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Question 86

Question

What are the security implications of using eval() and new Function()?

Answer

Security Implications:

Both eval() and new Function() allow you to execute arbitrary code dynamically. This flexibility comes with a significant security risk if you don't handle it carefully:

  • Code Injection: The most serious danger is the possibility of code injection attacks. If an attacker can control the string passed to eval() or new Function(), they could inject malicious code into your application. This code could steal sensitive data, modify website content, take control of user accounts, or perform other harmful actions.

  • Unpredictable Behavior: Evaluating untrusted code can lead to unexpected behavior and vulnerabilities. Code might try to access private variables, modify system settings, or trigger unintended side effects that compromise your application's integrity.

Illustrative Example (Vulnerability):

Imagine a website with a feature where users can submit custom JavaScript snippets for display:

const userCode = document.getElementById("userInput").value;
eval(userCode); // Extremely dangerous!

If an attacker enters malicious code like document.cookie = "username=hacker";, it would be executed, potentially stealing the user's cookie data.

Mitigating Risks:

  • Avoid Use Whenever Possible: The best practice is to avoid eval() and new Function() altogether. Find alternative, safer ways to achieve your goal.

  • Sanitize Input: If you absolutely must use them, rigorously sanitize any user-provided input before passing it to these functions. This involves removing potentially harmful characters or code snippets. Libraries like DOMPurify can help with this process.

  • Isolate Execution: Consider running untrusted code in a sandboxed environment (e.g., using Web Workers) to limit its access to sensitive data and system resources.

Recommendations:

  • Prioritize writing your code without relying on dynamic code execution. Use built-in JavaScript features and libraries whenever possible.

  • If you encounter eval() or new Function() in existing code, carefully assess the context and consider refactoring it for security.

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Last updated 6 months ago