XSS Unpacked: What It Is, How It Works, and How to Stop It

Published in

InfoSec Write-ups

7 min readApr 13, 2024

Cross-Site Scripting (XSS) is a prevalent security vulnerability that poses a significant threat to web applications worldwide. It involves the injection of malicious scripts into otherwise benign and trusted websites. When users visit a compromised site, the injected script executes within their browser, potentially leading to unauthorized access to sensitive information or other malicious activities.

Definition and Significance

XSS allows attackers to execute scripts in the victim’s browser, which can hijack user sessions, deface websites, or redirect the user to malicious sites. The danger of XSS lies in its ability to propagate unnoticed and affect multiple users quickly. As web applications become increasingly integral to personal and professional activities, the impact of XSS vulnerabilities also escalates, affecting everything from individual privacy to business operations.

Concept and Mechanism

The basic concept behind XSS is simple yet powerful: an attacker finds a way to place a piece of JavaScript or other script into a webpage that other users will load and execute. This is typically done in places where user input, such as form fields, URL parameters, or message boards, is directly included in the web pages without proper sanitization.

For example, if a user inputs a script tag into a comment field that is not properly sanitized by the website, that script could be served and executed in another user’s browser when they view the comment. This script can then perform actions on behalf of the user, such as stealing cookies or other data, without the user’s knowledge.

This introduction sets the stage for a deeper exploration into the types of XSS, their specific mechanisms, and examples to illustrate how they can be both executed and mitigated. Understanding the fundamentals of XSS is the first step towards defending against it effectively.

Cross-Site Scripting (XSS) vulnerabilities can manifest in several forms, each with unique characteristics and potential risks. We will explore the three primary types of XSS: Stored XSS, Reflected XSS, and DOM-Based XSS.

Stored XSS

Stored XSS, also known as persistent XSS, occurs when a malicious script is permanently stored on target servers, such as in a database, message forum, visitor log, or comment field. When other users access the stored data, the malicious script executes. For instance, if a user inputs a script into a comment on a blog, and the blog does not sanitize this input, the script will be stored and later served to every user who views that comment.

Stored XSS is particularly dangerous because it can affect multiple users over a prolonged period without the need for ongoing interaction from the attacker. It can lead to extensive information theft or long-term account compromise.

Suppose a website allows users to post comments without properly sanitizing the input. An attacker could submit a comment like this:

<script>alert('XSS');</script>

If this script is stored in the database and then displayed as-is on the website for other users to see, it will execute in their browsers, showing an alert box. This demonstrates a Stored XSS attack.

Reflected XSS

Reflected XSS occurs when an attacker sends a malicious script through a link, which reflects the script off of a web server, such as in a search result or error message. The victim must click on a malicious link that includes the script in the URL. For example, an attacker might send a link that leads to a search engine, where their malicious script is part of the search query. When clicked, the script executes within the victim’s browser.

Although reflected XSS requires tricking the user into clicking a link, it can be very effective in phishing attacks and can lead to immediate data theft or malicious actions on behalf of the user.

Imagine a search functionality on a website that echoes back user input in the results page without proper sanitization or encoding. An attacker could craft a URL like this:

http://example.com/search?query=<script>alert('XSS');</script>

When a user clicks on this link (possibly disguised as a legitimate link), the script will execute in their browser, showing an alert box. This is an example of Reflected XSS.

DOM-Based XSS

DOM-Based XSS is a type of XSS where the entire payload is executed as a result of modifying the DOM (Document Object Model) in the client’s browser, usually with client-side scripts such as JavaScript. It occurs when the data provided by a web client is used by a web application to write to the DOM without proper sanitization. An example would be manipulating URL parameters that are then used within the script to dynamically change the DOM.

This type of XSS attack can manipulate the web page being viewed by the user, without any data being sent back to the server. This can lead to a variety of issues, including displaying false or malicious information to the user and redirecting them to other malicious websites.

Consider a web page that uses URL parameters to dynamically change content on the page using JavaScript without properly sanitizing the input. Here’s how an attacker might exploit this:

1.The attacker crafts a URL like this:

http://example.com/#data=<script>alert('XSS');</script>

2.The JavaScript on the page then reads the data parameter from the URL and uses it directly to modify the DOM:

document.getElementById('someElement').innerHTML = decodeURIComponent(window.location.hash.split('=')[1]);

When a user visits this URL, the malicious script within the data parameter executes, illustrating a DOM-Based XSS attack.

Cross-Site Scripting (XSS) attacks can have severe consequences, compromising both individual privacy and organizational security. Understanding these impacts is crucial for appreciating the importance of proactive security measures.

Potential Consequences

XSS attacks can enable attackers to perform a range of malicious activities, such as:

Stealing Cookies: Attackers can access session tokens or other sensitive information stored in cookies, potentially hijacking user sessions.
Capturing Keystrokes: Scripts injected via XSS can log keystrokes on web forms, capturing sensitive information like passwords and credit card numbers as users type them.
Spreading Malware: Malicious scripts can redirect users to malware-laden sites or even directly inject malware into their systems.
Phishing: XSS can be used to alter the appearance of a website to trick users into entering personal information into fake login forms.

To effectively counter XSS attacks, developers and security teams must implement a combination of proactive security practices and use appropriate tools. Here’s how these attacks can be mitigated:

Secure Coding Practices

Input Sanitization: Ensure all user input is sanitized, meaning potentially harmful scripts are stripped out before being processed or stored. This prevents malicious scripts from entering the application ecosystem.
Output Encoding: When displaying user input, encode the output so that it is rendered harmless. For example, characters like <, >, ", and ' should be converted to HTML entities. This ensures that any script is not executed but displayed as plain text.
Use of Secure Frameworks: Many modern web development frameworks come with built-in protections against XSS. These frameworks, such as React or Angular, automatically handle many of the security concerns related to XSS by properly encoding user input by default.

Content Security Policies (CSP)

Implementing Content Security Policy (CSP) can significantly reduce the risk of XSS. CSP is a browser security mechanism that helps detect and mitigate certain types of attacks, including data injection attacks and XSS. By defining the sources of trusted content and instructing the browser to only execute or render resources from those sources, CSP prevents the execution of unauthorized scripts.

Security Tools

Several tools can assist in detecting and mitigating XSS vulnerabilities:

Static Application Security Testing (SAST): These tools analyze source code for potential security vulnerabilities before the application is run. They can identify places where XSS could be exploited.
Dynamic Application Security Testing (DAST): These tools test the application while it is running, mimicking the actions of an attacker to find exploitable vulnerabilities.
Web Application Firewalls (WAFs): WAFs can help block XSS attacks by filtering out malicious data based on predefined or dynamically learned rules.

Cross-Site Scripting (XSS) remains one of the most common and dangerous security vulnerabilities affecting web applications today. By exploiting these vulnerabilities, attackers can steal sensitive information, compromise user accounts, and even take control of affected systems. The examples and impacts discussed highlight the potentially devastating consequences of XSS attacks.

It is imperative for developers, security professionals, and all stakeholders involved in web application development to prioritize security at every stage of the development lifecycle. Understanding the types of XSS and their mechanisms, implementing robust preventive measures, and continually educating oneself on the latest security practices are essential steps towards safeguarding applications from such threats.

Stay vigilant, stay informed, and stay secure!