From: "David J. Bianco" <bianco@itribe.net>
Date: Fri, 22 Sep 95 07:10:59 PDT
To: cypherpunks@toad.com
Subject: SSL Man-in-the-middle
Message-ID: <199509221407.KAA23176@gatekeeper.itribe.net>
MIME-Version: 1.0
Content-Type: text/plain


Has anyone given much thought to the feasability of a man-in-the-middle
attack against an SSL (or other similar) transaction?  To me, the
possibility seems obvious, so I figure it must have been discussed before,
though I haven't seen it.

The basic idea is pretty simple, really a flaw in the user interface of the
browser more than a flaw in SSL.  Neither browsers nor servers routinely
validate that they are communicating with the entity they think they are.
 Sure, with netscape you can ask for the document information window, which
shows the server's public key information, but this isn't a common action
among users, and certainly isn't something you'd want to do for every page
you viewed.

The only readily accessible information about security is that blue key at
the bottom of the netscape window.  Netscape docs tell you that if that key
is blue, your transaction is "secure."  In reality, the only thing that key
means is that you've negotiated a session key and are encrypting your
communications.  It says nothing about the fact that you're actually
communicating with the correct party.  Authentication is a large part of
security, and Netscape doesn't make that information conveniently
available.

Consider the following example.  Bob wants to communicate securely with
Alice.  He fires up his "secure" browser, looks up Alice's address in the
DNS and makes a connection.  He then sends Alice a document and
disconnects.

Now consider the following attack on the scenario:  Bob still wants to
 communicate with Alice.  He fires up his browser and looks up Alice in the
DNS.  Mallet, who wants the information Bob's sending, has subverted
Alice's DNS server and replaced Alice's IP address with his own, making a
note of the proper value.  Thus, when Bob looks up Alice's address in DNS,
he gets the wrong information and contacts Mallet instead.  Mallet performs
the SSL protocol with Bob, pretending to be the server, and then with
Alice, pretending to be the client.

Since neither the browser nor the server perform any authentication checks,
neither Bob nor Alice know they are really speaking to Mallet.  The best
Alice can do is check the IP address of the client she's speaking to, but
if Mallet has his own DNS, he can make the IP address map to whatever name
he wants, including Bob, in order to fool alice.  Even if Alice doesn't
depend on the DNS for IP resolution, probably doesn't know that the IP
address in question is really Mallet's, since it looks just like any other
IP address to her.

In this scenario, Bob gets a warm fuzzy since his key is blue and he knows
his information is being encrypted as it goes out.  Alice has a smaller
fuzzy, since she believes the transaction is secure from prying eyes.
 Mallet has a *really big* fuzzy, since he's able to read the data Bob
sends, decrypt it, save it, then re-send it to Alice.

I've read through the SSL spec, and it provides authentication for both the
server and the client, but these features are rarely used, probably because
they are somewhat inconvenient for the user.  A good first step would be to
include the IP address of the server in the certificate signed by VeriSign.
 In this way, browsers could perform automatic checks that the IP address
in the certificate is actually the one that's being communicated with.
 This does raise other questions (such as protecting from IP spoofing), but
IMHO would be a good way of providing an automatic "first check" without
inconveniencing users.  The added inconvenience of obtaining a new
certificate when your server's IP address changes is fairly minor, and
could be viewed as necessary overhead for doing secure transactions via the
Net.



--
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David J. Bianco			| Web Wonders, Online Oddities, Cool Stuff
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