JavaScript Security: A Comprehensive Guide for 2025

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JavaScript, powering the vast majority of websites and web applications, is a crucial component of the modern internet. Its flexibility and versatility make it a popular choice for developers. However, this widespread adoption also makes it a prime target for malicious actors. This comprehensive guide provides a deep dive into JavaScript security, covering common vulnerabilities, secure coding practices, and essential tools to help you build robust and secure applications in 2025.

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While JavaScript boasts memory safety due to its garbage collection, its dynamic and loosely typed nature can introduce security risks if not handled carefully. Understanding these risks and implementing appropriate safeguards is paramount to protecting your users and your data. This guide will equip you with the knowledge and skills necessary to write secure JavaScript code.

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According to recent statistics, XSS (Cross-Site Scripting) and injection attacks remain among the most prevalent web application vulnerabilities, often exploiting weaknesses in JavaScript code. For example, a 2024 report by OWASP highlighted that XSS accounted for approximately 40% of reported web application vulnerabilities. This underscores the critical need for developers to prioritize security throughout the entire development lifecycle.

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This guide is designed for developers of all skill levels, from beginners to experienced professionals. We will cover fundamental concepts, practical examples, and advanced techniques to help you master JavaScript security. Let's begin!

Common Security Vulnerabilities in JavaScript

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Understanding common vulnerabilities is the first step towards building secure JavaScript applications. Here are some of the most prevalent threats:

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Vulnerability	Severity	Description	Example
Cross-Site Scripting (XSS)	High	Allows attackers to inject malicious scripts into websites viewed by other users. This can lead to session hijacking, data theft, and website defacement.	An attacker injects a script into a comment field that displays the script to all users who view the comment.
Cross-Site Request Forgery (CSRF)	Medium	Forces authenticated users to perform unintended actions on a web application.	An attacker embeds a malicious image tag in an email that, when viewed, triggers a request to change the user's password on a vulnerable website.
Injection Attacks (SQL, Command, etc.)	Critical	Exploits vulnerabilities in data handling to inject malicious code, potentially granting attackers unauthorized access to databases or system resources.	An attacker crafts a malicious SQL query through a vulnerable form field, allowing them to bypass authentication or extract sensitive data.
Denial of Service (DoS) / Distributed Denial of Service (DDoS)	High	Overwhelms a server with traffic, making it unavailable to legitimate users.	An attacker sends a large number of requests to a server, consuming its resources and preventing it from responding to legitimate requests.
Clickjacking	Medium	Tricks users into clicking on something different from what they perceive, potentially leading to unintended actions.	An attacker overlays a transparent iframe containing a malicious button over a legitimate button on a website, tricking users into clicking the malicious button.
Prototype Pollution	High	Allows attackers to modify the prototype of built-in JavaScript objects, potentially affecting the behavior of the entire application.	An attacker modifies the `Object.prototype` to inject malicious code that executes whenever a new object is created.
Open Redirects	Low to Medium	Redirects users to malicious websites, often used in phishing attacks.	A website uses a URL parameter to determine the redirect target, but doesn't properly validate the input, allowing an attacker to redirect users to a phishing site.

Built-in Security Features

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While JavaScript itself doesn't offer explicit built-in security features in the same way that some languages do, the browser environment and modern JavaScript engines provide several mechanisms that contribute to security:

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• Same-Origin Policy (SOP): A critical security mechanism that restricts scripts from one origin (domain, protocol, and port) from accessing resources from a different origin. This helps prevent malicious websites from accessing sensitive data from other websites.
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• Content Security Policy (CSP): An HTTP header that allows website administrators to control the resources that the user agent is allowed to load for a given page. CSP helps prevent XSS attacks by restricting the sources from which scripts can be loaded.
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• Subresource Integrity (SRI): A security feature that allows browsers to verify that files fetched from CDNs haven't been tampered with. SRI uses cryptographic hashes to ensure the integrity of the fetched resources.
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• Secure Contexts (HTTPS): Using HTTPS ensures that communication between the browser and the server is encrypted, preventing eavesdropping and man-in-the-middle attacks. Modern browsers often restrict access to certain features (e.g., geolocation, microphone) to secure contexts.
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• WebAssembly (Wasm) Security: While not strictly a JavaScript feature, WebAssembly provides a sandboxed execution environment that can enhance security by isolating code from the rest of the application.
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It's important to note that these features are not foolproof and require careful configuration and implementation to be effective. Developers should always be aware of the limitations of these features and implement additional security measures as needed.

Secure Coding Best Practices

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Adopting secure coding practices is essential for mitigating security risks in JavaScript applications. Here are some key practices to follow:

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Practice	Description	Example
Input Validation	Always validate and sanitize user input to prevent injection attacks and other vulnerabilities.	Use regular expressions to validate email addresses and other input fields. Sanitize user-provided HTML to prevent XSS.
Output Encoding	Encode output data to prevent XSS attacks. Use appropriate encoding methods based on the context (e.g., HTML encoding, URL encoding).	Use a library like `DOMPurify` to sanitize HTML output.
Principle of Least Privilege	Grant users only the minimum necessary permissions to perform their tasks.	Implement role-based access control (RBAC) to restrict access to sensitive resources.
Regular Security Audits	Conduct regular security audits to identify and address potential vulnerabilities.	Use automated security scanners and manual code reviews to identify security flaws.
Keep Dependencies Up-to-Date	Regularly update dependencies to patch security vulnerabilities.	Use tools like `npm audit` or `yarn audit` to identify and fix vulnerable dependencies.
Use Secure Libraries and Frameworks	Choose libraries and frameworks that have a strong security track record.	Use a framework like React or Angular, which provide built-in security features and follow secure coding practices.
Avoid eval() and Function()	Avoid using `eval()` and `Function()` to execute arbitrary code, as they can introduce security vulnerabilities.	Use alternative methods for dynamic code execution, such as template literals or pre-compiled functions.

Input Validation in JavaScript

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Input validation is a crucial security practice that involves verifying and sanitizing user-provided data before processing it. This helps prevent various attacks, including injection attacks, XSS, and data corruption.

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Why is Input Validation Important?

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• Prevents Injection Attacks: By validating input, you can prevent attackers from injecting malicious code into your application.
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• Protects Against XSS: Sanitizing input helps prevent attackers from injecting malicious scripts into your website.
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• Ensures Data Integrity: Validation ensures that the data you receive is in the expected format and range, preventing data corruption.
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Types of Input Validation:

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• Client-Side Validation: Performed in the browser using JavaScript. Provides immediate feedback to the user and improves the user experience. However, it should not be relied upon as the sole form of validation, as it can be bypassed.
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• Server-Side Validation: Performed on the server. Provides a more secure and reliable form of validation, as it cannot be bypassed by the user.
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Best Practices for Input Validation:

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• Validate All Input: Validate all user-provided data, including form fields, URL parameters, and cookies.
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• Use a Whitelist Approach: Define a list of allowed characters and formats, and reject any input that doesn't match.
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• Sanitize Input: Remove or escape potentially harmful characters from the input data.
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• Use Regular Expressions: Use regular expressions to validate complex input formats, such as email addresses and phone numbers.
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• Implement Both Client-Side and Server-Side Validation: Use client-side validation for immediate feedback and server-side validation for security.
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Example: Validating an Email Address

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\nfunction validateEmail(email) {\n  const regex = /^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/;\n  return regex.test(email);\n}\n\nif (validateEmail(userInput)) {\n  // Process the email address\n} else {\n  // Display an error message\n}\n

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Example: Sanitizing HTML Input

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\nconst clean = DOMPurify.sanitize(dirty);\ndocument.getElementById('output').innerHTML = clean;\n

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By implementing robust input validation, you can significantly reduce the risk of security vulnerabilities in your JavaScript applications.

Authentication & Authorization

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Authentication and authorization are fundamental security concepts that control access to resources and protect sensitive data. Authentication verifies the identity of a user, while authorization determines what resources the user is allowed to access.

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Authentication:

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• Purpose: To verify the identity of a user.
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• Methods: Common authentication methods include passwords, multi-factor authentication (MFA), and social login.
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• Best Practices:\n
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  • Use strong passwords and enforce password policies.
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  • Implement multi-factor authentication for enhanced security.
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  • Store passwords securely using hashing and salting.
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  • Protect against brute-force attacks by implementing rate limiting and account lockout mechanisms.
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Authorization:

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• Purpose: To determine what resources a user is allowed to access.
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• Methods: Common authorization methods include role-based access control (RBAC) and attribute-based access control (ABAC).
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• Best Practices:\n
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  • Implement role-based access control (RBAC) to restrict access to sensitive resources based on user roles.
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  • Use attribute-based access control (ABAC) for more fine-grained control over access permissions.
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  • Enforce the principle of least privilege, granting users only the minimum necessary permissions to perform their tasks.
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Authentication and Authorization in JavaScript:

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In JavaScript applications, authentication and authorization are typically handled on the server-side. However, client-side JavaScript can play a role in enhancing the user experience and providing additional security measures.

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Example: Implementing Role-Based Access Control (RBAC)

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\n// Example of role-based access control\nif (user.role === 'admin') {\n  // Allow access to admin features\n} else {\n  // Restrict access\n}\n

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Example: Using JSON Web Tokens (JWT) for Authentication

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\n// Example of verifying a JWT\nconst jwt = require('jsonwebtoken');\n\nconst token = req.headers.authorization.split(' ')[1];\n\njwt.verify(token, 'secret', (err, decoded) => {\n  if (err) {\n    // Token is invalid\n  } else {\n    // Token is valid\n    req.user = decoded;\n    next();\n  }\n});\n

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By implementing robust authentication and authorization mechanisms, you can protect your JavaScript applications from unauthorized access and data breaches.

Cryptography Best Practices

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Cryptography plays a vital role in securing JavaScript applications by protecting sensitive data and ensuring secure communication. However, using cryptography correctly is essential to avoid introducing vulnerabilities.

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Key Concepts:

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• Encryption: The process of converting data into an unreadable format to protect its confidentiality.
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• Hashing: The process of generating a fixed-size representation of data, used for verifying data integrity.
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• Digital Signatures: Used to verify the authenticity and integrity of data.
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Best Practices:

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• Use Established Cryptographic Libraries: Avoid implementing your own cryptographic algorithms. Use well-vetted and established libraries like `crypto-js` or `node-forge`.
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• Use Strong Encryption Algorithms: Choose strong encryption algorithms like AES-256 or ChaCha20.
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• Use Strong Hashing Algorithms: Choose strong hashing algorithms like SHA-256 or SHA-3.
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• Use Random Salt for Hashing: Always use a random salt when hashing passwords or other sensitive data.
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• Store Keys Securely: Protect cryptographic keys from unauthorized access.
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• Use HTTPS: Ensure that all communication between the browser and the server is encrypted using HTTPS.
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Cryptography in JavaScript:

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JavaScript provides built-in cryptographic functions through the `crypto` module in Node.js and the Web Crypto API in browsers. However, it's important to use these functions correctly and avoid common pitfalls.

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Example: Hashing a Password with Salt

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\nconst crypto = require('crypto');\n\nfunction hashPassword(password, salt) {\n  const hash = crypto.createHmac('sha256', salt);\n  hash.update(password);\n  return hash.digest('hex');\n}\n\nconst salt = crypto.randomBytes(16).toString('hex');\nconst hashedPassword = hashPassword('password123', salt);\n\nconsole.log('Salt:', salt);\nconsole.log('Hashed Password:', hashedPassword);\n

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Example: Encrypting Data

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\nconst crypto = require('crypto');\n\nfunction encrypt(text, key) {\n  const iv = crypto.randomBytes(16);\n  const cipher = crypto.createCipheriv('aes-256-cbc', Buffer.from(key), iv);\n  let encrypted = cipher.update(text);\n  encrypted = Buffer.concat([encrypted, cipher.final()]);\n  return { iv: iv.toString('hex'), encryptedData: encrypted.toString('hex') };\n}\n\nconst key = crypto.randomBytes(32);\nconst encryptedData = encrypt('Sensitive Data', key);\n\nconsole.log('Encrypted Data:', encryptedData);\n

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By following these best practices, you can effectively use cryptography to protect sensitive data in your JavaScript applications.

Managing Dependencies Securely

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JavaScript projects often rely on numerous third-party libraries and frameworks, known as dependencies. These dependencies can introduce security vulnerabilities if not managed carefully. A vulnerability in a single dependency can compromise the entire application.

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Why is Dependency Security Important?

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• Vulnerabilities in Dependencies: Third-party libraries can contain security vulnerabilities that can be exploited by attackers.
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• Transitive Dependencies: Dependencies can have their own dependencies (transitive dependencies), which can also introduce vulnerabilities.
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• Supply Chain Attacks: Attackers can compromise the supply chain by injecting malicious code into popular libraries.
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Best Practices for Managing Dependencies Securely:

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• Use a Package Manager: Use a package manager like npm or yarn to manage your dependencies.
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• Keep Dependencies Up-to-Date: Regularly update your dependencies to patch security vulnerabilities.
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• Use Security Scanners: Use security scanners like `npm audit` or `yarn audit` to identify vulnerable dependencies.
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• Use Dependency Locking: Use dependency locking to ensure that you're using the exact versions of your dependencies that you've tested.
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• Monitor Dependencies for Vulnerabilities: Monitor your dependencies for new vulnerabilities using tools like Snyk or Dependabot.
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• Use Subresource Integrity (SRI): Use SRI to verify the integrity of files fetched from CDNs.
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Example: Using npm Audit to Identify Vulnerable Dependencies

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\nnpm audit\n

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Example: Using npm Audit to Fix Vulnerable Dependencies

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\nnpm audit fix\n

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Example: Using Dependency Locking with npm

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\nnpm install --package-lock\n

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By following these best practices, you can effectively manage your dependencies and reduce the risk of security vulnerabilities in your JavaScript applications.

Security Tools & Scanners

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Leveraging security tools and scanners is crucial for identifying and mitigating vulnerabilities in JavaScript applications. These tools can automate the process of finding security flaws, allowing developers to address them proactively.

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Types of Security Tools:

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• Static Application Security Testing (SAST): Analyzes source code to identify potential vulnerabilities without executing the code.
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• Dynamic Application Security Testing (DAST): Analyzes running applications to identify vulnerabilities by simulating real-world attacks.
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• Software Composition Analysis (SCA): Identifies vulnerabilities in third-party libraries and dependencies.
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• Interactive Application Security Testing (IAST): Combines SAST and DAST techniques to provide more comprehensive security testing.
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Popular Security Tools for JavaScript:

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Tool Name	Type	Description
Snyk	SCA	Identifies and fixes vulnerabilities in dependencies.
OWASP ZAP	DAST	A free and open-source web application security scanner.
SonarQube	SAST	Analyzes code quality and identifies potential security vulnerabilities.
ESLint with Security Plugins	SAST	A JavaScript linter that can be configured with security plugins to identify common security flaws.
Retire.js	SCA	Detects the use of JavaScript libraries with known vulnerabilities.
Secably	SAST/DAST	A cloud-based security scanner that provides automated security testing for JavaScript applications.

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Example: Using ESLint with Security Plugins

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\nnpm install eslint eslint-plugin-security --save-dev\n

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Example: Running OWASP ZAP

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\nzap.sh -cmd -quickurl http://example.com -quickprogress\n

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By incorporating security tools and scanners into your development workflow, you can significantly improve the security posture of your JavaScript applications.

Frequently Asked Questions (FAQ)