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Subject: Re: [xacml-users] Policy with subject specific resources

Thank you for your response. I have expanded on the question below.

> On tis, 2012-02-28 at 12:49 +0000, Richard.Wilkinson@tessella.com wrote:
>> I have been asked to investigate how we should implement a XACML
>> policy to authorise a user to access a personal section of a web
>> application, and would be grateful for comments. I am aware of the
>> method described in
>> http://lists.oasis-open.org/archives/xacml/200404/msg00076.html, which
>> addresses the type of problem I am concerned with. It is preferred to
>> not use the subject-id as part of the URL, so that the URLs cannot be
>> guessed. ....
> Does that mean that if you guess the right URL you can circumvent access
> control? That doesn't seem very secure to me.

No - the PEP, PDP and policy would control access. This requirement came to me
second hand and I may be overstating it. The simplest policy would be one that
grants access for some action to user authenticated with subject-id Alice to,
say, resource-id:

However, the subject-id may be such that a URL constructed as above is not
appropriate (it could contain '/'s or characters that are illegal in URIs). In
this case, some form of translation is needed (which could simply be URL
encoding the subject-id). The question is then where and how this translation
is performed. If it occurs in the PEP, the PEP has to know about the specifics
of the application, i.e., that a user has a private resource with URL
contructed from the subject-id in a particular way. Our PEP is a filter that
normally has no knowledge of the application. Another possibility is to use a
translation function in the policy, but again, this would have to be
non-standard. If the function performed, URL encoding, it isn't specific to the
application, but would be a custom function. If it had to look up something in
a database shared with the application, it is more specific. My question was
really about how to minimise the customisation of the PDP and/or PEP; ideally,
it would be possible to avoid customising either.

>> Is there a better method (where "better" includes requiring less/no
>> custom modification to the PDP or PEP, scaling well with number of
>> users and being manageable)?
> Don't you have an authenticated session or something similar with the
> web app? You could save a reference to the subject-id in a cookie and
> retrieve it from a Map at the server side using the session context.
> ...

We already have an authentication system that makes the subject-id available.
Existing policies use subject attributes obtained from a PIP.

> Hope it helps,
> Ludwig
> --
> Ludwig Seitz, PhD
> Swedish Institute of Computer Science
> Ideon Science Park
> Building Beta 2 3v
> Scheelevägen 17
> SE-223 70 Lund

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