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Re: Websense Enterprise 6.3.3 Policy Bypass
From: <dink () mrhinkydink com>
Date: Sun, 30 May 2010 11:13:02 -0700

When you look at the "MUSTs" for Via in RFC 2616, there are only three. 
None of them seem to be applicable here.  And, of course, nowhere does
it say a client MUST NOT fake a Via header. ;-)

So you have an assumption that if an HTTP request with a Via header
passes through your device it must have gotten there legitimately and it
must be treated accordingly.

In other words, this trick may have untapped potential.

-------- Original Message --------
Subject: Re: [Full-disclosure] Websense Enterprise 6.3.3 Policy Bypass
From: Christian Sciberras <uuf6429 () gmail com>
Date: Sun, May 30, 2010 1:40 pm
To: "Thor (Hammer of God)" <Thor () hammerofgod com>
Cc: "dink () mrhinkydink com" <dink () mrhinkydink com>, 
"full-disclosure () lists grok org uk" <full-disclosure () lists grok org uk>

My first thought on this find was "cool".

However the more I think about this, the more I think there's something
seriously amiss.

I would assume that there is some inside code which specifically tells
it to turn off everything when "via" is used?
 If this is so, what stops us from concluding this was some sort of
The only other reason for this would be them trying to support "via" but
did a(n extremely) bad job about it.

 Christian Sciberras

On Sun, May 30, 2010 at 6:30 PM, Thor (Hammer of God)
<Thor () hammerofgod com> wrote:
 Adding "Via:" completely bypasses monitoring too??  That is bad.  I've
never used Websense,  so pardon my ignorance, but this wouldn't apply to
with ISA's native monitoring and logging,  so I'm just curious about
what's going on under the covers.   "Via:" bypassing the filter is "not
good" but bypassing monitoring (and presumably logging) is really bad. 
Nice find.
 I am curious as to what your thoughts are regarding Windows Auth as a
mitigation.   While it's nice that ISA could help solve a problem with
Websense, I'm don't see how that would work.  How would requiring auth
solve Websense's inability to filter "Via:" headers?

-----Original Message-----
From: full-disclosure-bounces () lists grok org uk
bounces () lists grok org uk] On Behalf Of dink () mrhinkydink com
Sent: Saturday, May 29, 2010 8:25 PM
To: full-disclosure () lists grok org uk
Subject: [Full-disclosure] Websense Enterprise 6.3.3 Policy Bypass

discovered by mrhinkydink

PRODUCT: Websense Enterprise v6.3.3

EXPOSURE: Trivial Web Policy Bypass


By adding a "Via:" header to an HTTP request it is possible for a user
completely bypass filtering and monitoring in a Websense Enterprise
6.3.3/Microsoft ISA Server (2004 or 2006) proxy integration


The following works in a Websense 6.3.3 Enterprise system using the
Server integration product and transparent authentication. It is
assumed it will
work with other proxy integration products, but this has not been

I. Install Firefox >= 3.5

II. Obtain and install the Modify Headers plug-in by Gareth Hunt

III. Configure the plug-in to add a valid "Via:" header to every

Example: "Via: 1.1 VIAPROXY"

IV. Browse to a filtered Web site

V. All content is allowed without monitoring




The Modify Headers plug-in does not work with SSL. However, in
practice a
user could browse to a so-called (by Websense) "Proxy Avoidance" Web
and use the SSL capabilities of the remote proxy.


Properly configured, a downstream SQUID proxy can send requests to the
upstream ISA server and all requests will pass through without
blocking or
monitoring. No evidence of activity will be logged by Websense. This
was in
fact how this vulnerability was originally discovered.
Considering the simplicity of the attack, the author suspects this
technique is already well-known in certain circles.

Also, it is trivial to modify proxy-enabled Linux utilities to
leverage this bypass.
The author has recompiled (that is, HACKED) OpenVPN, connect-proxy,
PuTTY, stunnel, and others to take advantage of this policy bypass.

Obviously, the risk of undetected (by Websense, at least) covert
tunnels is
high in a vulnerable installation of this product.

Linux platforms using this method in this specific environment will
also enjoy
bypassing Websense's transparent authentication requirement.


For this specific installation scenario (Websense 6.3.3 + ISA 2004/6 +
transparent authentication), none are known. The following may work:

* Use Windows Integrated Authentication on the ISA Server

* Upgrade to Websense 7.x

* Do not use a proxy integration product


10/09/2009 - vendor notified

05/29/2010 - PoC published



c. MMX mrhinkydink

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 Charter: http://lists.grok.org.uk/full-disclosure-charter.html
 Hosted and sponsored by Secunia - http://secunia.com/

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