Circumventing quarantine control in Windows 2003 and ISA 2004

["Memet Anwar" <mmta.gm@xxxxxxxxx>]

For those unfamiliar with MS quarantine control, you can read Jon Hassel's 
tutorial on Windows 2003 Network Access Quarantine Control (NAQC) 
[http://www.securityfocus.com/infocus/1794], and the ISA 2004 VPN Quarantine 
(ISAQ) feature [http://www.securityfocus.com/infocus/1799].

A simplistic mechanism used in both NAQC and ISAQ enable users to bypass the 
requirement set by administrators (such as XP must run SP2, should have 
latest virus-def, etc.). The problem is due to how the requirements are 
validated, it is trivial for users to trick RRAS/ISA into believing that the 
client's system are always aligned with the requirements, regardless the 
actual condition.

To illustrate my point, I will use Jon's article part-1 mentioned above, 
because it is much the same with Microsoft description on the subject that I 
see on MOC-2824B training material. Please refer to 'A Step-by-Step Overview 
of NAQC'-part of the article.

There, step 1-7 will put the client connection into quarantine mode, which 
is fine. Step 8-9 shows that the CMAK profile will execute a client-side 
script to validate client's configuration based on the preconfigured 
baseline. If the client meets the requirement, the script should call 
rqc.exe with appropriate parameter. In Step 10-14, rqc.exe in the client 
send its result status to rqs.exe (the listener) on the server, along with 
its script version string. The listener then compare the script version 
string with its reference, before reconfiguring the session to a normal 
access.

Now I see two weaknesses there.

First, it is trivial for users to ensure that rqc.exe will always report 
success back to rqs.exe, regardless the actual condition of his/her system. 
The script (or any executables used) can be modified or replaced, and it 
will always work as long as the replacement knows what parameter must be 
obtained from CMAK, and what should be passed to rqc.exe.

Second, in step 12, rqs.exe only performs string comparison of the script 
version to verify wether the correct script is in use by client.

For example, if the admin-supplied script is a .cmd file, a user can replace 
the content with something like the following. Note that .vbs or .exe files 
can also be replaced, as long the same functionality is provided.

 @echo off
 @rem Use %ServiceDir% to locate rqc.exe.
 SET RQCLOC=%1\rqc.exe
 SET REMOVAL=get_this_from_the_orig_script
 %RQCLOC% %2 %3 7250 %4 %5 %REMOVAL%

I've reported this issue to MSRC as a design flaw that could allow what they 
call 'ungranted trust' [1]. Part of their response was:

==========
1) Regardless of whether the Quarantine Control returns success or not, the 
actual "authentication and authorization" is handled correctly.  You are 
correct, the Quarantine Control could be circumvented, however it is not a 
"security" feature. It is merely a tool to help administrators ensure the 
vast majority of their users will be held to a standard.  In the worst case 
scenario, a user with an "infected" or "unpatched" machine could be allowed 
access.  A malicious user would still be able to connect to the network with 
a "clean" machine and subsequently do something malicious.

2) <From ISA.chm::/FW_VPNSecurity.htm:>
Security recommendations for a VPN
* Use the ISA Server Quarantine Control feature, to provide phased network 
access for remote VPN clients. With Quarantine Control, clients are 
restricted to a quarantine mode before allowed access to the network. 
Although Quarantine Control does not protect against attackers, computer 
configurations for authorized users can be verified and, if necessary, 
corrected before they can access the network. For more information, see VPN 
and Quarantine
<MS-ITS:ISA.chm::/FW_QuarantineOver.htm> .
* The quarantine feature does not protect against malicious users on the VPN 
Clients network.
==========

MSRC repeatedly stressed that according to ISA 2004 online help, the 
quarantine control 'is not a security feature'. And since this is not 
vulnerability, they may not provide a fix or advice against the use of 
rqs.exe and rqc.exe on Windows 2003 RRAS or ISA 2004 VPN quarantine.

Security feature or not, it certainly not working as many admins would 
expect. What's the purpose of having a quarantine control, if by-design, it 
can be circumvented ? ;)

Regards,
Memet

[1] Definition of a Security Vulnerability, 
http://www.microsoft.com/technet/archive/community/columns/security/essays/vulnrbl.mspx