CVE-2019-9942 in Twig
Summary
by MITRE
A sandbox information disclosure exists in Twig before 1.38.0 and 2.x before 2.7.0 because, under some circumstances, it is possible to call the __toString() method on an object even if not allowed by the security policy in place.
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Analysis
by VulDB Data Team • 08/04/2023
The vulnerability described in CVE-2019-9942 represents a critical sandbox escape mechanism within the Twig templating engine that affects versions prior to 1.38.0 and 2.x versions before 2.7.0. This issue stems from a fundamental flaw in how Twig handles object serialization and method invocation within its security sandbox environment, creating a pathway for unauthorized information disclosure. The vulnerability operates by exploiting the security policy enforcement mechanisms that are designed to prevent direct access to sensitive objects and methods, yet allows for indirect exploitation through the __toString() magic method invocation.
The technical root cause of this vulnerability lies in the improper handling of object method calls within Twig's sandboxed execution environment. When Twig processes templates, it maintains a security policy that restricts access to certain object methods and properties to prevent malicious code execution. However, under specific circumstances, the security policy fails to properly prevent the invocation of the __toString() method on objects that should be restricted. This occurs because the sandbox implementation does not adequately validate whether calling __toString() on a restricted object would expose sensitive information or bypass security boundaries. The flaw essentially allows attackers to bypass the intended security restrictions by leveraging the __toString() method as a vector for information disclosure.
The operational impact of this vulnerability is significant as it enables attackers to extract sensitive information from objects that should remain protected within the sandboxed environment. This information disclosure can potentially include internal object state, memory addresses, or other confidential data that would normally be inaccessible to template developers or end users. Attackers can construct malicious templates that trigger the __toString() method on restricted objects, thereby exposing information that could aid in further exploitation attempts. The vulnerability particularly affects applications that use Twig for rendering user-provided content, as it allows attackers to craft templates that reveal internal system information, configuration details, or other sensitive data that should remain hidden.
This vulnerability aligns with CWE-200, which addresses "Information Exposure," and specifically relates to improper restriction of information flow within sandboxed environments. The issue also maps to ATT&CK technique T1059.001, "Command and Scripting Interpreter: PowerShell," and T1068, "Exploitation for Privilege Escalation," as it enables attackers to gather information that can be used for further exploitation. The flaw demonstrates a classic sandbox escape pattern where an attacker identifies a method that bypasses security controls, creating an information disclosure pathway that can be leveraged for more sophisticated attacks.
The recommended mitigations for CVE-2019-9942 include immediate upgrade to Twig versions 1.38.0 or 2.7.0, which contain the necessary security fixes to properly enforce sandbox restrictions. Organizations should also implement comprehensive security reviews of their template rendering processes, particularly focusing on how user-provided content is handled and validated before being processed by Twig. Additional protective measures include implementing strict input validation for template content, monitoring for unusual object method calls within template processing, and conducting regular security assessments of templating engine configurations. Security teams should also consider implementing network segmentation and access controls to limit potential impact if exploitation occurs, while ensuring that all applications using Twig are regularly updated to maintain current security patches.