RE: AD in the DMZ . . . OK?

["Roger A. Grimes" <roger () banneretcs com>]

Karl, why I can't say I'm an expert on the subject, all I can say is to
use caution and think about the risks that are involved (which you are
already doing by sending out this email).  If I were to expose any AD
domain to the DMZ, I would take great pains to secure it using
additional methods (i.e. IPSec, SSL with client authentication
certificates, VPN, RRAS, Network Access Quarantine Control, etc.) to
secure and authenticate the communication channel.  For a couple of
reasons:

1.  First AD with W2K and above, likes to use Kerberos as the default
user authentication protocol.  Kerberos is significantly stronger than
its predecessors (LM, NTLM, and NTLMv2).  If users connect to your AD on
the DMZ and don't have a secure VPN tunnel that supports Kerberos, then
they will connect using one of the earlier protocols, all of which have
been successfully attacked using brute force methods.  Unless you have
LM hashing turned off, I maybe able to capture LM password hashes in the
traffic and compromise passwords.

2.  Unless you have SID filtering turned on, it may be possible for a
lesser authenticated security principal account (other requirements
apply) to elevate their privileges using the SID History trick.

3.  Unless you have your anonymous enumeration permissions set securely,
a remote hacker may be able to enumerate your AD objects.

4.  If I was a malicious hacker and I knew you were authenticating your
network user accounts on your DMZ, I would try my best to successfully
compromise your DMZ and sniff traffic.

This is just a few things I would worry about.  A secure communication's
tunnel and/or a properly designed .NET app can minimize the risk.  So
the real answer is that yes, putting AD on the DMZ elevates your risk of
compromise, but that elevated risk can be minimized by taking additional
countermeasures.  And security risk is always just a cost/benefit trade
off.

Roger

************************************************************************
***
*Roger A. Grimes, Banneret Computer Security, Computer Security
Consultant 
*CPA, CISSP, MCSE: Security (NT/2000/2003/MVP), CNE (3/4), A+
*email: roger () banneretcs com
*cell: 757-615-3355
*Author of Malicious Mobile Code:  Virus Protection for Windows by
O'Reilly
*http://www.oreilly.com/catalog/malmobcode
*Author of upcoming Honeypots for Windows (Apress)
************************************************************************
****

-----Original Message-----
From: karl [mailto:opium () runningriver co uk] 
Sent: Wednesday, July 28, 2004 6:49 AM
To: security-basics () securityfocus com
Subject: AD in the DMZ . . . OK?

Hello

One of the developers I work with has come up with a wild and crazy
notion to write a .NET app that sits on a DMZ Web server but gets user
information from the Active Directory on the other side of the
firewall..

I'm inexperienced with this, so did some research and found that this
kind of thing is possible (plenty of articles on putting Exchange
servers in the DMZ), but found myself wondering if this ever happens,
i.e. do people actually have their networks set up this way?  Do folk
expose/replicate AD to the DMZ in practice?

It's all very well that this stuff is possible, but if it's perceived as
insecure and not implementable in the real world . . . . . . .

Thanks for any advice . . . . .

Karl


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Ethical Hacking at the InfoSec Institute. Mention this ad and get $545 off
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to facilitate one-on-one interaction with one of our expert instructors.
Attend a course taught by an expert instructor with years of in-the-field
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