Example of a dom xss attack vulnerability

How DOM-based XSS Attacks Work and How to Prevent Them

Cross-Site Scripting (XSS) attacks are one of the most common types of web application vulnerabilities. They occur when an attacker injects malicious code into a web page, which is then executed by a victim's browser. One variation of XSS attacks is DOM-based XSS, which can be particularly tricky to detect and prevent.

What is DOM-based XSS?

DOM-based XSS attacks occur when an attacker can inject malicious code into a web page that is processed by the victim's browser's Document Object Model (DOM). The DOM is a programming interface for web pages that allows JavaScript to access and manipulate the content of the page. If an attacker can inject malicious code into the DOM, they can manipulate the page's behavior and steal sensitive information.

How does a DOM-based XSS attack work?

DOM-based XSS attacks typically start with a vulnerable web page that processes user input without proper validation or sanitization. For example, the web page might use JavaScript to extract a value from the URL and display it on the page. If the user input is not properly sanitized or validated, an attacker can inject malicious code into the URL and execute it on the victim's browser.

Here is an example of a vulnerable web page:



    <title>DOM-based XSS Example</title>



    <h1>Welcome to my website!</h1>


      var username = window.location.hash.substr(1);

      document.write("Hello " + username + "!");




In this example, the page takes the username from the URL hash and displays a personalized greeting on the page. For example, if the URL is http://example.com/#Alice, the page will display "Hello Alice!".

However, an attacker can craft a URL that includes malicious JavaScript code in the hash:


When a victim visits this URL, the JavaScript code in the hash is executed in their browser, resulting in an alert box that says "XSS!".

How to prevent DOM-based XSS attacks

To prevent DOM-based XSS attacks, web developers should implement the following best practices:

Sanitize user input: All user input should be validated and sanitized to prevent malicious code from being injected into the page.

Use proper encoding and escaping techniques: All user input that is displayed on the page should be properly encoded and escaped to prevent it from being interpreted as code.

Use a Content Security Policy (CSP): A CSP is a security feature that allows web developers to specify which resources (such as scripts and stylesheets) are allowed to be loaded by a web page. A CSP can help prevent XSS attacks by blocking the execution of any scripts that are not explicitly allowed.

Use a browser security extension: There are several browser extensions available that can help prevent XSS attacks by blocking malicious code from being executed.


DOM-based XSS attacks are a serious threat to web applications and can be difficult to detect and prevent. By implementing the best practices outlined above, web developers can significantly reduce the risk of their web applications being vulnerable to these types of attacks. It is important to stay vigilant and keep up-to-date with the latest security best practices to protect against new and emerging threats.

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