XSS Explained

Cross-site scripting, also known as XSS is a security vulnerability typically found in web applications. It’s a type of injection which can allow an attacker to execute malicious scripts and have it execute on a victim’s machine. A web application is vulnerable to XSS if it uses unsanitized user input. XSS is possible in Javascript, VBScript, Flash and CSS. There are three main types of cross-site scripting:

  1. Stored/Persistent XSS - the most dangerous type of XSS. This is where a malicious string originates from the website’s database. This often happens when a website allows user input that is not sanitised (remove the "bad parts" of a users input) when inserted into the database.

  2. Reflected XSS - the malicious payload is part of the victims request to the website. The website includes this payload in response back to the user. To summarise, an attacker needs to trick a victim into clicking a URL to execute their malicious payload.

  3. DOM-Based XSS - DOM stands for Document Object Model and is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style and content. A web page is a document and this document can be either displayed in the browser window or as the HTML source.

XSS Payloads

Remember, cross-site scripting is a vulnerability that can be exploited to execute malicious Javascript on a victim’s machine. Check out some common payloads types used:

  • Popup's (<script>alert(“Hello World”)</script>) - Creates a Hello World message popup on a users browser.

  • Writing HTML (document.write) - Override the website's HTML to add your own (essentially defacing the entire page).

  • XSS Keylogger (http://www.xss-payloads.com/payloads/scripts/simplekeylogger.js.html) - You can log all keystrokes of a user, capturing their password and other sensitive information they type into the webpage.

  • Port scanning (http://www.xss-payloads.com/payloads/scripts/portscanapi.js.html) - A mini local port scanner (more information on this is covered in the TryHackMe XSS room).

XSS-Payloads.com is a website that has XSS related Payloads, Tools, Documentation and more. You can download XSS payloads that take snapshots from a webcam or even get a more capable port and network scanner.

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Change elements on the page:
<script>document.querySelector('#thm-title').textContent = 'I am a hacker'</script>

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