What Is Cross Site Scripting XSS & How Does It Work?


Heather Bennett

What Is Cross Site Scripting (XSS) & How Does It Work?

Cross-Site Scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious scripts into websites viewed by unsuspecting users. By exploiting XSS vulnerabilities, attackers can gain unauthorized access to user sessions, steal sensitive information, manipulate website content, and even distribute malware.

The Anatomy of XSS Attacks

XSS attacks typically occur when a web application does not properly validate user input or fails to sanitize it before rendering it on the website. There are three main types of XSS attacks:

  • Stored XSS: Also known as persistent XSS, this attack involves injecting malicious code that is permanently stored on the Target server. When a user visits the compromised page, the script is executed without their knowledge.
  • Reflected XSS: In this type of attack, the injected malicious code is embedded in URL parameters or form inputs.

    When the victim clicks on a crafted link or submits a vulnerable form, the server includes the injected code in its response, executing it in the victim’s browser.

  • DOM-based XSS: This attack occurs when client-side scripts manipulate HTML documents’ structure or content in an unsafe way. If an attacker can control certain parts of a web page’s DOM (Document Object Model), they can inject malicious scripts that are executed by the victim’s browser.

How Does XSS Work?

XSS attacks exploit the trust relationship between a website and its users’ browsers. Here’s a step-by-step breakdown:

  1. The attacker identifies a vulnerable web application that does not properly validate or sanitize user input.
  2. The attacker crafts a malicious payload, usually in the form of JavaScript code, that will be executed by the victim’s browser.
  3. The attacker finds a way to deliver the payload to the Target users. This can be done through various means, such as injecting the payload into user-generated content (comments, forum posts), sending it via email or social engineering techniques.
  4. When an unsuspecting user visits the compromised web page or interacts with the manipulated content, their browser unknowingly executes the injected malicious code.
  5. The malicious code can perform actions on behalf of the victim, such as stealing sensitive data (login credentials, credit card information), manipulating web page content, redirecting users to phishing sites, or even spreading malware.

Protecting Against XSS Attacks

To defend against XSS attacks and protect both your website and its users:

  1. Input Validation and Sanitization: Always validate and sanitize user input before accepting and displaying it on your website. Implement strict input validation mechanisms to filter out any suspicious or potentially harmful characters.
  2. Content Security Policy (CSP): Implement a Content Security Policy that restricts which types of scripts are allowed to run on your website.

    Consider using nonces or hashes for inline scripts to prevent unauthorized script execution.

  3. Escape and Encode Data: When rendering user input or any dynamic content on your web pages, make sure to properly escape and encode all untrusted data. This prevents browsers from interpreting it as executable code.
  4. Use HTTP-Only Cookies: Set cookies with the “HttpOnly” flag enabled. This prevents client-side scripts from accessing sensitive session cookies, reducing the risk of session hijacking.
  5. Stay Up-to-Date: Regularly update your web application frameworks, libraries, and plugins to ensure you have the latest security patches and protections against known vulnerabilities.

In conclusion, Cross-Site Scripting (XSS) is a dangerous security vulnerability that can lead to various malicious activities. Understanding how XSS attacks work and implementing proper security measures is crucial for safeguarding your website and protecting your users’ sensitive information.

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