the forged request to the vulnerable Drupal site. Since the browser includes session cookies automatically, the Drupal server processes the request as if it came from the legitimate user.

What makes this vulnerability particularly concerning is its potential to chain with other vulnerabilities. The improper form input handling can lead to additional security issues beyond the initial CSRF, amplifying the attack surface and potential damage.

Root Cause

The root cause of CVE-2020-13663 lies in insufficient CSRF token validation within Drupal Core's Form API. The Form API does not properly verify that form submissions originate from legitimate user interactions on the same origin. This allows cross-origin requests to bypass security controls that should prevent unauthorized form submissions, enabling attackers to forge requests that appear to come from trusted users.

Attack Vector

The attack vector for this vulnerability is network-based and requires user interaction. An attacker must convince a victim who is authenticated to the target Drupal site to visit a malicious webpage or click a crafted link. The attack flow typically involves:

1. The attacker identifies a vulnerable Drupal installation and determines target forms to exploit
2. The attacker creates a malicious webpage containing hidden form elements or JavaScript that automatically submits requests to the target Drupal site
3. The victim, while authenticated to the Drupal site, is tricked into visiting the attacker's page
4. The malicious page triggers form submissions to the Drupal site, which are processed with the victim's session credentials
5. Unauthorized actions are executed on the Drupal site under the victim's identity

The vulnerability mechanism exploits the trust relationship between the Drupal server and authenticated user sessions. Without proper form token validation, the server cannot distinguish between legitimate user-initiated requests and forged cross-site requests. Refer to the Drupal Security Advisory for complete technical details.

Detection Methods for CVE-2020-13663

Indicators of Compromise

• Unexpected form submissions or administrative actions performed without user knowledge
• Unusual referrer headers in web server logs indicating requests originating from external domains
• Anomalous patterns of user account activity that users cannot recall initiating
• Evidence of configuration changes or content modifications without corresponding administrative sessions

Detection Strategies

• Monitor web application logs for form submissions with suspicious or missing referrer headers
• Implement Content Security Policy (CSP) headers and monitor for policy violations
• Deploy web application firewall (WAF) rules to detect and block potential CSRF attack patterns
• Review audit logs for actions performed by authenticated users that appear inconsistent with normal usage patterns

Monitoring Recommendations

• Enable detailed logging for all form submissions in Drupal, including referrer information and timestamp correlation
• Implement real-time alerting for administrative actions performed outside of expected time windows or from unusual IP addresses
• Monitor for sudden spikes in form submission rates which may indicate automated CSRF exploitation attempts
• Configure SentinelOne Singularity to monitor web server processes for anomalous request handling patterns

How to Mitigate CVE-2020-13663

Immediate Actions Required

• Update Drupal Core to the latest patched version immediately using drush updatedb or Composer update commands
• Review recent administrative actions and user account changes for signs of unauthorized activity
• Implement additional CSRF protection measures such as SameSite cookie attributes
• Enable Drupal's built-in security modules and ensure they are properly configured

Patch Information

Drupal has released security updates to address this vulnerability as documented in the Drupal Security Advisory SA-CORE-2020-004. Administrators should update to the latest stable release of their Drupal major version to receive the security fix. The patch implements proper form token validation to prevent cross-site request forgery attacks through the Form API.

Workarounds

• Implement Web Application Firewall (WAF) rules to block requests with suspicious referrer patterns until patching is complete
• Configure SameSite=Strict or SameSite=Lax attributes on session cookies to limit cross-site request capabilities
• Review and restrict administrative user sessions to known IP addresses where feasible
• Consider temporarily disabling vulnerable form functionality if immediate patching is not possible

# Configuration example - Drupal settings.php cookie configuration
# Add SameSite attribute to session cookies
$settings['session_cookie_samesite'] = 'Lax';

# Update Drupal Core using Composer
composer update drupal/core --with-dependencies

# Clear cache after updates
drush cache:rebuild