Re: [Full-disclosure] Possible RDP vulnerability

["Thor (Hammer of God)" <Thor@xxxxxxxxxxxxxxx>]

I’m not sure what you are referring to.  I never made any recommendation to 
‘sidestep’ desktop restrictions at all.   Rather, I indicated that “desktop 
restrictions” (and thank you for the use of that far more appropriate term) 
were not actual security access controls, but rather UI access mechanisms, and 
that they should not be relied upon to enforce security policy and/or access 
authorization.

I actually recommend deploying UI restrictions in RDP environments where you 
must give the user direct control of the desktop.  To be sure, prior to 
deploying such solutions, I recommend that RemoteApp or similar types of 
application-specific publishing scenarios are considered first (though they 
also carry risks to be aware of).  So while not specific security controls, I 
do put them under the Security in Depth umbrella and recommend that due 
diligence in planning and execution of RDP access models is exercised, which 
would include such additional restriction mechanisms.  I apologize if my 
statements were interpreted as “don’t use desktop restrictions.”

The initial recommendation was to treat the security of an RDP session as one 
would local desktop access; that being consideration of overall permissions, 
SAFER/AppLocker application, and other desktop-based host security measures.

t

From: Dan Kaminsky [mailto:dan@xxxxxxxxxxx]
Sent: Saturday, March 27, 2010 9:28 AM
To: Thor (Hammer of God)
Cc: wicked clown; Full-Disclosure@xxxxxxxxxxxxxxxxx
Subject: Re: [Full-disclosure] Possible RDP vulnerability

You say this, but I note with interest and approval that your recommendation 
was to sidestep desktop 'restrictions' entirely in favor of certificate based 
access control.

Quantization of security and all that.

On Mar 27, 2010, at 11:56 AM, "Thor (Hammer of God)" 
<Thor@xxxxxxxxxxxxxxx<mailto:Thor@xxxxxxxxxxxxxxx>> wrote:

No, they don’t “always” win.  Maybe back with windows 2000, or XP, but not with 
Windows 7 anyway.  Of course, none of this does anything to stop them from 
booting off a CD and owning the box that way.

However, I do agree that people need to fully understand exactly what they are, 
and more importantly, are NOT doing insofar as security is concerned when it 
comes to access to local assets.

t

From: Dan Kaminsky [mailto:dan@xxxxxxxxxxx]
Sent: Saturday, March 27, 2010 7:37 AM
To: wicked clown
Cc: Thor (Hammer of God); 
Full-Disclosure@xxxxxxxxxxxxxxxxx<mailto:Full-Disclosure@xxxxxxxxxxxxxxxxx>
Subject: Re: [Full-disclosure] Possible RDP vulnerability

So it's a super common thing for schools to have 'locked down' Windows 
desktops, and even more common for even slightly nerdy kids to take the 
lockdown as a challenge to be  defeated.

The point of course is that the kids always win:  At the point somebody has the 
set of privileges exposed by any amount of desktop access, constraining 
execution for them is similar to getting the idea that, perhaps, it would be 
the responsible thing to open discussions around managing the open state of the 
barn door.



On Mar 27, 2010, at 7:39 AM, wicked clown 
<wickedclownuk@xxxxxxxxxxxxxx<mailto:wickedclownuk@xxxxxxxxxxxxxx>> wrote:
I think we are two different pages :)

what I was trying to show if you have a group policy that will only run a 
certain applications for example notepad.exe, the user is unable to access my 
computer, run or the start button or any other application. There would be a 
shortcut on the desktop for just notepad.exe for the user to execute.

The user use RDP to connect to the terminal server so they can access 
notepad.exe but if you change the application in the programs tab under the RDP 
client the user is now able to run any application on the terminal server, the 
user then execute internet explorer and download a modified cmd.exe and save it 
in the c:\windows\temp (even if you denied access to the hard drive users can 
still write to the temp folder) now I log off the rdp client change the program 
to point to c:\windows\temp\cmd.exe, I how have access to the command prompt 
with access to the command prompt I can run any other application or access 
other parts of the server they are not allowed to access.

That is what my video was try to demostrate that even denying access to 
applications on the server you can still execute applications from that server.

But as been mention if you put that single click in the RDP-tcp on the server 
then the user is unable to execute other applications.

I have been doing some further checks and I can confirm I have seen this affect 
about 90% of so called secure systems with group policy, but will execute 
normally block applications. I have even found systems on the Internet that are 
vulnerable to this.

I think we may have to agree to disagree on this subject. But thank you for you 
views and comments.
On Fri, Mar 26, 2010 at 9:29 PM, Thor (Hammer of God) 
<Thor@xxxxxxxxxxxxxxx<mailto:Thor@xxxxxxxxxxxxxxx>> wrote:
I think you still misunderstand.

The option you refer to has nothing to do with “locking down” the server.  When 
you say things like “a locked down group policy that is tighter than a ducks 
bum” what exactly are you talking about?

Selecting “don’t allow a startup program to be run” simply forces the desktop 
to be shown as opposed to an application one may specify.  If I initiate a 
session and tell it to run calc.exe, then calc.exe is what it presented upon 
connection.  It’s a shortcut for the user.  If at the server I don’t allow 
applications to be specified, then it won’t run them and will default to the 
desktop.  But I can still go “start, run, calc” and it will run fine if I have 
permissions to run it.  AppLocker is a great way to lock down the host 
environment, whether RDP or not.

And you are quite incorrect about “no user based control” stopping you.  As 
mentioned, AppLocker could have prevented it had it been deployed “properly.”  
Well, it would help, anyway.   Depends on the manner in which the attack was 
carried out, of course, but that has nothing whatsoever to do with the setting 
in RDP.  Deploying RDP to untrusted users or malicious users is not good 
policy; as such, you need to take extra care in securing RDP hosts by using 
permissions and other restrictions.

I think you need to relax a little and think about what you post – saying 
things like “a GPO tighter to a ducks bum” and “open to total pwnage” and 
“nothing would stop me” sounds a bit hyperbolic (in addition to being 
incorrect).

To summarize, your concerns have nothing to do with RDP security settings as 
you have presented them.  MS10-015 is certainly an important issue for 
local-host based attacks, of which RDP is one.  One’s mitigation efforts should 
indeed include RDP hosts.  The takeaway from that is to apply more due 
diligence to securing RDP deployments as one would with any asset you give 
users local access to.   RDP should not be viewed as a security mechanism, but 
rather, an access mechanism.  There are MANY ways to secure RDP, limit access, 
publish applications in singularity, create remote workspaces, etc, but you 
need to educate yourself on these solutions.

The behavior you describe is expected, by design behavior.

t

From: 
full-disclosure-bounces@xxxxxxxxxxxxxxxxx<mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx>
 
[mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx<mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx>]
 On Behalf Of wicked clown
Sent: Friday, March 26, 2010 8:31 AM

To: Full-Disclosure@xxxxxxxxxxxxxxxxx<mailto:Full-Disclosure@xxxxxxxxxxxxxxxxx>
Subject: Re: [Full-disclosure] Possible RDP vulnerability

Thank you for your comment.

What I was referring to it being scary is that if you create a locked down 
group policy that is tighter than a ducks bum and you forget that single tick 
(I admit I didn't knew of that option and I bet lots of other people didn't 
know about it) you leave your system to total pwnage!! It's simple mistakes 
like that which compromises systems.

If I found this before MS10-015 patch was released I could of download that 
exploit and gain system level permission, so no user based permission or access 
control would of stopped me.


On Fri, Mar 26, 2010 at 2:13 PM, Thor (Hammer of God) 
<Thor@xxxxxxxxxxxxxxx<mailto:Thor@xxxxxxxxxxxxxxx>> wrote:
There’s nothing “scary” about it.   I believe you are incorrectly asserting 
that the inclusion of the “start the following program on connection” has 
something to do with “locking down the server” and/or “only allow(ing) users 
who connect to your server to run certain applications.”   I would suggest that 
you study up on what RDP is and how it works before posting things like this.

Consider “locking down RDP” a process similar to “locking down a local host.”  
Use permissions and other host/OS based controls to secure what a user can and 
can’t do on a host.

t



From: 
full-disclosure-bounces@xxxxxxxxxxxxxxxxx<mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx>
 
[mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx<mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx>]
 On Behalf Of wicked clown
Sent: Friday, March 26, 2010 3:33 AM

To: Full-Disclosure@xxxxxxxxxxxxxxxxx<mailto:Full-Disclosure@xxxxxxxxxxxxxxxxx>
Subject: Re: [Full-disclosure] Possible RDP vulnerability

Cheers for that,

I take it back that I haven't found an vulnerability :(, but by default this 
isn't enabled which is scary !!
On Fri, Mar 26, 2010 at 9:57 AM, Mr. Hinky Dink 
<dink@xxxxxxxxxxxxxxx<mailto:dink@xxxxxxxxxxxxxxx>> wrote:
There is a section in RCP-Tcp Properties on the server under "Environment" for 
"Do not allow an initial program to be launched.  Always show the desktop".

----- Original Message -----
From: wicked clown<mailto:wickedclownuk@xxxxxxxxxxxxxx>
To: Full-Disclosure@xxxxxxxxxxxxxxxxx<mailto:Full-Disclosure@xxxxxxxxxxxxxxxxx>
Sent: Friday, March 26, 2010 5:04 AM
Subject: [Full-disclosure] Possible RDP vulnerability

Hi Guys,

I think I possible may have found a vulnerability with using RDP / Terminal 
services on windows 2003.

If you lock down a server and only allow users who connect to your RDP 
connection to run certain applications, users can bypass this and run ANY 
application they want. You can do this by modifying the RDP profile / shortcut 
and add your application to the alternate shell and the shell working directory.

When the user connects now to the RDP server the banned application will 
execute upon logging on even though the user isn’t allowed to execute the 
application if the user logs on normally. This doesn’t work with cmd.exe but I 
have been able to execute internet explorer, down a modified cmd version, 
modify the RDP profile to execute the new cmd and it works like a charm.

I have only been able to tested this on windows 2003 using a local policy and 
works like a treat. Even in the wild!

I have done a quick basic video which can been seen here;
http://www.tombstone-bbs.co.uk/v1d30z/rdp-hack2.swf

Instead of modifying the RDP profile, I just added my application to the 
program tab.. I know the video is crappy but it’s just meant to give you an 
idea what I am talking about :)

So in short, if anybody can access your server via RDP they are NOT restricted 
by the policy. I would be interested in any feed back about this possible 
exploit / vulnerability even if you don’t think it is.. or even better if 
someone knows how to defend againest it!! LOL! :)

Cheers

Wicked Clown.
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_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/



_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/

_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/