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

CVE-2020-13663 Overview

CVE-2020-13663 is a Cross-Site Request Forgery (CSRF) vulnerability in the Drupal Core Form API. The Form API does not properly handle certain form input originating from cross-site requests. An attacker who tricks an authenticated user into visiting a malicious page can submit forms on the user's behalf, potentially chaining into other vulnerabilities within the Drupal application.

The weakness maps to CWE-352: Cross-Site Request Forgery and affects Drupal Core installations. Drupal published advisory SA-CORE-2020-004 addressing the issue.

Critical Impact

Successful exploitation allows attackers to perform privileged form submissions against a victim's session, leading to potential compromise of confidentiality, integrity, and availability of the Drupal site.

Affected Products

• Drupal Core 7.x prior to the SA-CORE-2020-004 patched release
• Drupal Core 8.8.x prior to the SA-CORE-2020-004 patched release
• Drupal Core 8.9.x and 9.0.x prior to the SA-CORE-2020-004 patched releases

Discovery Timeline

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

Technical Details for CVE-2020-13663

Vulnerability Analysis

The Drupal Form API is the framework used to define, render, and process HTML forms across Drupal modules. It relies on form tokens and origin checks to ensure that submitted requests come from legitimate user-initiated sessions. CVE-2020-13663 stems from improper handling of specific form input when submissions arrive from cross-origin requests.

The flaw weakens the CSRF protections that the Form API is expected to enforce. An attacker can craft a malicious page that triggers a state-changing form submission against a Drupal site where the victim holds an active session. Because the Form API does not correctly validate the cross-site origin of the input, the server may process the request as if it were intentional.

Exploitation requires user interaction, such as visiting an attacker-controlled page while authenticated to the target Drupal site. The vulnerability can be chained with other module-specific weaknesses to escalate impact beyond simple state changes.

Root Cause

The root cause lies in Form API input handling that fails to reject or sanitize specific form parameters supplied through cross-site requests. The validation logic does not consistently bind form processing to the expected anti-CSRF token flow for all input paths. This gap allows specially constructed form submissions to bypass the cross-site origin checks the API is designed to enforce.

Attack Vector

The attack vector is network-based and requires the victim to be authenticated to a vulnerable Drupal site. The attacker hosts a page containing an auto-submitting form or scripted request targeting a Drupal Form API endpoint. When the authenticated victim loads the page, the browser issues the cross-site request carrying the user's session cookies. The Drupal site then processes the action under the victim's identity. See the Drupal Security Advisory SA-CORE-2020-004 for technical details.

Detection Methods for CVE-2020-13663

Indicators of Compromise

• Unexpected form submissions or content changes performed by privileged users without corresponding browser navigation in access logs.
• Cross-origin Referer or Origin headers on POST requests to Drupal form endpoints such as /user/edit, /admin/*, or module-specific form paths.
• Spikes in failed or anomalous form token validation entries within Drupal watchdog logs.

Detection Strategies

• Audit Drupal watchdog and webserver access logs for state-changing POST requests that lack a same-origin Referer header.
• Correlate authenticated session activity with off-site referrers immediately preceding privileged form submissions.
• Run the Drupal drush pm:security or status report page to confirm Core version is at or above the SA-CORE-2020-004 patched release.

Monitoring Recommendations

• Forward Drupal application logs and web server access logs to a centralized SIEM for correlation against authenticated user sessions.
• Alert on POST requests to administrative routes that originate from external referrers or carry mismatched Origin headers.
• Monitor for new or modified administrative accounts, role grants, and content changes outside of expected change windows.

How to Mitigate CVE-2020-13663

Immediate Actions Required

• Upgrade Drupal Core to the patched releases referenced in SA-CORE-2020-004 for the 7.x, 8.8.x, 8.9.x, and 9.0.x branches.
• Invalidate active sessions for privileged users after patching to force re-authentication.
• Review audit logs for suspicious privileged actions performed during the exposure window and revert unauthorized changes.

Patch Information

Drupal addressed CVE-2020-13663 in the Core releases announced in SA-CORE-2020-004. Administrators should apply the latest Core update for their installed branch and verify the version reported on /admin/reports/status matches or exceeds the fixed release.

Workarounds

• Restrict access to administrative paths using web server or reverse proxy rules limiting source IP ranges where feasible.
• Enforce a strict SameSite=Lax or SameSite=Strict attribute on Drupal session cookies to reduce cross-site submission risk.
• Require re-authentication for sensitive administrative operations and shorten session lifetimes for privileged accounts.

# Update Drupal Core via Composer to apply SA-CORE-2020-004
composer update drupal/core --with-dependencies
drush updatedb -y
drush cache:rebuild
drush status | grep "Drupal version"