XSS (Cross-Site Scripting)

A web security vulnerability allowing attackers to inject malicious scripts into web applications viewed by users.

Understanding XSS (Cross-Site Scripting)

Cross-Site Scripting (XSS) is a web security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. XSS occurs when web applications do not properly sanitize user input before rendering it in the browser. The injected script can execute within the context of a victim’s browser, enabling attackers to steal sensitive information, hijack user sessions, deface websites, or launch phishing attacks.

Types of XSS Attacks

Stored XSS (Persistent XSS)

Malicious scripts are permanently stored on the server, such as in a database or message board.
Every time a user visits the affected page, the script executes automatically in their browser.
Example: An attacker injects a malicious <script> tag into a comment section, which then executes whenever another user views the comment.

Reflected XSS (Non-Persistent XSS)

The attack script is embedded in a URL or request and executed when the victim interacts with it.
Typically exploited through phishing emails or malicious links.
Example: A link containing JavaScript code is sent to a victim, and when clicked, it executes in their browser.

DOM-Based XSS

Occurs when JavaScript on the client-side dynamically modifies the web page based on untrusted data without proper validation.
Example: An attacker manipulates the DOM (Document Object Model) by injecting malicious input into a form field that later executes as part of the page's script.

Prevention and Mitigation Strategies

Input Validation and Output Encoding
- Validate and sanitize all user inputs to remove or neutralize harmful characters.
- Encode outputs using functions like htmlspecialchars() (PHP) or encodeURIComponent() (JavaScript).
Use Content Security Policy (CSP)
- Implement CSP headers to restrict the execution of inline scripts and external scripts from untrusted sources.
Escape User Input in JavaScript
- Avoid using innerHTML, document.write(), or eval() with untrusted input.
- Use textContent instead of innerHTML for displaying user input.
Sanitize User Input with Security Libraries
- Use libraries like DOMPurify for safe HTML sanitization.
- Frameworks like Angular and React have built-in XSS protections.
Use HTTPOnly and Secure Cookies
- Prevent JavaScript from accessing sensitive cookies by setting the HttpOnly flag.
Regular Security Audits and Penetration Testing
- Conduct security assessments and penetration testing to detect potential XSS vulnerabilities.