Reflected XSS in a JavaScript URL with some characters blocked – Explaining the Payload

Reflected XSS (Cross-Site Scripting) is a common security vulnerability found in web applications that allows an attacker to execute malicious scripts in the context of a user’s web browser. In this article, we will discuss a specific scenario where a JavaScript URL with some characters blocked can still be vulnerable to XSS and explain the payload used to exploit it.

The Vulnerability

When a web application accepts user input and reflects it back to the user without proper validation or sanitization, it becomes vulnerable to XSS attacks. A common defense mechanism against XSS is to block certain characters or patterns that are commonly used in malicious scripts, such as <, >, and “. However, in some cases, these filters can be bypassed by using alternative encoding techniques or other creative methods.

The Payload

Let’s consider a scenario where a web application allows users to input their name in a search field and reflects it back in a JavaScript URL. The following code snippet demonstrates how the input is reflected in the URL:

        
            var searchQuery = " + userInput + ";
            var url = "https://example.com/search?query=" + searchQuery;
            window.location = url;
        
    

Now, let’s assume that the web application filters out the characters <, >, and ” from the userInput. However, an attacker can still exploit this by using the following payload:

        
            %3Cscript%3Ealert('XSS')%3C/script%3E
        
    

When the above payload is input as the search query, it gets reflected in the JavaScript URL as follows:

        
            var searchQuery = "%3Cscript%3Ealert('XSS')%3C/script%3E";
            var url = "https://example.com/search?query=" + searchQuery;
            window.location = url;
        
    

As a result, when the user visits the search page, the malicious script will be executed in the context of their browser, leading to an XSS attack.

Conclusion

In conclusion, even when a web application blocks certain characters or patterns, it may still be vulnerable to XSS if the input is not properly validated or sanitized. It’s important for developers to implement robust input validation and output encoding to prevent XSS attacks. Additionally, security researchers and developers should continuously monitor and update their defenses against evolving XSS techniques to ensure the security of their web applications.