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Re: Possible RDP vulnerability
From: "Thor (Hammer of God)" <Thor () hammerofgod com>
Date: Sat, 27 Mar 2010 15:53:09 +0000

Well, it's not really an "agree to disagree" thing.  You are technically incorrect in your dissertation, whether we 
both agree or not.  We can agree not to care what each other's opinions are, but the tech is the tech.

Many people have misconceptions of how RDP "security" works and what it is; when I see posts like this, I think it is 
important to use these types of misunderstandings as "learning opportunities."

I've asked a couple of times now, but you've not given any details on what "group policy" object you are using to 
ensure that "only a certain application will run" in your "so called secure systems with group policy."

You are clearly confusing authorization to execute a program with a "start menu" item.   You say "the user is unable to 
access my computer, run or the start button or any other application" but you have "a shortcut to notepad."  You are 
simply describing a menu system for the user to execute notepad.exe, and NOT anything that enforces that only certain 
programs run.

You are not "denying access" to any programs here.  You are just taking away the default menu system for the user to 
get to the program.  Even in this case, if you took notepad.exe off the desktop, all I have to do in my RDP session is 
hit CTRL+ALT+END and run Task Manager.  I can the go File, New Task, and run whatever I want.  You are further 
incorrect about "if you put a single click in the RDP-tcp on the server then the user is unable to execute other 
applications."  I can still EXECUTE whatever I have permissions to EXECUTE.

You really need to understand that what you are doing is just playing around with the windows/menu systems that the 
users "sees."  You are doing NOTHING to "secure" the box in these cases.   You keep using the words "only run certain 
programs" and "have access to other parts of the server they are not allowed access" and "execute normally blocked 
programs."  If this is the case, then you are obviously logging onto the box as an administrator, and/or you simply 
don't have basic security policies in place, and/or running on a FAT drive or something.

If you don't want them to run "notepad.exe" then deny access permissions to "notepad.exe" and they can't execute it.  
Period.  If you allow them to write to windows\temp, then they can write to windows\temp; as such, you have clearly 
"allowed" that.  You somehow think that accessing the box via RDP "magically" protects you from what the user could do 
if they were sitting at the console.   If I didn't want you to run your "altered cmd.exe" then I would use Applocker to 
only let you run the "real" cmd.exe via hash or certificate (not that cmd.exe has a cert ) but my point is that you 
have a tremendous number of ways to secure your installation that you are not using - instead you think that removing a 
menu items that points to the .exe someone sets security on the .exe itself.  It doesn't.

Menu item restriction and custom interfaces and simply user experience/ UI changes to make it "easy" for the user.  Do 
not mistake them for security access controls.  They are not (as you have seen).  Stop running around showing videos of 
the obvious when you COULD be actually securing the system.  You have to GRANT access to your RDP server. It's not by 
default.  You have to ALLOW users to do the things you want them to do.  You seem surprised that since you have given 
remote access to a computer to someone who has permissions to run programs that they can actually do that.  If you only 
wanted to allow them to run notepad, then giving them RDP access to that box without actually imposing security access 
controls on what they can do was, well, not really smart.

I understand the fact that you WANT there to be a vulnerability here. You want to be able to post videos of how you 
found some "hole."  Even when you said "looks like this might not be a vulnerability" you followed up with a frown.   
Thing is, there just isn't.  You can make it seem like there is, but there's not.   I suggest that you spend your time 
actually learning how to secure RDP and not looking for ways to make menu items seem like security issues.

t

From: wicked clown [mailto:wickedclownuk () googlemail com]
Sent: Saturday, March 27, 2010 4:39 AM
To: Thor (Hammer of God)
Cc: Full-Disclosure () lists grok org uk
Subject: Re: [Full-disclosure] Possible RDP vulnerability

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 () hammerofgod com<mailto:Thor () hammerofgod com>> 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 () lists grok org uk<mailto:full-disclosure-bounces () lists grok org uk> 
[mailto:full-disclosure-bounces () lists grok org uk<mailto:full-disclosure-bounces () lists grok org uk>] On Behalf Of 
wicked clown
Sent: Friday, March 26, 2010 8:31 AM

To: Full-Disclosure () lists grok org uk<mailto:Full-Disclosure () lists grok org uk>
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 () hammerofgod com<mailto:Thor () hammerofgod com>> 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 () lists grok org uk<mailto:full-disclosure-bounces () lists grok org uk> 
[mailto:full-disclosure-bounces () lists grok org uk<mailto:full-disclosure-bounces () lists grok org uk>] On Behalf Of 
wicked clown
Sent: Friday, March 26, 2010 3:33 AM

To: Full-Disclosure () lists grok org uk<mailto:Full-Disclosure () lists grok org uk>
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 () mrhinkydink com<mailto:dink () mrhinkydink com>> 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 () googlemail com>
To: Full-Disclosure () lists grok org uk<mailto:Full-Disclosure () lists grok org uk>
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
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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/

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