Cross-Site Scripting (XSS)

In this section, we will explore cross-site scripting, detailing its various forms, and providing guidance on detection and prevention.

What is Cross-Site Scripting (XSS)?

Cross-site scripting (XSS) is a web security vulnerability that enables attackers to compromise user interactions with vulnerable applications. By circumventing the same-origin policy, which is designed to isolate websites from each other, XSS allows attackers to impersonate users, perform actions on their behalf, and access their sensitive data. If a user has elevated privileges within the application, an attacker could potentially gain complete control over the application’s functionality and data.

How Does XSS Work?

XSS attacks function by manipulating a website to deliver malicious JavaScript code to its users. When this malicious code is executed within a victim’s browser, it grants the attacker the ability to fully compromise the user’s interaction with the application.

Labs

For those already familiar with XSS vulnerabilities and seeking practical experience in exploiting them on realistic, intentionally vulnerable targets, the labs for this topic are accessible via the link below.

XSS Proof of Concept

You can typically validate an XSS vulnerability by injecting a payload that triggers arbitrary JavaScript execution in your own browser. The alert() function has been a long-standing choice for this purpose due to its simplicity, harmlessness, and clear indication of successful execution. In fact, most XSS labs can be solved by successfully invoking alert() in a simulated victim’s browser.

However, Chrome version 92 and later (released July 20, 2021) prevent cross-origin iframes from calling alert(). As these iframes are utilized in some advanced XSS attacks, an alternative proof-of-concept payload may be necessary in certain scenarios. In such cases, the print() function is a recommended alternative. For more information on this change and the rationale behind recommending print(), refer to our blog post on the subject: alert is dead long live print.

Given that the simulated victim environment in our labs uses Chrome, the affected labs have been adapted to accommodate solutions using print() as well. This is indicated in the instructions where relevant.

Types of XSS Attacks

There are primarily three categories of XSS attacks:

Reflected Cross-Site Scripting

Reflected XSS, the most basic form of XSS, occurs when an application receives data within an HTTP request and immediately incorporates that data into the response in an unsafe manner.

Consider this example of a reflected XSS vulnerability:

https://insecure-website.com/status?message=All+is+well. <p>Status: All is well.</p>

Since the application processes the data without sanitization, an attacker can easily craft an attack like this:

https://insecure-website.com/status?message= <p>Status: </p>

If a user clicks on the attacker’s crafted URL, the malicious script embedded by the attacker will execute within the user’s browser, within the context of their session with the application. This enables the script to perform any action and retrieve any data accessible to the user.

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Stored Cross-Site Scripting

Stored XSS (also known as persistent or second-order XSS) arises when an application stores data from an untrusted source and subsequently includes this data in later HTTP responses in an unsafe way.

This data could be submitted to the application through HTTP requests, such as blog post comments, chat room usernames, or customer order details. Alternatively, the data might originate from other untrusted sources, such as emails displayed by a webmail application, social media posts in a marketing application, or network traffic data in a network monitoring application.

Here’s a simple example of a stored XSS vulnerability. Imagine a message board application that allows users to post messages, which are then displayed to other users:

<p>Hello, this is my message!</p>

If the application doesn’t sanitize the input, an attacker can post a message containing malicious code that will then attack other users viewing the message board:

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DOM-Based Cross-Site Scripting

DOM-based XSS (or DOM XSS) vulnerabilities occur when client-side JavaScript within an application processes data from an untrusted source in an unsafe manner, typically by writing this data back to the Document Object Model (DOM).

In the following example, JavaScript code reads a value from an input field and writes it into an HTML element:

var search = document.getElementById('search').value; var results = document.getElementById('results'); results.innerHTML = 'You searched for: ' + search;

If an attacker can control the input field’s value, they can inject malicious code that will be executed:

You searched for: <img alt="" src="http://1">

Frequently, the input field’s value is derived from a part of the HTTP request, such as a URL query string parameter. This enables attackers to launch attacks using malicious URLs, similar to reflected XSS attacks.

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What Can XSS Be Used For?

Exploiting an XSS vulnerability allows an attacker to:

• Impersonate the victim user.
• Execute any action the user can perform.
• Access any data the user can access.
• Steal the user’s login credentials.
• Perform virtual website defacement.
• Inject malicious functionality into the website.

Impact of XSS Vulnerabilities

The severity of an XSS attack is generally determined by the application’s nature, its functionality, the sensitivity of its data, and the privileges of the compromised user. For instance:

• In a simple brochure website where users are anonymous and all content is public, the impact is often minimal.
• In applications handling sensitive data like banking transactions, emails, or health records, the impact can be severe.
• If the compromised user holds administrative privileges, the impact is typically critical, potentially allowing the attacker to gain full control of the application and compromise all users and data.

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How to Find and Test for XSS Vulnerabilities

The majority of XSS vulnerabilities can be efficiently and reliably detected using automated web vulnerability scanners like Burp Suite Scanner.

Manual testing for reflected and stored XSS usually involves submitting unique, simple input (like a short alphanumeric string) into every application entry point. Then, identify all locations where this input is reflected in HTTP responses. Each location should be tested to see if crafted input can execute arbitrary JavaScript. This process helps determine the context of the XSS vulnerability, allowing for the selection of an appropriate exploit payload.

Testing for DOM-based XSS from URL parameters is similar: insert unique input in the parameter, use browser developer tools to search the DOM for this input, and test each location for exploitability. However, detecting other forms of DOM XSS is more challenging. Finding DOM-based vulnerabilities in non-URL-based inputs (e.g., document.cookie) or non-HTML-based sinks (like setTimeout) often requires JavaScript code review, which can be very time-consuming. Burp Suite Scanner combines static and dynamic JavaScript analysis to automate DOM-based vulnerability detection effectively.

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Content Security Policy

Content Security Policy (CSP) is a browser security mechanism designed to lessen the impact of XSS and similar vulnerabilities. If an application with CSP exhibits XSS-like behavior, the CSP might hinder or prevent the vulnerability’s exploitation. However, CSP can often be bypassed to exploit the underlying vulnerability.

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Dangling Markup Injection

Dangling markup injection is a technique used to capture cross-domain data when a full XSS exploit is prevented by input filters or other defenses. It can often be exploited to capture sensitive information visible to other users, including CSRF tokens that can be used to perform unauthorized actions on their behalf.

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How to Prevent XSS Attacks

Preventing cross-site scripting varies in complexity, ranging from straightforward to challenging, depending on the application’s complexity and how it manages user-controlled data.

Effective XSS prevention typically involves a combination of these measures:

• Input Filtering on Arrival: Filter user input as strictly as possible at the point of entry, based on expected or valid input formats.
• Output Encoding: Encode user-controlled data when outputting it in HTTP responses to prevent its interpretation as active content. The required encoding might involve combinations of HTML, URL, JavaScript, and CSS encoding, depending on the output context.
• Appropriate Response Headers: For HTTP responses not intended to contain HTML or JavaScript, use Content-Type and X-Content-Type-Options headers to ensure browsers interpret responses as intended.
• Content Security Policy (CSP): As a final defense layer, implement Content Security Policy to minimize the impact of any XSS vulnerabilities that might still occur.

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Common Questions About Cross-Site Scripting

How common are XSS vulnerabilities? XSS vulnerabilities are highly prevalent, likely ranking as the most frequent type of web security vulnerability.

How common are XSS attacks? Real-world XSS attack data is difficult to obtain reliably, but exploitation is likely less frequent compared to other vulnerabilities.

What is the difference between XSS and CSRF? XSS involves manipulating a website to deliver malicious JavaScript, whereas CSRF (Cross-Site Request Forgery) tricks a user into performing unintended actions.

What is the difference between XSS and SQL injection? XSS is a client-side vulnerability targeting application users, while SQL injection is a server-side vulnerability targeting the application’s database.

How do I prevent XSS in PHP? Sanitize inputs using whitelists of allowed characters and utilize type hinting or casting. Escape outputs with htmlentities and ENT_QUOTES for HTML contexts, or JavaScript Unicode escapes for JavaScript contexts.

How do I prevent XSS in Java? Filter inputs with character whitelists and use libraries like Google Guava for HTML-encoding outputs in HTML contexts, or JavaScript Unicode escapes for JavaScript contexts.

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