W3C home > Mailing lists > Public > public-appformats@w3.org > October 2007

Re: [access-control] non-GET threat model and authorization choreography

From: Bjoern Hoehrmann <derhoermi@gmx.net>
Date: Sun, 21 Oct 2007 00:26:19 +0200
To: Ian Hickson <ian@hixie.ch>
Cc: public-appformats@w3.org
Message-ID: <umvkh356pk7aprts3am5jkjsqaudtn012p@hive.bjoern.hoehrmann.de>

* Ian Hickson wrote:
>Referer-Root gets sent with every request; how would it be used to 
>distinguish the two?

I didn't think you would. I take it that would again be motivated by the
list-too-long case, but sending the header on every request very poorly
accounts for that. The content of the header is highly sensitive, if you
make a couple of popular web services that anyone can embed, you'd get a
steady stream of information about what web sites people visit including
intranet sites that make use some of the services. That you don't get to
know exactly what pages they visit makes little difference. So sending
it out so gratiously would quickly lead people to filter it out.

Further, the whole mechanism would be intransparent: you have no way of
saying a particular access has been "granted" or "denied" on a case by
case basis, which is especially important in the access check request
case. In Anne's cache the Referer-Root is not actually stored there,
leading to false positives if you do not prevent caching of the result,
or to false negatives if you do add it. Preventing caching is not really
possible either, at best you could try to set a bogus expiration date
to circumvent your keep-it-cached-for-a-little-bit policy.

The draft does not actually mention case-by-case policies as an option,
so it would seem we have an overly intrusive, wasteful, awkward, under-
documented mechanism that's supposed to simplify some minority of cases.
Those cases seem way too few to simplify them and I have doubts they're
really being simplified by Referer-Root; but I am happy to read up on a
detailed analysis of the problem meant to be solved here, if anyone can
point me to it. That'd also help to come up with alternate solutions if
they are actually needed.

>Could you elaborate on this? Some examples might be helpful as I'm not 
>sure I follow. I thought the proposal was to have a separate cache 
>(non-HTTP) for the pre-flight test requests.

Yes, and the response to that request, as well as ordinary requests, may
be served from a cache, which may very well serve the wrong variant if
you don't properly indicate that there are multiple variants and prevent
caching of the variant as necessary. Authors usually implement this in-
correctly, especially if they are trying to minimize the number of times
the document is fetched as they would in your expensive-to-compute case
(or the more general dont-want-to-waste-traffic case due to using GET.)

>Redirects seem inefficient, requiring two network accesses and 
>corresponding access checks, and are also far more prone to 
>inconsistencies due to the extra checks.

I didn't think you would follow redirects to do an access check request.

>Actually the "Allow" line above is directly copied and pasted from the 
>response headers sent in response to an OPTIONS to this URI:
>   http://software.hixie.ch/utilities/cgi/test-tools/echo
>...which is a CGI script.

Very old versions of the Apache server don't let mod_cgi scripts handle
OPTIONS requests without additional configuration. That was changed two
years ago, and you have a range of other options to satisfy the request,
such as using mod_perl or mod_php, mod_headers, mod_rewrite, etc.

>The XML PIs are an important convenience because they work in all other 
>cases for this API and consistency here is important (consistency in 
>security APIs is critical -- anything surprising in security APIs will 
>almost always lead to security holes).

Access check requests are completely different from all other cases; the
purpose, from my perspective, of the processing instruction is to hinder
hostile applications from reading confidential data from a document. Now
with access check requests you never make the document you get available
to hostile applications, so reading it makes little sense to me.

Consistency and avoiding surprises is certainly important. If you want
to find out what access options are available for a resource, you use
the OPTIONS method to find out, certainly you would in the same origin
case. Using GET instead is a surprise. The Allow header is also usually
only used with OPTIONS, using it with GET instead is another surprise.
You actually have Method-Check to work around adverse effects of these

During the access check request, you are also only interested what the
access options for one particular resource are. Above you indicate you
would follow redirects; doing that can only tell you something about the
options available for other resources, so this is yet another surprise.
Of course you probably follow redirects in "all other cases", so that
might explain that, though I don't quite see how it'd work (may I post
to X? Please go to Y instead. May I post to Y? Yes. Then post to X!?)

>Sending the information in the "ultimate request" seems like it would make 
>it somewhat difficult for the original request to have the information, 
>which is required, as I explained in my last e-mail.

The information is redundant at that point. The purpose of the access
check request is to find out what the server is prepared to handle. If
it answers accurately, there cannot be a problem afterwards; in the
worst case it would tell the client to proceed and reject the following
request. Generally, it will have to do that anyway.
Björn Höhrmann · mailto:bjoern@hoehrmann.de · http://bjoern.hoehrmann.de
Weinh. Str. 22 · Telefon: +49(0)621/4309674 · http://www.bjoernsworld.de
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Received on Saturday, 20 October 2007 22:26:33 UTC

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