**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Security Best Practices in Express.js **Topic:** Common security vulnerabilities (XSS, CSRF, SQL Injection) **Introduction:** As a developer, it's crucial to understand common web security vulnerabilities and how to protect your application against them. In this topic, we'll explore three common security vulnerabilities: Cross-Site Scripting (XSS), Cross-Site Request Forgery (CSRF), and SQL Injection. **Cross-Site Scripting (XSS):** Cross-Site Scripting (XSS) is a type of web application security vulnerability that allows attackers to inject malicious code, usually in the form of JavaScript, into a web page. This malicious code is executed by the victim's browser, allowing the attacker to steal sensitive information or perform unauthorized actions. **Types of XSS attacks:** 1. **Stored XSS:** The malicious code is stored on the server, and when a user requests the webpage, the code is executed. 2. **Reflected XSS:** The malicious code is not stored on the server but is instead injected through a request (e.g., through a URL or form input). 3. **DOM-based XSS:** The malicious code is executed on the client-side, and the server is not involved. **Example:** Suppose we have a simple Express.js route that echoes user input: ```javascript app.get('/echo', (req, res) => { const input = req.query.input; res.send(`<h1>${input}</h1>`); }); ``` An attacker can inject malicious JavaScript code by visiting the URL `http://example.com/echo?input=<script>alert('XSS')</script>`. **Prevention:** 1. **Use a template engine:** EJS, Pug, and other template engines automatically escape input data, preventing XSS attacks. 2. **Escape user input:** Use functions like `escape()` or `encodeURIComponent()` to escape user input before displaying it on the page. 3. **Use Content Security Policy (CSP):** Define a CSP to restrict the sources of JavaScript code that can be executed on your page. **Cross-Site Request Forgery (CSRF):** Cross-Site Request Forgery (CSRF) is an attack that forces a user to perform unintended actions on a web application without their knowledge or consent. **How CSRF works:** 1. An attacker creates a malicious website that contains a form or link that performs an action on your application. 2. The victim visits the malicious website, which initiates the unwanted action on your application. **Example:** Suppose we have a simple Express.js route that deletes a user account: ```javascript app.delete('/users/:id', (req, res) => { const id = req.params.id; // Delete the user account }); ``` An attacker can create a malicious website with a form that submits a request to `http://example.com/users/123`, deleting the user account with ID 123. **Prevention:** 1. **Use a token-based approach:** Generate a random token for each user session and verify it on every request. 2. **Use a Cookie-based approach:** Set a cookie with a secret value and verify it on every request. **SQL Injection:** SQL Injection is a type of web application security vulnerability that allows attackers to inject malicious SQL code, which can be executed on the database. **How SQL Injection works:** 1. An attacker injects malicious SQL code into user input. 2. The application executes the SQL query without proper escaping, allowing the attacker to access sensitive data or manipulate the database. **Example:** Suppose we have a simple Express.js route that retrieves a user's data: ```javascript app.get('/users/:id', (req, res) => { const id = req.params.id; const query = `SELECT * FROM users WHERE id = '${id}'`; // Execute the query }); ``` An attacker can inject malicious SQL code by visiting the URL `http://example.com/users/123 OR 1=1`, retrieving all user data. **Prevention:** 1. **Use parameterized queries:** Use a library like Mongoose or Sequelize to create parameterized queries. 2. **Escape user input:** Use functions like `escape()` or `encodeURIComponent()` to escape user input before executing the SQL query. **Conclusion:** In this topic, we explored three common web security vulnerabilities: Cross-Site Scripting (XSS), Cross-Site Request Forgery (CSRF), and SQL Injection. By understanding these vulnerabilities and implementing the prevention measures discussed, you can significantly improve the security of your Express.js application. **Additional Resources:** * OWASP WebGoat: A web application security testing environment that allows you to practice identifying and exploiting common vulnerabilities. (<https://owasp.org/www-project-webgoat/>) * OWASP Cheat Sheet Series: A collection of cheat sheets that provide concise guidance on various security topics. (<https://cheatsheetseries.owasp.org/>) **Next Topic:** In the next topic, we'll explore how to use Helmet.js for setting HTTP headers to secure Express apps. **Leave a comment or ask for help:** If you have any questions or need further clarification on any of the topics discussed in this section, please leave a comment below.