Re: [TLS] AIA cert fetching seen as harmful

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

>> 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.
> 
> Are you all aware of this paper, now making a stir?
> 
>     https://www.cynops.de/techzone/http_over_x509.html
> 
> It claims that fetching CA certs from URLs found in AIA extensions in certs
> that have not yet been validated is a vulnerability.  At least one browser
> organization known to me agrees.

What I suggested is that the information about which URL's are safe
for the client certificate URL extension could be embedded in the
-root- certificate, which you trust.  When a client sends you a URL
in place of a certificate, you would compare it to the information
in each of your root certificates.  If the URL matches one of them,
you know it's safe to retrieve it; otherwise you best not.  Also,
theoretically it would be wasted effort since you would not be able
to build a complete certificate chain.  So this could also improve
performance in cases where client authentication will fail.

Mike
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