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Re: ISSUE-4: Policy combination

From: Giorgio Maone <g.maone@informaction.com>
Date: Thu, 08 Dec 2011 21:10:50 +0100
Message-ID: <4EE119CA.1050400@informaction.com>
To: public-webappsec@w3.org
+1 for A, first seen wins.

-- G

Adam Barth wrote, On 08/12/2011 20.35:
> One of our open issues is about how to deal with multiple CSP policies
> for a given resource.  At TPAC, one resolution we discussed was the
> following:
> 
> 1) If a resource has multiple HTTP headers containing CSP policies,
> enforce all of the policies.  Because CSP policies only reduce
> privileges (never grant privileges), that effectively means that an
> action is allowed only if it is allowed by all the CSP policies.
> 
> 2) If a resource has a CSP policy from an HTTP header, then we ignore
> any CSP policies that might be contained in <meta> elements.
> Otherwise, the user agent enforces all the CSP policies found in
> <meta> elements.
> 
> Another resolution (which I advocate) is the following:
> 
> A) The first CSP policy the user agent encounters for a document wins.
> 
> IMHO, approach (A) is better than approach (1+2) for two reasons.
> First, it's simpler.  CSP is already more complex that it should be.
> Adding more complexity is costly, both now in terms of implementation
> and in the future in terms of constraints.
> 
> Second, approach (1+2) constrains future evolution of CSP.  For
> example, suppose we wanted to include
> http://wiki.whatwg.org/wiki/Meta_referrer as a CSP directive.  How
> would we define the combination of policies containing referrer
> directives?  We'd have to define some ordering like "never < origin <
> always", but where does default fit in?
> 
> These are, in some sense, the same concern.  We can implement
> combination today, but it imposes constrains on the future that we
> might wish we didn't have later.
> 
> Adam
> 
Received on Thursday, 8 December 2011 20:11:10 GMT

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