What Causes Cross-Site Scripting Vulnerability?


Scott Campbell

What Causes Cross-Site Scripting Vulnerability?

Cross-Site Scripting (XSS) is a common web application vulnerability that allows attackers to inject malicious scripts into trusted websites, thereby compromising the security of users’ data. Understanding the causes of XSS vulnerabilities is essential for developers and website owners to prevent such attacks.

1. Lack of Input Validation

One of the primary causes of XSS vulnerabilities is the lack of proper input validation.

When user-supplied data, such as form inputs or URL parameters, is not properly validated or sanitized, it becomes susceptible to XSS attacks. Attackers can exploit this by injecting malicious scripts into the application, which are then executed on the victim’s browser.

2. Improper Output Encoding

Another common cause of XSS vulnerabilities is improper output encoding.

Output encoding refers to the process of converting special characters into their HTML entities before displaying them on a webpage. If output encoding is not applied correctly, user-supplied data containing HTML or JavaScript code can be interpreted by the browser as executable code rather than plain text.


Consider a comment section on a blog where users can enter their comments. If the input is not properly encoded before being displayed on the webpage, an attacker can inject a script that steals sensitive information from other users or performs unauthorized actions on their behalf.

3. Cross-Site Script Inclusion (XSSI)

Cross-Site Script Inclusion (XSSI) occurs when an application includes untrusted content from a different domain without proper validation or sanitization. This can happen when developers include external scripts or resources without considering potential security risks.


If an application includes a JavaScript file hosted on a different domain, an attacker can compromise that external resource and inject malicious code into it. When the application loads and executes the compromised script, it becomes a vector for XSS attacks.

4. DOM-Based XSS

DOM-Based XSS is a type of XSS vulnerability that occurs when client-side scripts manipulate the Document Object Model (DOM) incorrectly. This allows attackers to modify the webpage’s structure or behavior, leading to potential security breaches.


An attacker can manipulate JavaScript code that dynamically updates the webpage’s content based on user inputs. If the script fails to properly sanitize and validate user inputs before modifying the DOM, it becomes vulnerable to XSS attacks.

5. Insecure Direct Object References (IDOR)

Insecure Direct Object References (IDOR) can also lead to XSS vulnerabilities in certain scenarios. This occurs when an application exposes internal references or identifiers directly in its URLs or forms, allowing attackers to tamper with them and inject malicious scripts.


If an application uses a user ID as part of its URL structure without proper authorization checks, an attacker can modify the user ID parameter and inject malicious scripts into the page.


To mitigate Cross-Site Scripting vulnerabilities, developers must implement proper input validation and output encoding techniques. Additionally, careful handling of external resources and preventing insecure direct object references are crucial steps in preventing XSS attacks. By understanding these causes and following best practices for web security, developers can significantly reduce the risk of XSS vulnerabilities in their applications.

Discord Server - Web Server - Private Server - DNS Server - Object-Oriented Programming - Scripting - Data Types - Data Structures

Privacy Policy