Re: [TLS] Implementation survey: Client Certificate URL extension

[Mike <mike-list at pobox.com>]

>>>> This vulnerability of Client Certificate URL is already described in
>>>> the Security Considerations text in RFC 4366, so it isn't anything
>>>> particularly new.
> 
> Security Considerations, section 6.3 of rfc4366 already describes the
> problems and implications in quite detail (I wasn't aware of that).
> But maybe that isn't scary enough?  Personally, I feel quite uncomfortable
> with such a dangerous extension in an IETF standard.  *I* can NOT think
> of a safe mode of operation for the client certificate URL extension.
> As soon as the admins enables it, his server is toast.  Do we really need
> to standardize something that is so extremely dangerous?

This could be made safe with some help from PKIX (if X.509 doesn't
already support it -- I haven't read RFC 3280 or -bis in a while).
If root certificates listed constraints on what constitutes a valid
URL for retrieving issued certificates, then a server could scan
the combined list from each trusted root to determine if it is safe
to fetch a client certificate.

Mike
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