Understanding The 4 Types of Cross-Site Scripting (XSS) Vulnerabilities

Imagine you can see through the eyes of a bank manager without them knowing. You watch as they input the security code, open the gate, and unlock the vault brimming with cash.  

This invisible observation is akin to how Cross-Site Scripting (XSS) operates in the digital realm and can be just as lucrative for attackers.  

In this article, we’ll review what XSS is, the four types of XSS, and common scenarios where XSS vulnerabilities can occur. 

What is Cross-Site Scripting (XSS)?  

The advent of JavaScript in the mid-1990s marked a significant evolution in web browsing, transforming static web pages into dynamic, interactive experiences. This innovation enabled code execution within the browser itself. However, this breakthrough also introduced new security vulnerabilities: scripts from one website could be executed within the context of another, posing risks of unauthorized data access and manipulation.  

Netscape implemented the Same-Origin Policy (SOP) to regulate access to the Document Object Model (DOM) and provide robust protection against security threats by restricting how scripts interact with sensitive elements in the browser environment. This policy has been expanded and refined over time to balance the need for dynamic web capabilities with user data and privacy protection. 

The SOP had limitations that restricted cross-origin interactions. Cross-Origin Resource Sharing (CORS) was developed to overcome this as a more flexible system that allows controlled access to resources outside a website's domain while maintaining security protocols. 

While the SOP and CORS play pivotal roles in defending against attacks, it's essential to recognize that they are not foolproof, allowing XSS attacks to bypass these security measures. Therefore, a deep understanding of how attackers can circumvent these policies is crucial for web developers and security professionals.  

How Does Cross-Site Scripting (XSS) Work?  

XSS exploits vulnerabilities within a website, causing it to return malicious JavaScript code to users inadvertently. This manipulation is often subtle and goes unnoticed by the site's security mechanisms.  

When a user accesses such a compromised site, the malicious script executes within their browser. This execution can have severe consequences, enabling the attacker to hijack the victim's interaction with the application.  

Depending on the nature of the script, this could lead to a range of malicious activities, such as: 

• Stealing Session Cookies 
• Harvesting Personal Data 
• Manipulating Victim's Actions Within the Application 

This level of compromise underscores the insidious and potentially wide-ranging impact of XSS attacks, making them a significant concern in web security.  

There are four different types of XSS vulnerabilities, and each has their own way to attack your app. 

Four Types of Cross-Site Scripting (XSS) Vulnerabilities:  

1. Reflected XXS: This occurs when an application receives data in a request and includes it in the response unsafely. For instance, a search function that returns a message like "No results found for [user input]" without adequately escaping the user input. The attacker's script is 'reflected' back to the user's browser.  
2. Stored XXS: This malicious script is permanently stored on the target servers, such as in a database, message forum, visitor log, comment field, etc. The malicious script is transmitted to their browsers when other users access the stored data. It's more dangerous than reflected XXS because it doesn't require tricking the user into clicking a link.  
3. DOM-based XXS: This type of vulnerability exists in the Document Object Model (DOM) and does not involve the server-side code. The attacker's payload is executed as a result of modifying the DOM environment in the victim's browser. This can happen, for example, when JavaScript takes data from an untrusted source (like the URL) and processes it without proper sanitization.  
4. Document Object Model (DOM) Manipulation: If a web application uses JavaScript to dynamically update the page's DOM based on user input (like in AJAX applications) without proper input handling, it can lead to DOM-based XSS.  

Common Scenarios Where XSS Can Occur 

Cross-site scripting (XSS) vulnerabilities can occur in various scenarios, mainly where web applications mishandle user input. Here are some common scenarios where XSS can occur:  

• User Input Fields: XSS can occur whenever a web application includes user input directly in its output without proper validation or escaping. This includes form fields, URL parameters, or any other input areas.  
• Comments and Message Boards: Websites that allow users to post comments or messages are at risk if the content is not properly sanitized before being displayed to other users.  
• Email and Messaging Systems: An email or messaging application displaying messages containing HTML or JavaScript without adequate sanitization can become a vector for XSS attacks.  
• Search Functions: An attacker could inject scripts through search queries if a search function reflects user input back to the browser without escaping it.  
• URL Manipulation: Applications that use data from URL parameters to modify the DOM can be vulnerable to DOM-based XSS if they don't properly sanitize the parameters.  
• Third-Party Widgets and Plugins: Incorporating third-party content, like widgets or plugins, without proper security checks can introduce XSS vulnerabilities.  
• Rich Content Editors: Applications allowing users to create rich content (like HTML editors in CMS systems) might inadvertently enable inserting malicious scripts.  
• Error Messages: Improperly handled error messages that display user input can be exploited for XSS attacks.  
• HTTP Headers: Applications that include untrusted data from HTTP headers (like the Referrer header) in their responses could be vulnerable.  

XSS attacks, while often straightforward in their execution, can have severe and far-reaching consequences. This is evident in notable incidents such as the MySpace Samy Worm, the TweetDeck Vulnerability, and eBay's Stored XSS Attack.  

These incidents are a stark reminder of the importance of diligent web security measures and the potential risks of underestimating seemingly basic vulnerabilities. 

Read More: How Do You Train Developers In Secure SDLC Practices? 

Are Your Apps Safe? 

Cross-Site Scripting (XSS) remains one of the most prevalent and pernicious security vulnerabilities in the web today, posing significant risks to users and organizations. Despite its longstanding presence in the cybersecurity landscape, XSS vulnerabilities continue to surface, primarily due to evolving web technologies, the complexity of modern web applications, and too little focus on preventing the vulnerability. 

As web technologies advance, so too must our strategies for securing them. We can build a safer, more trustworthy digital world by fostering a deeper understanding of XSS attacks and prioritizing security in development. 

Secure Coding Training can help your developers write secure code the first time, protecting your information from malicious XSS attacks.