CVE-2024-55638: Drupal Core RCE Vulnerability

CVE-2024-55638 Overview

CVE-2024-55638 is a Deserialization of Untrusted Data vulnerability in Drupal Core that enables Object Injection attacks. The vulnerability exists due to the presence of a gadget chain within Drupal core—a sequence of methods that can be exploited when an insecure deserialization vulnerability exists on the site. While the gadget chain itself presents no direct threat, it serves as an attack vector that can be leveraged to achieve remote code execution if the application deserializes untrusted data due to another vulnerability.

This vulnerability is particularly concerning for Drupal installations running affected versions, as it lowers the barrier for attackers who discover or exploit separate deserialization flaws within the same application.

Critical Impact

This vulnerability enables attackers to potentially achieve remote code execution through PHP Object Injection when combined with other deserialization vulnerabilities, compromising the confidentiality, integrity, and availability of affected Drupal installations.

Affected Products

• Drupal Core versions 7.0 through 7.101
• Drupal Core versions 8.0.0 through 10.2.10
• Drupal Core versions 10.3.0 through 10.3.8

Discovery Timeline

• 2024-12-10 - CVE-2024-55638 published to NVD
• 2025-06-02 - Last updated in NVD database

Technical Details for CVE-2024-55638

Vulnerability Analysis

The vulnerability stems from Drupal Core containing a chain of exploitable methods known as a "gadget chain." In PHP applications, gadget chains are sequences of class methods that can be chained together during the deserialization process. When an attacker can control the serialized data being passed to PHP's unserialize() function, they can instantiate arbitrary objects and invoke magic methods such as __wakeup(), __destruct(), or __toString().

In the context of CVE-2024-55638, Drupal Core's codebase contains classes with methods that, when chained together during deserialization, can lead to dangerous operations including arbitrary code execution. The vulnerability relates to both CWE-502 (Deserialization of Untrusted Data) and CWE-915 (Improperly Controlled Modification of Dynamically-Determined Object Attributes).

Root Cause

The root cause lies in the existence of exploitable gadget chains within Drupal Core's PHP class structure. These chains exist due to the presence of magic methods in various classes that perform sensitive operations when invoked. While Drupal itself may not directly deserialize untrusted input in the affected code paths, the presence of these gadget chains creates a latent vulnerability that can be triggered if any other part of the application (including contributed modules or custom code) performs insecure deserialization.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction to exploit. An attacker would need to identify a deserialization sink within the target Drupal application—a location where user-controlled data is passed to unserialize(). Once identified, the attacker can craft a malicious serialized payload that leverages Drupal Core's gadget chain to achieve code execution.

The exploitation process typically follows this pattern:

1. The attacker identifies an insecure deserialization entry point in the target application
2. A malicious serialized PHP object payload is crafted using Drupal Core's gadget chain
3. The payload is delivered to the deserialization sink via network request
4. Upon deserialization, PHP instantiates the attacker-controlled objects
5. Magic methods are invoked, triggering the gadget chain
6. The chain culminates in arbitrary code execution on the server

For detailed technical information about the vulnerability mechanism and gadget chain specifics, refer to the Drupal Security Advisory SA-CORE-2024-008.

Detection Methods for CVE-2024-55638

Indicators of Compromise

• Unusual PHP serialized data appearing in web requests, logs, or database fields containing serialized object notation (e.g., O: prefix for objects)
• Unexpected outbound network connections from the web server following incoming requests
• Anomalous PHP process behavior including execution of system commands or file system modifications
• Web server error logs containing PHP unserialize warnings or errors with stack traces involving Drupal core classes

Detection Strategies

• Deploy Web Application Firewall (WAF) rules to detect and block serialized PHP object patterns in request parameters, cookies, and POST bodies
• Implement runtime application self-protection (RASP) solutions capable of monitoring PHP deserialization operations
• Enable verbose logging for PHP applications to capture deserialization attempts and associated stack traces
• Utilize SentinelOne's Singularity platform to detect post-exploitation behavior such as suspicious process spawning from PHP workers

Monitoring Recommendations

• Monitor web server access logs for requests containing encoded or serialized PHP objects
• Implement file integrity monitoring on critical Drupal core and configuration files
• Track process creation events originating from web server processes for signs of command execution
• Alert on any modifications to Drupal's code base or unexpected PHP file creations in web-accessible directories

How to Mitigate CVE-2024-55638

Immediate Actions Required

• Immediately upgrade Drupal Core to version 7.102, 10.2.11, or 10.3.9 or later depending on your installed branch
• Audit all custom and contributed modules for insecure uses of PHP's unserialize() function
• Implement input validation and sanitization for any data that may be passed to deserialization functions
• Consider disabling or removing unnecessary contributed modules until they can be audited for deserialization vulnerabilities

Patch Information

Drupal has released security updates that address this vulnerability by removing or modifying the exploitable gadget chain within Drupal Core. The patched versions are:

• Drupal 7.x: Upgrade to version 7.102 or later
• Drupal 10.2.x: Upgrade to version 10.2.11 or later
• Drupal 10.3.x: Upgrade to version 10.3.9 or later

For complete patch details and upgrade instructions, see the Drupal Security Advisory SA-CORE-2024-008.

Workarounds

• If immediate patching is not possible, implement strict Web Application Firewall rules to block serialized PHP object patterns in all request vectors
• Disable any functionality that accepts serialized data from untrusted sources until patches can be applied
• Employ network segmentation to limit the blast radius of a potential compromise
• Implement PHP configuration hardening using disable_functions directive to restrict dangerous functions that may be called through gadget chains

# Drupal upgrade commands
# For Composer-managed Drupal installations:
composer update drupal/core --with-dependencies
drush updatedb
drush cache:rebuild

# Verify the installed version after upgrade:
drush status | grep "Drupal version"