Re: Preventing tunnels through HTTPS proxies

[Jared Curtis <jared () w00ttech com>]

If you forced all traffic through a proxy server which then would
respond back with a spoofed cert for a domain signed by a local CA you
could then inspect the traffic.  This would be fairly resource
intensive process, 2 SSL connections per session, plus generating
certs on the fly, and traffic inspection.  On top of all that you're
being pretty nasty with your users and opening potential security
issues.

Of course this is a bit crazy, if you're concerned about data being
pushed out via SSL-VPN then you should white list permitted
IPs/domains on your proxy and/or firewall and deny traffic to all
other destinations.

To identify a SSL-VPN connection from a standard HTTPS connection
would be difficult.  It would require some research but I would
suspect that a SSL-VPN connection is more active than an HTTPS
session.  If this is the case than you could create a process that if
an SSL session sends more than X data alert you with the source and
destination IPs.  Once you have a "suspect" destination IP you could
than investigate whether or not it's an SSL-VPN or valid HTTPS site.
Of course I'm making all this up without doing any research on it, so
take it with a huge grain of salt.

On Tue, Jun 16, 2009 at 5:48 PM, Michal Ludvig<mludvig () logix net nz> wrote:
> Hi all,
>
> as you probably know it's very easy to bypass egress filters on a
> network as soon as there's an internal HTTPS proxy available. There are
> many packages laying around for all kinds of operating systems that make
> setting up a tunnel or VPN through such proxies a breeze.
>
>
> I wonder how to prevent these abuses? Clearly the traffic pattern for a
> VPN will be distinguishable from a genuine HTTPS traffic - but how to
> detect it? Alternatively playing a man-in-the-middle on the proxy,
> decrypting all the traffic, inspecting that it's indeed HTTP and
> encrypting back with a key signed by a private CA that all the desktops
> in the corporation would trust may be another option. Any other ideas?
>
>
> It would, in fact, be enough to learn that it was a VPN traffic
> afterwards, we don't necessarily need to kill the tunnel in realtime
> (although it would be nice). Since this kind of proxy abuse is forbidden
> by the company IT policy the trespasser's managers would deal with it at
> the HR level anyway. However net ops will have to provide some evidence.
>
>
> Does anyone know of any tools that can be used for this detection?
> Ideally something open source (or commercial but not insanely expensive)
> that could be used in conjunction with a Squid proxy? Other suggestions
> are welcome as well.
>
>
> Thanks
>
> Michal
>
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