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Date: Sat May 29 2010 - 22:24:38 CDT
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
to completely bypass filtering and monitoring in a Websense Enterprise
6.3.3/Microsoft ISA Server (2004 or 2006) proxy integration environment.
PROOF OF CONCEPT
The following works in a Websense 6.3.3 Enterprise system using the ISA
Server integration product and transparent authentication. It is assumed
it will work with other proxy integration products, but this has not
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 request
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
site 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
bypass 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
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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