CVE-2024-51754: Twig Template Engine RCE Vulnerability

CVE-2024-51754 Overview

CVE-2024-51754 is a security policy bypass vulnerability in Twig, a popular template language for PHP. The vulnerability allows an attacker operating within a sandboxed Twig environment to invoke the __toString() method on objects even when this method is explicitly disallowed by the security policy. This bypass occurs when the target object is part of an array or included in an argument list, such as arguments passed to functions or filters.

Critical Impact

Attackers can bypass Twig sandbox security policies to execute restricted __toString() methods, potentially leading to information disclosure in sandboxed template environments.

Affected Products

• Twig versions prior to 3.11.2
• Twig versions 3.12.x through 3.14.0

Discovery Timeline

• 2024-11-06 - CVE CVE-2024-51754 published to NVD
• 2025-05-29 - Last updated in NVD database

Technical Details for CVE-2024-51754

Vulnerability Analysis

This vulnerability represents a sandbox escape condition classified under CWE-668 (Exposure of Resource to Wrong Sphere). The Twig templating engine provides a sandbox mode designed to restrict template authors from calling arbitrary methods on objects passed into templates. Security policies can be configured to explicitly deny access to specific methods, including PHP's magic __toString() method.

The flaw exists in how Twig handles object serialization when objects are embedded within arrays or passed as arguments to functions and filters. Under these conditions, PHP's implicit string conversion is triggered, causing __toString() to be invoked on the object. The sandbox security checks fail to intercept this implicit method call, allowing the disallowed method to execute despite the security policy configuration.

Root Cause

The root cause of this vulnerability lies in incomplete security policy enforcement within Twig's sandbox implementation. When objects are processed as part of composite data structures (arrays) or during argument resolution for function and filter calls, the sandbox fails to properly intercept implicit type coercion that triggers __toString() invocations. The security policy validation logic does not account for these implicit method calls that occur during PHP's automatic string conversion process.

Attack Vector

An attacker with the ability to create or modify Twig templates in a sandboxed environment can exploit this vulnerability by crafting templates that embed restricted objects within arrays or pass them as arguments to filters and functions. When these templates are rendered, the __toString() method is implicitly called on the objects during string conversion operations, bypassing the sandbox security policy. This is a network-accessible attack vector requiring high privileges and high attack complexity, as the attacker must have template authoring access to the vulnerable application.

The vulnerability allows information disclosure through the unauthorized execution of __toString() methods, which may return sensitive data that the security policy was designed to protect.

Detection Methods for CVE-2024-51754

Indicators of Compromise

• Unusual template execution patterns involving objects being passed within arrays to filters or functions
• Log entries showing sandbox security policy violations or unexpected template rendering behavior
• Application logs indicating __toString() method execution on objects that should be restricted

Detection Strategies

• Monitor Twig template rendering for objects being coerced to strings when embedded in arrays or argument lists
• Implement application-level logging to track method invocations on sandboxed objects
• Review templates for patterns that pass objects through filters or functions in potentially exploitable ways

Monitoring Recommendations

• Enable verbose logging for Twig sandbox security policy enforcement
• Audit template authoring activities and changes in environments using Twig sandboxes
• Implement alerting for any unexpected method invocations on objects within sandboxed template contexts

How to Mitigate CVE-2024-51754

Immediate Actions Required

• Upgrade Twig to version 3.11.2 or 3.14.1 immediately
• Audit existing templates for patterns that may have exploited this vulnerability
• Review objects passed to sandboxed templates to ensure __toString() methods do not expose sensitive information

Patch Information

The Twig development team has released patched versions that address this sandbox bypass vulnerability. Users should upgrade to Twig 3.11.2 or 3.14.1 to remediate this issue. The fix is available in commit 2bb8c2460a2c519c498df9b643d5277117155a73. For additional details, refer to the GitHub Security Advisory and the Debian LTS Announcement for distribution-specific updates.

Workarounds

• No known workarounds are available for this vulnerability according to the vendor advisory
• Upgrading to the patched versions is the only recommended remediation path
• Consider restricting template authoring privileges as a defense-in-depth measure until patching is complete