How Is Cross Site Scripting Performed?


Scott Campbell

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 can then execute in the victim’s browser, giving the attacker access to sensitive information or control over the user’s session. Understanding how XSS attacks are performed is crucial for developers to prevent such vulnerabilities in their applications.

Types of Cross-Site Scripting:
There are three main types of XSS attacks: Stored XSS, Reflected XSS, and DOM-based XSS. Each type has its own characteristics and methods of exploitation.

Stored XSS:
Stored XSS, also known as persistent or type 1 XSS, occurs when an attacker injects malicious code that gets permanently stored on the Target server. This code is then served to unsuspecting users whenever they access the affected page. Attackers often exploit user-generated content submission forms or comment sections to inject their scripts.

Reflected XSS:
Reflected XSS, also known as non-persistent or type 2 XSS, involves the injection of malicious code that gets reflected back to the user within a single HTTP response. This type of attack typically relies on tricking victims into clicking a specially crafted URL containing the injected script.

DOM-based XSS:
DOM-based XSS occurs when client-side JavaScript dynamically manipulates the Document Object Model (DOM) based on untrusted data. This allows an attacker to inject malicious code that is executed by the victim’s browser when certain conditions are met.

Performing Cross-Site Scripting Attacks:

Step 1: Identifying Vulnerable Entry Points

The first step in performing an XSS attack is identifying potential entry points where user-supplied data is not properly sanitized or validated by the application. Common vulnerable entry points include input fields, URL parameters, cookies, and HTTP headers.

Step 2: Crafting the Payload

Once a vulnerable entry point is identified, the attacker needs to craft a payload that will be injected into the application and executed by the victim’s browser. The payload typically consists of JavaScript code that performs malicious actions such as stealing session cookies or redirecting users to phishing websites.

Example Payload:

In this example, the payload uses the alert() function to display a pop-up alert with the message ‘XSS Attack’. However, an attacker can create more sophisticated payloads to achieve their specific goals.

Step 3: Injecting the Payload

The next step is injecting the crafted payload into the vulnerable entry point. This can be done by submitting malicious input through forms, modifying URL parameters, or manipulating cookies.

Step 4: Exploiting User Interaction

To successfully execute an XSS attack, attackers often rely on user interaction. For example, they may trick users into clicking on a specially crafted link that contains the injected payload or enticing them to visit a compromised website where the payload is injected.

Preventing Cross-Site Scripting Attacks:

To prevent XSS attacks, developers should implement proper input validation and output encoding practices. Here are some best practices to follow:

  • Input Validation: Validate and sanitize all user-supplied data before accepting it.
  • Output Encoding: Encode user-generated content before displaying it in web pages.
  • Content Security Policy (CSP): Implement CSP directives to restrict which scripts can execute on a webpage.
  • Avoid Dynamic Code Execution: Avoid executing user-supplied code or evaluating user-generated data as code.
  • Use Web Application Firewalls (WAFs): Implement WAFs to detect and block malicious requests.


Cross-Site Scripting (XSS) attacks can have severe consequences, compromising user data and damaging the reputation of web applications. By understanding how these attacks are performed and following best practices for prevention, developers can protect their applications and ensure the security of their users’ information. Stay vigilant, sanitize inputs, encode outputs, and keep your web applications secure from XSS vulnerabilities.

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