How Does Cross Site Scripting Attacks Work?


Larry Thompson

How Does Cross Site Scripting Attacks Work?

When it comes to web security, one of the most common vulnerabilities that developers need to be aware of is Cross Site Scripting (XSS). XSS attacks can have severe consequences, from stealing sensitive user information to compromising the integrity of a website. In this article, we will dive into how XSS attacks work and explore various types of XSS vulnerabilities.

What is Cross Site Scripting?

Cross Site Scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious scripts into web pages viewed by unsuspecting users. These scripts are typically written in JavaScript and can be executed in the user’s browser. The danger lies in the fact that the user’s browser cannot distinguish between legitimate website content and injected malicious scripts.

XSS attacks occur when a website does not properly validate or sanitize user input before displaying it on a web page. Attackers take advantage of this vulnerability by injecting code that gets executed when other users visit the affected page.

Types of Cross Site Scripting Attacks

There are three main types of XSS attacks:

  • Stored XSS: Also known as persistent XSS, this type of attack occurs when an attacker injects malicious code that gets permanently stored on a Target website’s server. When other users access the compromised page, they unknowingly execute the injected script.
  • Reflected XSS: In this type of attack, the malicious script is embedded within a URL and then sent to unsuspecting users via email or social engineering techniques.

    When the victim clicks on the manipulated URL, their browser executes the script.

  • DOM-based XSS: This type of attack Targets the Document Object Model (DOM) of a web page. The malicious script manipulates the DOM, causing it to execute unintended actions.

How Does Cross Site Scripting Work?

Let’s take a closer look at how an attacker might exploit a reflected XSS vulnerability:

  1. The attacker identifies a website that is vulnerable to XSS. This could be a website that fails to properly sanitize user input or doesn’t validate data before displaying it on web pages.
  2. The attacker crafts a URL that contains malicious JavaScript code.

    They may use techniques like URL encoding to obfuscate the script.

  3. The attacker lures unsuspecting users into clicking on the manipulated URL. This can be done through phishing emails, social media messages, or other deceptive means.
  4. When the victim clicks on the manipulated URL, their browser sends a request to the vulnerable website’s server with the injected script as part of the request parameters.
  5. The server processes the request and includes the user’s input in the response without proper validation or sanitization.
  6. The victim’s browser receives the response and renders it, executing the injected script in the process.

Preventing Cross Site Scripting Attacks

To prevent XSS attacks, developers should implement proper input validation and output encoding:

  • Input Validation: Ensure that all user input is properly validated on both client-side and server-side. Use whitelisting or strict validation techniques to only allow expected characters and formats.
  • Output Encoding: Before displaying user-generated content on web pages, make sure to properly encode special characters. This prevents browsers from interpreting them as HTML or JavaScript code.

Additionally, web application firewalls (WAFs) and security scanners can help detect and mitigate XSS vulnerabilities. Regular security audits and penetration testing are also recommended to identify and address any potential XSS flaws in your web applications.


Cross Site Scripting (XSS) attacks are a significant threat to web application security. By understanding how XSS attacks work and implementing proper input validation and output encoding, developers can significantly reduce the risk of falling victim to these attacks. It is crucial to stay up-to-date with the latest security best practices and continuously monitor your web applications for potential vulnerabilities.

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