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Re: Possible RDP vulnerability
From: Dan Kaminsky <dan () doxpara com>
Date: Sat, 27 Mar 2010 10:37:10 -0400

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 () googlemail com> 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 () 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 i s 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 specifi ed, then it won’t run them and will default to the desktop. But I c an 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 ho st 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 i t 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. De ploying RDP to untrusted users or malicious users is not good policy ; as such, you need to take extra care in securing RDP hosts by usin g permissions and other restrictions.



I think you need to relax a little and think about what you post – s aying 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 takea way from that is to apply more due diligence to securing RDP deploym ents as one would with any asset you give users local access to. R DP should not be viewed as a security mechanism, but rather, an acce ss mechanism. There are MANY ways to secure RDP, limit access, publ ish 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] On Behalf Of wicked clown
Sent: Friday, March 26, 2010 8:31 AM


To: 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 > 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 stud y 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] On Behalf Of wicked clown
Sent: Friday, March 26, 2010 3:33 AM


To: 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> 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

To: 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 e xecute the application if the user logs on normally. This doesn’t wo rk with cmd.exe but I have been able to execute internet explorer, d own a modified cmd version, modify the RDP profile to execute the ne w 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/

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