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Date: Sun, 30 May 2010 13:32:41 -0700
From: <>
To: "" <>
Subject: Re: Websense Enterprise 6.3.3 Policy Bypass

I wouldn't call breaking proxy chaining mitigation, either.  More like a
"quick fix", if and only if it works.  Or maybe you'd call it a
"work-around", which is what I called it in the first place.

No, there's nothing at all in the Websense database indicating you went
to  You are home free until someone decides to look at the
ISA logs (if logging is turned on) and finds it in there.  But you spent
good money on Websense and want pretty reports with charts and colors
and your company logo, right?

In that way, this is much better than my 2007 User-Agent hack (now
fixed).  Your indiscretions were logged, but not blocked or categorized.

Now, as far as stripping out the Via header at ISA goes, per RFC 2616...

"Multiple Via field values represents each proxy or gateway that has
forwarded the message. Each recipient <shout> MUST </shout> append its
information such that the end result is ordered according to the
sequence of forwarding applications."

"MUST append..." does not mean, in my understanding of the English
language (and RFC 2119), "delete the downstream device's Via header". 
If you do anything other than "append..." (which you MUST do), you're
breaking the RFC.  

And if you go around breaking RFCs, you're BAD, m'kay? ;-)

Link to 2007 User-Agent hack, just in case you missed it...

-------- Original Message --------
Subject: RE: [Full-disclosure] Websense Enterprise 6.3.3 Policy Bypass
From: "Thor (Hammer of God)" <>
Date: Sun, May 30, 2010 2:19 pm
To: "" <>
Cc: ""

Ah, authenticating at the web proxy *chain*. That wasn't intuitive from
the original post... "breaking" the chain by requiring an auth mechanism
that the downstream proxy doesn't support really isn't a "mitigation,"
but now I understand the basis of the statement.

But, if they fixed it in 7.x, then it obviously wasn't "B" below. ISA
wouldn’t work like that anyway. It doesn't "send requests to the
plug-in." You either use a filter or you don't. For instance, if I
simply used the web proxy filter instead, I could filter for "Via:" and
block it. But then again, if I had to do that, I wouldn't have purchased
Websense but rather handled all my blocking at ISA. 

Not that it really matters to me personally, but I am curious - is the
logging of the request completely dropped, or is it just not logged as a
"filtered request." IOW, if I'm behind the downstream proxy, and I go to, Web sense logs the request and part of the logging is that
it was "filtered" or "blocked" or something. But if I set "Via" in the
downstream proxy (or at the client via something like firefox) and go to, not only do I reach the site, but there is no record
whatsoever that I went to playboy? If it is the latter, then they would
HAVE to fix it in 6.3.3 IMO. 


>-----Original Message-----
>From: []
>Sent: Sunday, May 30, 2010 10:47 AM
>To: Thor (Hammer of God)
>Subject: RE: [Full-disclosure] Websense Enterprise 6.3.3 Policy Bypass
>Chaining downstream proxies to ISA and requiring Windows Integrated Auth
>has been an issue for a long time (it generally breaks the chain, so that fixes
>the bypass problem right there), but frankly I'm guessing.
>Windows Auth brings a lot of incompatibilities with it. I wouldn't recommend
>it unless it was absolutely required, but its proxy-chain-breaking properties
>are legendary.
>The ISA server will continue to log, even though Websense won't, so you
>do have that. But ISA won't filter, so you're back to square one. And
>comparing the two databases for discrepancies can get ugly. By the time you
>get around to comparing the databases, the damage has already been done.
>It becomes a forensics exercise at that point.
>What I think is going on here is either:
>A) The Websense ISA plug-in sees that the request has come in by proxy and
>assumes it has already been filtered by the originating proxy
>B) ISA sees the request has come in by proxy and therefore doesn't send the
>request to the Websense ISA plug-in for filtering.
>If it's "B", then it's a Microsoft issue and it may never get fixed (and it
>becomes marketing bullet point for ISA Server TMG).
>If the same problem occurs in a SQUID integration of Websense 6.3.3, then it's
>definitely "A".
>I have a feeling Websense fixed it in the 7.x series, so they're probably not
>motivated to fix it in 6.x. Again, I don't have the resources to test that theory
>(and I asked Dan Hubbard politely for a temporary license for research
>My hunch is they did fix it in 7.x because they pretty much ignored me after
>the first e-mail I sent back in October 2009.
>-------- Original Message --------
>Subject: RE: [Full-disclosure] Websense Enterprise 6.3.3 Policy Bypass
>From: "Thor (Hammer of God)" 
>Date: Sun, May 30, 2010 12:30 pm
>To: "" , "full-
>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-----
>>[mailto:full-disclosure-] On Behalf Of
>>Sent: Saturday, May 29, 2010 8:25 PM
>>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
>>to completely bypass filtering and monitoring in a Websense Enterprise
>>6.3.3/Microsoft ISA Server (2004 or 2006) proxy integration environment.
>>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
>been tested.
>>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
>>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
>>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
>>Full-Disclosure - We believe in it.
>>Hosted and sponsored by Secunia -

Full-Disclosure - We believe in it.
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