CVE-2017-16088 in safe-eval
Summary
by MITRE
The safe-eval module describes itself as a safer version of eval. By accessing the object constructors, un-sanitized user input can access the entire standard library and effectively break out of the sandbox.
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Analysis
by VulDB Data Team • 02/15/2020
The safe-eval module vulnerability represents a critical sandbox escape flaw that undermines the fundamental security assumptions of JavaScript sandboxing mechanisms. This vulnerability exists within a module designed to provide a "safer" alternative to the native eval function, which typically poses significant security risks when handling untrusted input. The module's security model relies on restricting access to dangerous JavaScript features while maintaining functionality for legitimate use cases. However, the implementation contains a critical design flaw that allows attackers to bypass these restrictions through object constructor access patterns.
The technical exploitation occurs when un-sanitized user input is processed by the safe-eval module, enabling attackers to access object constructors and subsequently gain access to the entire standard library. This vulnerability stems from improper input validation and insufficient sandbox boundary enforcement, allowing malicious code to traverse the JavaScript execution environment and access privileged functions that should remain isolated. The flaw operates by leveraging prototype chains and constructor properties to escape the restricted execution context, effectively nullifying the sandbox protection mechanisms that the module claims to provide.
The operational impact of this vulnerability extends far beyond simple code execution, as it enables full system compromise through the exploitation of the standard library access. Attackers can leverage this vulnerability to perform arbitrary code execution, access sensitive data, modify system behavior, and potentially escalate privileges. The vulnerability affects any application that relies on the safe-eval module for processing untrusted input, making it particularly dangerous in web applications, server-side environments, and any context where user-provided data is evaluated dynamically. This type of vulnerability is classified under CWE-254 as a "Security Feature" weakness, specifically involving insufficient sandboxing mechanisms.
Mitigation strategies for this vulnerability require immediate remediation through module updates, input sanitization, and alternative security approaches. Organizations should implement comprehensive input validation, avoid using vulnerable modules altogether, and consider alternative approaches such as AST-based evaluation or proper sandboxing solutions that do not rely on constructor-based access restrictions. The vulnerability aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript" and represents a classic example of sandbox escape through prototype manipulation. Security teams must conduct thorough code reviews to identify all instances where the vulnerable module is used and implement proper security controls to prevent exploitation. The incident highlights the importance of rigorous security testing for sandboxing mechanisms and demonstrates how seemingly benign security modules can contain critical design flaws that compromise entire systems.