From: Simon Spero <ses@tipper.oit.unc.edu>
Date: Sat, 30 Sep 95 19:20:06 PDT
To: Eric Murray <ericm@lne.com>
Subject: Re: NetScape's dependence upon RSA down for the count!
In-Reply-To: <199509302336.QAA23456@slack.lne.com>
Message-ID: <Pine.SOL.3.91.950930185650.4661A-100000@chivalry>
MIME-Version: 1.0
Content-Type: text/plain


On Sat, 30 Sep 1995, Eric Murray wrote:

> What you have described is the classic 'man in the middle' attack.
> Netscape claims that SSL V.3 is immune to the MITM attack in
> appendix D.4 of the SSL V.3 spec. 

I think you're misunderstanding the claim somewhat - what D.4 does is warn 
implementors that they should perform  some means of associating 
hostnames and certificates. You can't just use SSL out of the box without 
doing anything in the application to check. 

My currently recommended approach is to enforce the verisign requirement 
that all valid hostnames for the server be included in the certificate as 
CN values. This allows the check to be made below the application layer. 
Unfortunately a lot of currently issued certificates are non-compliant 
(even Verisign and netscape :-); any fully automated implementation needs 
a static table of hostnames aliases- interactive applications can 
display certificates for manual review.

> 
> You will need to get the 'filter' (MITM) key signed by Verisign.
> Or hack Verisign's server-key-signing key.

This is not really much protection. Getting hold of any key is much 
easier than getting a specific key, and don't forget there are a number 
of vulnerable keys floating around until their expiration dates pass. 

This attack has been known about for a long time, and is too obvious to 
be really interesting; I implemented a demonstration of the attack as 
part of developing a protection. If anybody is interested I can mail them my
report. 

The variant of the attack I implemented is useless for anything but 
demonstration purposes, or I'd ask sameer for a T-shirt. The external fix 
is a bit more useful, but there doesn't seem to be a patchnetscape alias :-)

Simon