Wednesday, August 18, 2010

Websense/ISA "Via:" Bypass Redux

discovered by mrhinkydink

PRODUCT: Websense Enterprise

EXPOSURE: Trivial Web Policy Bypass (III)


On May 29, 2010 I demonstrated that 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. This was addressed in Websense Knowledge Base article #5117.

However, anyone familiar with the Via bypass technique would have noticed this remediation was insufficient.


The following works in a Websense Enterprise system using the ISA Server integration product in a Cache Array Routing Protocol (CARP, sometimes referred to as "CRAP") configuration, which requires at least two ISA servers.

Assuming there a two ISA servers configured as per Websense Knowledge Base article #5117, one at IP address and another at, perform the following:

I. Install Firefox >= 3.5

II. Configure Firefox to use one of the proxy servers in the CARP array (

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

IV. Configure the plug-in to add a valid "Via:" header pointing to the other server in the array.

    Example: "Via: 1.0"

V. Browse to a filtered Web site

VI. All content is allowed without monitoring or filtering


All restrictions of the original Via Bypass article apply.



Limited only by your imagination! You do have an imagination, don't you?



Install Hotfix 17 provided by Websense.


06/25/2010 - vendor notified

08/13/2010 - vendor releases Hotfix 17

08/18/2010 - PoC published

c. MMX mrhinkydink

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