Cross-Site Scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious scripts into webpages viewed by other users. It occurs when an attacker is able to inject client-side scripts (usually JavaScript) into web pages, which are then executed in the context of another user's browser. This can lead to a variety of malicious outcomes, such as stealing user data, hijacking sessions, defacing websites, or spreading malware.

Types of XSS:

1. Stored XSS (Persistent XSS):

   • In this type of attack, the malicious script is permanently stored on the server, typically in a database, and is served to users who access a particular page or resource.
   • Example: An attacker submits a malicious script in a comment section, and the script is stored in the website's database. When other users view the comment, the malicious script is executed.

2. Reflected XSS (Non-Persistent XSS):

   • In this case, the malicious script is reflected off the server immediately in the response to a request, usually via a URL or query parameter.
   • Example: An attacker sends a link with a malicious script embedded in the URL, and when the victim clicks on the link, the script runs in their browser.

3. DOM-based XSS:

   • This occurs when the vulnerability is in the client-side code (JavaScript) rather than in the server-side code. The attack is triggered when the browser's Document Object Model (DOM) is manipulated by malicious input.
   • Example: If user input (like a URL) is inserted into the DOM without sanitization, an attacker could inject a script that gets executed when the page is loaded.

How XSS Works:

1. An attacker identifies a website or web application that does not properly validate or sanitize user input.
2. The attacker crafts a payload (malicious script), typically JavaScript, and injects it into a form, URL, or other input field.
3. When other users view the page with the injected script, the browser executes the malicious code, which can then steal cookies, session data, or perform other harmful actions.

Potential Consequences of XSS:

• Session Hijacking: Stealing session cookies, allowing attackers to impersonate a user.
• Data Theft: Capturing sensitive information like usernames, passwords, or credit card details.
• Malware Distribution: Redirecting users to malicious sites or injecting malicious code that infects their machines.
• Phishing Attacks: Displaying fake login forms or other forms of social engineering to steal user credentials.

Prevention of XSS:

1. Input Validation and Output Encoding:

   • Ensure that any user input (e.g., form fields, query parameters) is properly validated and sanitized before being processed or rendered on the page. Use context-specific output encoding (e.g., escaping HTML, JavaScript, URL).

2. Use Content Security Policy (CSP):

   • Implement a Content Security Policy to restrict the sources of executable scripts on your site, helping to mitigate the impact of XSS attacks.

3. Sanitize Input:

   • Use libraries or frameworks that automatically sanitize input (such as OWASP's Java HTML Sanitizer or similar tools).

4. Avoid Inline JavaScript:

   • Avoid using inline JavaScript (e.g., <script> tags directly in HTML or event handlers like onclick) and use external script files instead.

5. HTTP-only and Secure Cookies:

   • Mark session cookies as HTTP-only to prevent JavaScript from accessing them, and use Secure flags to ensure they are only sent over HTTPS.

6. Use Frameworks with Built-in Protection:

   • Many modern web frameworks (like React, Angular, and Vue.js) have built-in mechanisms for preventing XSS attacks by automatically escaping user input when rendering it on the page.

Example of XSS Attack:

An attacker might input the following script into a comment box on a vulnerable website:

If the website displays this comment without properly sanitizing the input, the script will run in the browser of anyone who views that comment, showing an alert box.