Re: [TLS] AIA cert fetching seen as harmful

[Nelson B Bolyard <nelson at bolyard.com>]

Eric Rescorla wrote, On 2008-04-10 19:15:
> At Thu, 10 Apr 2008 18:28:54 -0700,
> Nelson B Bolyard wrote:
>> Eric Rescorla wrote, On 2008-04-10 18:08:
>>> At Thu, 10 Apr 2008 17:45:06 -0700,
>>> Nelson B Bolyard wrote:
>>>> Mike wrote, On 2008-04-10 09:01:
>>>>
>>>>> 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
>>> Yes, Martin cited this paper a few weeks ago.
>>>
>>>> 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.
>>> How does that organization feel about inline images in HTML pages?
>> The problem isn't so much when browsers initiate fetches for certs from
>> servers.  The major concerns are:
>> a) servers fetching URLs from unvetted client auth certs, and
>> b) mail clients fetching certs to verify signatures in emails from strangers.
>>
>> Some email clients, in particular, are good at not fetching remote content
>> from html emails, which confirms email addresses to spammers.  AIA cert
>> fetching weakens their ability to defend against such attempts to validate
>> email addresses.
>>
>> Servers see them selves as similarly weakened.
>>
>> I'm receiving inquiries about white listing CA URLs for AIA fetching. :(
> 
> I assume these people are up in arms about DKIM, then?

No, they're just not doing DKIM.  Otherwise they might be. :)
With respect to email, their concern is presently limited to S/MIME v3.
Their concern is: take an existing S/MIME v3 MUA and upgrade the cert
verification library to one that can do AIA cert fetching: vulnerable.
But of course, this email concern is a bit off topic for this list. :)
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