SquareLemon Blog

mitm the mitmers

Last week I mentioned that James Arlen and I gave the closing keynote at SCCongress Toronto. We had planned to do a live demo as part of the talk, but after reaching the venue and connecting to the wifi we found that it would not work as planned, specifically because the venue wifi was “correcting” my tampering of the DNS on the demo victim, they were still visiting the real website.

And so a new demo was born, the “How this venue wifi is tampering with our, and your, connection right now” demo.

It appeared that DNS on this network was not used to determine which webserver to contact when browsing to a page, it was merely a method to ensure that connections were routed to anything other than the local network, at which point something else took over. This is pretty easy to demonstrate, by setting up an /etc/hosts entry (used to locally override DNS for a particular FQDN) for a website, directing it to an unused RFC1918 address which was not on the local network, and by definition not on the internet either, we can test how DNS is used. The site still loaded up fine in a browser, this leads me to suspect that a transparent proxy is inline somewhere, intercepting all traffic and then using the host header in the HTTP request to determine which pages to deliver as opposed to using the destination addres in the of the IP header.

Using tcptraceroute on port 80 we also find that every website tested was apparently located on the local LAN, which of course they are not:

    $ sudo tcptraceroute www.google.com 80
    Selected device en0, address, port 55333 for outgoing packets
    Tracing the path to www.google.com ( on TCP port 80 (http), 30 hops max
     1 [open]  0.576 ms  0.744 ms  0.515 ms

Finally using any packetsniffer (wireshark in this case), we can capture the payload of a transaction and look for artifacts which may give away what is happening. In this instance it was very easy to spot:

{% blockquote %} X-Cache: MISS from localhost X-Cache-Lookup: MISS from localhost:3128 Via: 1.1 localhost:3128 (squid/2.7.STABLE9) {% endblockquote %}

These lines in the HTTP response header are not sent by the webserver, I was visiting my own webserver, so I’m pretty sure of this fact. Anyone who has experience in this field knows that this screams transparent proxy. And if you look up the version which is so helpfully displayed, you will notice that it screams horribly out of date and probably not patched proxy.

The point that we made at the time is that a high percentage of users in the audience are probably on this network as we spoke, that they were being MiTM’d without their knowledge. If someone had compromised that old version of squid that they could have been doing any number of things to those http connections and the devices using them.

As security professionals we talk about the dangers of conference and coffee shop wifi all the time, from the point of view of other users of the network being malicious. It’s always worth remembering that you can’t trust the venues themselves to be completely clean either.