RE: Can you impersonate a client side cert??

["charl van der walt" <charl () sensepost com>]

Hi,

> I have been reading a paper which was published back in Feb 2001 by a
> company call Sensepost which says that there is a way to impersonate a
> users
> client side cert by using the same common name. Does anybody have any
> experience of doing this or is it even possible considering that the
> users
> public part of the cert would be installed on the web server?


I'm the author of the paper being referred to...

We really have had such experience, and it is very possible.

The mistake you're making is in thinking that the user's public key is
stored on the web server; its not. The CA's public key is stored on the web
server, along with a number of other public keys from other CAs that are
installed by default.

In authenticating a user using certificates, the server has to ask itself
two questions (simplest case):

1) Does the 'Common Name' (CN) of this cert have access?
2) Do I trust the CA that issued this certificate

The second question is answered by looking in the list of trusted CAs that
have been configured. *Any* CA that has been left remaining on the list will
qualify as 'trusted'. This typically includes a number of public CAs like
Verisign and Thawte.

Thus, unless the administrator has specifically removed non-trusted CAs from
the list then such an attack is possible.

Ofcourse, there are some challenges:

a) You have to be able to derive the CN of the user you are trying to
impersonate. On an NT system there's a fair chance that there may be a user
called 'Administrator'.
b) You have to convince the CA to issue you a certificate in the format that
you require. For example: I know that Verisign no longer issues certificates
where the surname field is not blank. I'm not sure about other CAs though...
c) Etc...

Regards

charl

========================================================================
charl van der walt      +27 83 454 4203 SensePost Information Security
www.hackrack.com
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