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Certificate Validation Error in Netscape Browsers...
From: mattison () WEBOVISION COM (Dennis W. Mattison (Little Wolf))
Date: Wed, 15 Mar 2000 17:43:32 -0800


This may not be a normal "BugTraq" issue, since it is more a flaw in trust
in a security design then it is an actual bug in software...but
none-the-less I think it is something that should be discussed.  I haven't
checked this with Microsoft IE, I just noticed it as being a flaw in
Netscape (submitted a bug report to them earlier but they are either
really busy or have chosen to ignore the report.)  Tested in browsers from
4.07 - 4.72, all which operated in the same fashion.

What is the issue?

The scenerio is that a user accesses a website for which they do not
currently have trust for the signer of the certificate.  They are asked
whether they would like to trust the server certificate (until it
expires,) which if they respond yes, the web site signer certificate will
be stored in the certificate database.  You can check on these
certificates by clicking on the Security Icon on the browser, then select
the Website item from the menu.  Once stored in the database, any future
access to this site is permitted without warning.  The error occurs when
the web site certificate is expired and the new site certificate is valid,
Netscape never checks to see if the certificate is expired and replaced
with a new certificate, and thus the user can continue to access the site
without a warning stating that the certificate is expired and that a new
certificate exists for the site (it apparently only checks to see if the
new certificate isn't expired.)  Manually verifying the old certificate in
the database will prove that the certificate is invalid.  When the site is
properly reissued a certificate, Netscape automatically trusts the new
certificate based on the previous certificate...if the previous
certificate is removed from the database and the website is re-accessed,
the standard warning appears asking the user if they wish to trust the
certificate.  Since the new certificate is cryptographically different
from the old certificate, no trust relationship should exist (only the
signer is the same.)

Netscape does not replace the old expired certificate with the new
certificate, and does not add the new certificate to the database.  Nor
does it tell the user that the new certificate a site is sending does not
match a previous certificate.

Why is this a problem?

The problem is that there is an inherited trust between an expired
certificate and an active certificate, where there really shouldn't be.
If any trust should be there, it certainly shouldn't be with an expired
certificate.  The idea here is that Netscape should complain about a site
which has a certificate different than what Netscape has in its database.
When you accept a certificate from a website which you do not already hold
a trust with the signer of the certificate, you should be warned if that
certificate is no longer valid or when the server has been issued a new
one.  You are trusting that certificate and its signer, not that site.  If
the site's certificate changes, you should be warned about the change and
asked if you still want to trust the site.  If a hacker manages to gain
access to the key and the certificate, and changes the key and the
certificate, a warning may be the only thing to protect you from that
hacker becoming a man in the middle to the attack.

What should be the solution?

An option, in the browser, to allow the user to be warned the first time a
certificate changes on a webserver.  If the previous certificate is
expired, and the current certificate on a site is different, the user
should be warned of the change, and asked whether they wish the new
certificate to replace the previous one.  That way, paranoid users like
myself can be warned when a certificate changes, so that we can decide
whether the new certificate should be trusted.  Of course, if I already
trust the certificate signer, then I shouldn't be prompted about the
certificate.

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