========================================================================
Internet Explorer SSL Vulnerability 08/05/02
Mike Benham <moxie@thoughtcrime.org>
http://www.thoughtcrime.org

========================================================================
Abstract

Internet Explorer's implementation of SSL contains a vulnerability that
allows for an active, undetected, man in the middle attack.  No dialogs
are shown, no warnings are given.

========================================================================
Description

In the normal case, the administrator of a web site might wish to provide
secure communication via SSL.  To do so, the administrator generates a
certificate and has it signed by a Certificate Authority.  The generated
certificate should list the URL of the secure web site in the Common Name
field of the Distinguished Name section.

The CA verifies that the administrator legitimately owns the URL in the CN
field, signs the certificate, and gives it back.  Assuming the
administrator is trying to secure www.thoughtcrime.org, we now have the
following certificate structure:

[CERT - Issuer: VeriSign / Subject: VeriSign]
-> [CERT - Issuer: VeriSign / Subject: www.thoughtcrime.org]

When a web browser receives this, it should verify that the CN field
matches the domain it just connected to, and that it's signed using a
known CA certificate.  No man in the middle attack is possible because it
should not be possible to substitute a certificate with a valid CN and a
valid signature.

However, there is a slightly more complicated scenario.  Sometimes it is
convenient to delegate signing authority to more localized authorities.
In this case, the administrator of www.thoughtcrime.org would get a chain
of certificates from the localized authority:

[Issuer: VeriSign / Subject: VeriSign]
-> [Issuer: VeriSign / Subject: Intermediate CA]
   -> [Issuer: Intermediate CA / Subject: www.thoughtcrime.org]



When a web browser receives this, it should verify that the CN field of
the leaf certificate matches the domain it just connected to, that it's
signed by the intermediate CA, and that the intermediate CA is signed by a
known CA certificate.  Finally, the web browser should also check that all
intermediate certificates have valid CA Basic Constraints.

You guessed it, Internet Explorer does not check the Basic Constraints.

==========================================================================
Exploit

So what does this mean?  This means that as far as IE is concerned, anyone
with a valid CA-signed certificate for ANY domain can generate a valid
CA-signed certificate for ANY OTHER domain.

As the unscrupulous administrator of www.thoughtcrime.org, I can generate
a valid certificate and request a signature from VeriSign:

[CERT - Issuer: VeriSign / Subject: VeriSign]
-> [CERT - Issuer: VeriSign / Subject: www.thoughtcrime.org]

Then I generate a certificate for any domain I want, and sign it using my
run-of-the-mill joe-blow CA-signed certificate:

[CERT - Issuer: VeriSign / Subject: VeriSign]
-> [CERT - Issuer: VeriSign / Subject: www.thoughtcrime.org]
   -> [CERT - Issuer: www.thoughtcrime.org / Subject: www.amazon.com]

Since IE doesn't check the Basic Constraints on the www.thoughtcrime.org
certificate, it accepts this certificate chain as valid for
www.amazon.com.

Anyone with any CA-signed certificate (and the corresponding private
key) can spoof anyone else.

========================================================================
Severity



I would consider this to be incredibly severe.  Any of the standard
connection hijacking techniques can be combined with this vulnerability
to produce a successful man in the middle attack.  Since all you need is
one constant CA-signed certificate (and the corresponding private key), an
attacker can use that to generate spoofed certificates for other domains
as connections are intercepted (on the fly).  Since no warnings are given
and no dialogs are shown, the attacker has effectively circumvented all
security that an SSL certificate provides.

There is some level of accountability, though, since a user who randomly
chooses to view the certificate of the web site she's visiting will see
the attacker's certificate in the chain.  However, by that point the
damage has already been done.  All "secure" data has already been
transmitted.

=========================================================================
Affected Browsers

Netscape 4.x and Mozilla are NOT vulnerable.

IE 5 and 5.5 are vulnerable straight-up, and IE 6 is mostly vulnerable.

When VeriSign issues certificates, usually they leave out the CA Basic
Constraint information all together.  Thawte tends to explicitly put in a
Basic Constraint CA = FALSE with the critical bit set to TRUE.

When the CA Basic Constraint on the middle certificate is explicitly set
to false and marked as critical, IE 6 does not follow the chain.  When
it's not mentioned at all, IE 6 follows the chain and is vulnerable.

This just means that an attacker needs to use a VeriSign-issued
certificate to exploit IE 6.

=========================================================================
Working Exploit

I've set up a URL to demonstrate this problem.  If you want to test
browsers not listed above or need proof of this vulnerability, contact me
and I'll give you the information.

=========================================================================
Vendor Notification Status

Last week I saw Microsoft downplay and obfuscate the severity of the
IE vulnerability that Adam Megacz reported.  After seeing that, I don't
feel like wasting time with the Microsoft PR department.

- Mike

--
http://www.thoughtcrime.org