RSLinx Classic and FactoryTalk Linx Gateway - Privilege Escalation

# Title: RSLinx Classic and FactoryTalk Linx Gateway - Privilege Escalation
# Date: 2017-12-11
# Author: LiquidWorm
# Vendor: Rockwell Automation, Inc.
# Product web page: https://www.rockwellautomation.com
# Affected version: Rockwell Automation RSLinx Classic 3.90.01
#                 Rockwell Automation RSLinx Classic 3.73.00
#                 Rockwell Automation RSLinx Classic 3.72.00
#                 Rockwell Automation RSLinx Classic 2.58.00
#                 Rockwell Automation FactoryTalk Linx Gateway 3.90.00
# CVE: CVE-2018-10619
# Tested on: Microsoft Windows 7 Professional SP1 (EN)

# Summary: 
# The FactoryTalk Linx Gateway adds a Classic OPC DA and OPC UA server
# interface to deliver information collected by FactoryTalk Linx from Logix5000™
# and other Allen-Bradley® controllers to external OPC clients, permitting
# third-party software to coexist with FactoryTalk® software.

# PoC: 
The application suffers from an unquoted search path issue impacting
the service 'dnwhodisp' for Windows deployed as part of RSLinx and FactoryTalk.
This could potentially allow an authorized but non-privileged local user to
execute arbitrary code with elevated privileges on the system. 

A successful attempt would require the local user to be able to insert their 
code in the system root path undetected by the OS or other security applications
where it could potentially be executed during application startup or reboot. If
successful, the local user's code would execute with the elevated privileges
of the application.

C:\>sc qc dnwhodisp
[SC] QueryServiceConfig SUCCESS

SERVICE_NAME: dnwhodisp
        TYPE               : 10  WIN32_OWN_PROCESS
        START_TYPE         : 3   DEMAND_START
        ERROR_CONTROL      : 1   NORMAL
        BINARY_PATH_NAME   : C:\Program Files (x86)\Rockwell Software\RSLINX\dnwhodisp.exe
        LOAD_ORDER_GROUP   :
        TAG                : 0
        DISPLAY_NAME       : dnWhoDisp
        DEPENDENCIES       : RPCSS
        SERVICE_START_NAME : LocalSystem