---------- Forwarded message ---------- Return-Path: <wirepairat_private> Delivered-To: daat_private Received: (qmail 23076 invoked from network); 27 Sep 2002 16:37:44 -0000 Received: from outgoing2.securityfocus.com (HELO outgoing.securityfocus.com) (205.206.231.26) by mail.securityfocus.com with SMTP; 27 Sep 2002 16:37:44 -0000 Received: from gator.darkhorse.com (mail.darkhorse.com [209.95.33.140]) by outgoing.securityfocus.com (Postfix) with ESMTP id 534368F2C6 for <daat_private>; Fri, 27 Sep 2002 09:45:51 -0600 (MDT) Received: from [66.92.95.254] (account <wirepairat_private>) by gator.darkhorse.com (CommuniGate Pro WebUser 3.5.9) with HTTP id 7910895 for <daat_private>; Fri, 27 Sep 2002 09:42:17 -0700 From: "wirepair" <wirepairat_private> Subject: Re: Hacking Citrix Faq To: Dave Ahmad <daat_private> X-Mailer: CommuniGate Pro Web Mailer v.3.5.9 Date: Fri, 27 Sep 2002 09:42:17 -0700 Message-ID: <web-7910895at_private> In-Reply-To: <Pine.LNX.4.43.0209271010450.31567-100000at_private> MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Sure, I must admit I forget which list you moderate. If you would like you can just forward this message, Thanks. Hacking Citrix: - Introduction - How Citrix Works - Peeling Citrix Back - Securing Citrix... Sort of. .Introduction Citrix is a Remote Desktop application that is becoming widely popular. It is built off of Microsoft's Terminal Services RDP (Remote Desktop Protocol). Unlike Terminal Services, Citrixs' lines of products allow the administrator to specify certain applications to be run on the server. This allows them to control which programs they want to allow the end user to execute. There exists an interesting gray line for the security of citrix applications due to the mixing of both citrix technology, and microsoft technology. With an application that allows users remote access to not only published programs, but remote desktops, a serious threat arises. In this paper I will be outlining how citrix works, and how to take advantage of the way citrix handles user access to programs. Corrections have been provided to me by Brian Madden (Author of Citrix MetaFrame XP: Advanced Technical Design Guide.) Note: I'm not a Citrix Administrator some of the descriptions of the product maybe faulty but this is not the main focus of this paper. For corrections please email me at wirepairat_private .How Citrix Works There are multiple solutions that citrix has available and I will list them here. - Citrix MetaFrame Citrix MetaFrame has three different suits in its family; XPs, XPa, and XPe. Each come with a different set of tools and applications to help the administrator. XPe is fully featured with many different management options. XPa and XPe have slightly less. For this paper I will be discussing just XPe, but most of these tactics also apply to the other versions. Citrix uses XML over whatever port you want usually port 80 to begin communication. Citrix listens by default on port 1494 and must be connected to using the Citrix ICA client which can be downloaded for free from their web site. - Citrix NFuse/Citrix Secure Gateway Citrix NFuse allows the administrator to lock down applications even further by only allowing connections in through a web browser. NFuse itself sits on IIS 5.0 by default but I have seen papers detailing how to set it up on apache. You probably understand the concerns regarding NFuse sitting on IIS by default and we will touch on that subject later. All communication is done over SSL at 128-bit encryption, IF specified. A common setup with NFuse for remote access allows the administrator to also implement Citrix Secure Gateway. If they properly configure NFuse, the remote user should not be able to directly access the Citrix Server itself. Instead, all connections are filtered through the NFuse server through the Secure Gateway connection. Here's a diagram to make this a bit easier to understand. [user]--->[router]--->[firewall] | | | | | +-[DMZ]-->[Citrix NFuse Server] | | | | [Citrix Secure Gateway] | +----+ | | | +--------------+ | [Citrix Server] I hope that makes more sense... and you can follow my horrible diagram I probably confused you more. Now the user does not have a direct connection to the Citrix Server but instead follows through the DMZ -> internal network. In a configuration with the default NFuse setup (without Secure Gateway) the user first hits the NFuse web server, then once the published application is established, the user makes a direct connection to the Citrix Server. You can see the problem with this, the user can still *directly* access the Citrix Server and after using the published application scanner, gather their own list of published applications and then create their own .ica file containing the information they specify. This also means, if this configuration is for an external network, two holes are punched through the firewall, one for the IIS server *shudder* and one for Citrix. You can scan port 1494 to see if this holds true. Citrix MetaFrame usually has a very helpful management console which enables the administrator to set users and publish applications with relative ease. A citrix network without NFuse uses Independent Computing Architecture (.ICA) files which is a text based document that contains all of the configuration options and settings. These files are given to the end user who, after having the citrix client installed, clicks on the file and automatically connects to the server specified in the .ica file. If NFuse is in use, then these .ICA files are usually not required. A user opens a web browser, goes to the NFuse web server and logs in using his Domain/NT account. After this is successful the user is given a list of applications which they may use to access the servers or Citrix Farm. .Peeling Citrix Back There are so many ways to get a remote desktop on a citrix host its impossible for me to know where to begin. I imagine the first thing people want is remote access so let us begin there. Recently Ian Vitek of ixsecurity has released some very helpful perl tools. I use these a lot now and I must give him props on them. The tool I use most is Citrix published application scanner [you can find these tools at: http://www.securiteam.com/exploits/5CP0B1F80S.html]. This tool enumerates the published applications that are allowed on the remote server. This comes in handy when you're 'knocked out' of the regular login process, such as a script to automatically log a user out. After getting a list of the published applications, you can then modify your ICA file with the published application information. At this point you can attempt to brute force login attempts, I personally find backup accounts to be very helpful. If you find a 'test' account or backup account has a guess able password you're in luck. I find variants of test and citrixtest almost always existent. So lets assume at this point you have access to the citrix server. And lets suppose you are denied login, maybe guest doesn't have enough privileges to follow through the login process. Now, if you used the citrix application scanner and gathered a list of applications try this trick. I haven't seen this published and maybe this is a citrix flaw or maybe its expected. But citrix appears to only look at the [Application Name] in the .ICA file when verifying which application to run. So here is a regular published application file: [WFClinet] Version=2 TcpBrowserAddress=ip.ip.ip.ip [ApplicationServers] word= [word] Address=word InitialProgram=#word TransportDriver=TCP/IP WinStationDriver=ICA 3.0 DesiredHRES=800 DesiredVRES=600 So what happens if we modify the InitialProgram from #word to cmd.exe? or explorer.exe? Well, it happily runs what ever application we specify in that field. I've used this tactic numerous times to bypass faulty login processes. Just make sure that the application in the brackets and the Address is legitimate otherwise you'll get some interesting errors. So now we have a remote command prompt on the citrix server. Or if you like a gui and you decide you want this after you run cmd.exe, no problem just type in explorer.exe you'll get a little prompt stating that you are now starting a seamless remote desktop session :). So now you have a desktop. I'd hope at this point you would know enough to elevate your privileges so I will not cover that aspect. An interesting note, even when I logged in as guest to a citrix server, I was able to access the Administrative tools. Obviously this is rather alarming and to be honest I myself was rather surprised. I'd suggest looking at the user accounts to see if you can find more accounts that would most likely have default/easily guessed passwords. Never underestimate the power of shitty password choosing. Just so you know you are not *really* administrator, just try changing a password. So what if they are using NFuse with Citrix Secure Gateway and filtering all traffic through a firewall? Then you can not modify your .ica file in this aspect. Once again there are multiple ways of getting a remote desktop. A lot of times citrix administrators have published applications such as office. What a wonderful piece of software. If you have access to excel, look into vbscript macros for starting explorer.exe. Or if your lazy like me, just View as Web page. This will open Internet Explorer and allow you to execute C:\winnt\explorer.exe. This trick also works for Microsoft Word and I assume other office products. In fact, most programs have a help -> View online help option. So this will also execute Internet Explorer if it is set to the default browser. Another issue is 'winhelp32', most programs have this built in, and guess what? You can "Jump to URL." Now you can put cmd.exe on a remote web server, jump to the url and download it. If you have write access your set. Modify your .ica file to execute this upon logon and you now have your shell. If you compromise a machine that has citrix installed, and you have sufficient privileges, Citrix stores all of its connections for the Citrix Program Neighborhood in a well known directory. - In NT4 browse to %systemroot%\profiles\username\Application Data\ - In Win2k browse to C:\Documents and Settings\username\Application Data\ steal the ICAClient directory and copy it to your %systemroot%\profiles\youruser\Application Data\ or C:\Documents and Settings\profiles\youruser\Application Data\ and then open Citrix Program Neighborhood. You'll notice all of that users connections. If they are using Citrix chances are they are lazy as well and most likely cache their passwords. Use Revelation or another 'password reveler' and viola you have their password for that connection. Sniffing is yet another concern, dsniff has citrix sniffing capabilities and this may become a real problem when used in conjunction with arp cache poisoning. .Securing Citrix First off I'd like to say good luck. Here are some tips that may help an administrator locking down a Citrix Server. I spent the last few days attempting to come up with a magical Group Policy for windows that will lock programs down. The goal is to make it so a user may not use any programs which may be used to upload or transfer files to the citrix host. - Securing Externally 1. Use the NFuse / Citrix Secure Gateway configuration such as the one I displayed earlier in this text. 2. Make sure the IIS/Apache server is completely locked down and resides in the DMZ. Require .htaccess or NTLM authentication to even access the webroot of the NFuse server. 3. If possible, require remote users to use SecureID for authentication. This gets pricy so is most likely not going to be an option for most people. 4. Use a different browser than Internet Explorer per default. Most applications use the "default browser" for online help pages. If you install Netscape and set this as the default browser they will not be able to execute applications such as cmd.exe/explorer.exe. 5. Make a group and put all remote citrix users in this group. Deny access (using ntfs) to cmd.exe, ftp.exe, tftp.exe, rcp.exe, net.exe and any other programs that they may execute to transfer files to the citrix server. I did not test whether or not this may break login scripts so be sure to test this before it is put in to production. 6. Keep up to date on all patches for your citrix server, If you keep up to date on your patching you'll not need to worry about privilege escalation exploitation. 7. Since there are ways to bypass the 'default browser' such as winhelp32. You can attempt to set security settings under internet options to restrict to only allow certain web pages. This will hopefully disallow someone from downloading their own cmd.exe. 8. Although I've not attempted this and it may only work in certain situations. Disallow write access to the citrix server itself. This way they can not upload their own files. This may only be possible to achieve if you also disallow users mapping their drives to the server. Remember, users can map their drives and run their tools from their local machine. I'm quite sure this runs under the citrix localsystem context, I may be incorrect on this point. 9. If possible, disallow users from mapping their drives. You can do this via the Citrix Connection Configuration double click on ica-tcp, select 'client settings.' And select the necessary boxes. ADDITIONS: The following fixes have been provided to me by Brian Madden. One of these fixes will disallow the changing of the InitialProgram. I thank Brian again for bringing these to my attention. 1. Disable MetaFrame XP server broadcast response. CMC | Right-click on farm | MetaFrame Settings tab | Uncheck the two boxes in the "Broadcast Response" section. This will prevent that perl scanner from working. 2. Configure your MetaFrame XP servers so that users can only run published applications. (Citrix Connection Configuration | double-click "ica-tcp" | Advanced button | Check "Only run published applications" in the "Initial Program" section). I'd suggest first adding a published application for 'remote desktop' and only allow administrators to use this feature. 3. This exploit does work on MetaFrame XP with Service Pack 2 and Feature Release 2. (The most recent version) - Securing Internally Try using the same steps as listed for external, if you can segment your network access via a firewall + citrix secure gateway you are much better off then leaving it on the same segment. Other than that I know of no real good procedures for securing internally. .Questions Comments? This is my first paper regarding Citrix and most likely as I play with it more I'll add more additions. Feel free to email me at wirepairat_private Disregard the lame banner underneath my emails, its free, and its email... Hope this helps some people realize the serious flaws with using citrix. It's a good idea, just not when used in conjunction with Microsoft Windows :(. .Editorial Note: After attempting to contact Citrix regarding the cmd.exe insert flaw, they told me this rather amusing story. Since I do not have a tech support agreement with citrix, it would cost me 400$ per incident to report this. I was rather astounded and made quite sure this was true. Here is the dialogue: citrix: We are sorry sir, you do not currently have a tech support agreement with us. Without one, contacting tech support to report this issue will cost you 400$, would you like to charge this to a credit card? me: Wait, so let me get this straight, I pay you 400$ to do you a favor by telling you, you have a possible security flaw with your program? citrix: Yes *nervous giggle* we are sorry but it is the only way to contact tech support, they won't even let me transfer you over. me: Ok so I have to release this to everyone and have your engineers find and possibly scramble to fix this issue? citrix: Yes *nervous giggle again* *mentions something about going through my reseller* me: Ok looks like I'll release it, Thank you. _____________________________ For the best comics, toys, movies, and more, please visit <http://www.tfaw.com/?qt=wmf>
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