Valve Steam 3.42.16.13 - Local Privilege Escalation

# Exploit Title: Valve Steam 3.42.16.13 Local Privilege Escalation
# CVE-ID: CVE-2016-5237
# Date: 5/11/52016
# Exploit Author: gsX
# Contact: [email protected]
# Vendor Homepage: http://www.valvesoftware.com/
# Software Link: http://store.steampowered.com/about/
#Version: File Version 3.42.16.13, Built: Apr 29 2016, Steam API: v017, Steam package versions: 1461972496
# Tested on: Windows 7 Professional x64 fully updated.


1. Description:

The Steam directory located at C:\Program Files (x86)\Steam implement weak
file permissions
and allow anyone in the BUILTIN\Users windows group to modify any file in
the Steam directory and any of its child files and folders.

Since Steam is a startup application by default this makes it particularly
easy to achieve lateral/vertical privilege escalation and achieve code
execution against any user running the application.


2. Proof

C:\Program Files (x86)>icacls Steam
Steam BUILTIN\Users:(F)
      BUILTIN\Users:(OI)(CI)(IO)(F)
      NT AUTHORITY\SYSTEM:(F)
      NT AUTHORITY\SYSTEM:(OI)(CI)(IO)(F)
      NT SERVICE\TrustedInstaller:(I)(F)
      NT SERVICE\TrustedInstaller:(I)(CI)(IO)(F)
      NT AUTHORITY\SYSTEM:(I)(F)
      NT AUTHORITY\SYSTEM:(I)(OI)(CI)(IO)(F)
      BUILTIN\Administrators:(I)(F)
      BUILTIN\Administrators:(I)(OI)(CI)(IO)(F)
      BUILTIN\Users:(I)(RX)
      BUILTIN\Users:(I)(OI)(CI)(IO)(GR,GE)
      CREATOR OWNER:(I)(OI)(CI)(IO)(F)

Successfully processed 1 files; Failed processing 0 files


3. Exploit:

Simply backdoor/replace Steam.exe or any other related exe's/dll's  with
the code you want to
run.

I would like to note that I contacted Valve on several occasions
and gave them plenty of time to reply/fix the issue before releasing this
entry.