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Juice-Shop Write-up: Bonus Payload

Challenge Overview

Title: Bonus Payload
Category: Cross-Site Scripting (XSS)
Difficulty: ⭐ (1/6)

Following the previous DOM XSS challenge, this "Bonus Payload" challenge requires to inject a more complex XSS payload involving an iframe that embeds a SoundCloud player.

Tools Used

  • Web Browser: For executing the attack and observing its effects.

Methodology and Solution

Understanding the Payload

The given payload is an iframe designed to embed a SoundCloud music player into the application's DOM dynamically. This payload, while not malicious in nature, illustrates the potential for embedding arbitrary third-party content via XSS, which could lead to phishing attacks, unwanted advertisements, or malicious scripts.

Steps Taken to Solve the Challenge

  1. Craft and Encode the Payload:

    • The provided iframe code was URL-encoded to ensure it would be processed correctly by the web browser when passed as a query parameter.
    • The complete URL with the injected payload looked like this: http://127.0.0.1:3000/#/search?q=%3Ciframe%20width%3D%22100%25%22%20height%3D%22166%22%20scrolling%3D%22no%22%20frameborder%3D%22no%22%20allow%3D%22autoplay%22%20src%3D%22https:%2F%2Fw.soundcloud.com%2Fplayer%2F%3Furl%3Dhttps%253A%2F%2Fapi.soundcloud.com%2Ftracks%2F771984076%26color%3D%2523ff5500%26auto_play%3Dtrue%26hide_related%3Dfalse%26show_comments%3Dtrue%26show_user%3Dtrue%26show_reposts%3Dfalse%26show_teaser%3Dtrue%22%3E%3C%2Fiframe%3E

  2. Execute the Attack:

    • By navigating to the manipulated URL, the iframe successfully rendered within the web application, embedding the SoundCloud player as intended.

Solution Explanation

The challenge was completed by demonstrating that it was possible to inject a complex iframe element into the web application via an XSS vulnerability. The iframe, in this case, embedded a SoundCloud player, showing how attackers could utilize XSS to insert almost any content into a vulnerable web application.

Remediation

To protect against such XSS attacks, particularly those involving complex payloads like the one used in this challenge, follow these recommendations:

  • Content Security Policy (CSP): Implement a CSP that restricts the sources from which scripts and iframes can be loaded. This policy helps prevent the execution of unauthorized scripts and embedding of undesired external content.
  • Input Sanitization: Ensure all user inputs are sanitized before being included in the DOM. This includes encoding or stripping out HTML elements from user inputs.