CVE-2020-13664: Drupal Core RCE Vulnerability

CVE-2020-13664 Overview

CVE-2020-13664 is an arbitrary PHP code execution vulnerability affecting Drupal Core. An attacker could trick an administrator into visiting a malicious site that could result in creating a carefully named directory on the file system. With this directory in place, an attacker could attempt to brute force a remote code execution vulnerability. Windows servers are most likely to be affected by this vulnerability due to file system handling characteristics.

Critical Impact

This vulnerability enables remote code execution through a multi-stage attack combining social engineering with file system manipulation, potentially allowing complete server compromise on affected Drupal installations.

Affected Products

• Drupal Core 8.8.x versions prior to 8.8.8
• Drupal Core 8.9.x versions prior to 8.9.1
• Drupal Core 9.0.x versions prior to 9.0.1

Discovery Timeline

• 2021-05-05 - CVE-2020-13664 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-13664

Vulnerability Analysis

This vulnerability is classified as CWE-77 (Command Injection), where arbitrary PHP code execution becomes possible through a carefully orchestrated attack sequence. The attack requires user interaction—specifically, a Drupal administrator must be tricked into visiting a malicious website. This social engineering component is reflected in the attack characteristics, which require user interaction but no special privileges.

The vulnerability primarily affects Windows-based Drupal installations due to differences in how Windows handles file system operations compared to Unix-like systems. The attack succeeds by exploiting Drupal's file handling mechanisms to create specially crafted directories that can later be leveraged to achieve code execution.

Root Cause

The root cause lies in insufficient validation and sanitization of file system operations within Drupal Core. When an administrator is deceived into visiting a malicious site, the attacker can manipulate the application to create a directory with a carefully chosen name on the target server's file system. This directory placement sets the stage for subsequent brute force attempts against the remote code execution vulnerability. The weakness indicates improper neutralization of special elements used in commands, allowing attackers to inject and execute arbitrary PHP code.

Attack Vector

The attack unfolds in multiple stages through the network attack vector:

1. Social Engineering Phase: The attacker first identifies a Drupal administrator and crafts a malicious website designed to exploit this vulnerability.

2. Directory Creation: When the administrator visits the malicious site, the attack triggers the creation of a specifically named directory on the Drupal server's file system through Cross-Site Request Forgery or similar techniques.

3. Brute Force Exploitation: With the malicious directory in place, the attacker can then attempt to brute force the remote code execution vulnerability, leveraging the created directory structure to execute arbitrary PHP code.

4. Code Execution: Successful exploitation results in arbitrary PHP code execution on the target server with the privileges of the web server process.

The attack requires network access and user interaction, but no authentication is needed from the attacker's perspective. The impact is severe, potentially compromising confidentiality, integrity, and availability of the affected system.

Detection Methods for CVE-2020-13664

Indicators of Compromise

• Unexpected or anomalously named directories appearing in Drupal's file system, particularly in writable locations such as sites/default/files
• Administrator access logs showing visits to suspicious external URLs followed by file system modifications
• PHP execution from unusual directory paths not typically associated with Drupal module or theme operations
• Web server logs indicating brute force attempts targeting PHP files in non-standard locations

Detection Strategies

• Implement file integrity monitoring (FIM) to detect unauthorized directory creation within the Drupal installation path
• Monitor web server logs for suspicious patterns of administrator activity followed by file system changes
• Deploy web application firewall (WAF) rules to detect and block CSRF-style attacks targeting Drupal administrators
• Enable Drupal's watchdog logging and monitor for unusual file operation events

Monitoring Recommendations

• Configure real-time alerting for any new directory creation within Drupal's document root on Windows servers
• Establish baseline behavior for administrator accounts and alert on deviations such as unexpected file operations
• Monitor outbound connections from the Drupal server that may indicate successful compromise
• Review PHP error logs for execution attempts from non-standard directory paths

How to Mitigate CVE-2020-13664

Immediate Actions Required

• Update Drupal Core immediately to version 8.8.8, 8.9.1, or 9.0.1 depending on your current major version
• Audit the Drupal file system for any suspicious or unexplained directories that may have been created through exploitation
• Review administrator access logs to identify any potential compromise attempts
• Consider temporarily restricting administrator access to trusted IP addresses until patching is complete

Patch Information

Drupal has released security updates to address this vulnerability. The official patch information and upgrade instructions are available in the Drupal Security Advisory SA-CORE-2020-005. Organizations should upgrade to the following patched versions:

• Drupal 8.8.x users should upgrade to 8.8.8 or later
• Drupal 8.9.x users should upgrade to 8.9.1 or later
• Drupal 9.0.x users should upgrade to 9.0.1 or later

Workarounds

• Implement strict Content Security Policy (CSP) headers to limit the effectiveness of CSRF-style attacks
• Deploy a Web Application Firewall (WAF) with rules to detect and block malicious requests targeting file system operations
• Restrict administrator browsing activities by using separate browser profiles or virtual machines for Drupal administration
• Enable two-factor authentication for all Drupal administrator accounts to add an additional security layer
• On Windows servers, consider applying restrictive NTFS permissions on Drupal's writable directories to limit unauthorized directory creation

# Drupal update via Composer (recommended method)
composer update drupal/core --with-dependencies

# Verify current Drupal version after update
drush status | grep "Drupal version"

# Clear Drupal caches after update
drush cache-rebuild