Re: [websec] Coordinating Frame-Options and CSP UI Safety directives

David Ross <> Tue, 04 September 2012 05:06 UTC

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From: David Ross <>
To: Adam Barth <>, Tobias Gondrom <>
Thread-Topic: [websec] Coordinating Frame-Options and CSP UI Safety directives
Date: Tue, 04 Sep 2012 05:06:39 +0000
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Subject: Re: [websec] Coordinating Frame-Options and CSP UI Safety directives
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There was a bit of discussion earlier on the list w.r.t. multi-origin vs. single-origin for allow-from.  I'm very much in favor of single origin.  Just imagine a few years from now seeing a policy or header with 1500 origins -- if we allow that to happen, it will happen.

So I'm very happy to hear we can specify the single-origin syntax as suggested below.  If we're going to go with CSP for FO, I'd feel most comfortable if we can get some confirmation that this is the POR.

I agree that serving dynamic policy shouldn't be technically difficult.  But I do worry about this in a larger sense.  It would be good to brainstorm implications of dynamic policy.  Is there any impact to platforms, design patterns, etc. that may have been imagined / planned during the course of CSP's development?  It just feels like a non-trivial change to the way CSP was thought out.


-----Original Message-----
From: Adam Barth [] 
Sent: Monday, September 03, 2012 10:26 AM
To: Tobias Gondrom
Cc:;; David Ross
Subject: Re: [websec] Coordinating Frame-Options and CSP UI Safety directives

On Mon, Sep 3, 2012 at 9:35 AM, Tobias Gondrom <> wrote:
> On 02/09/12 23:40, Adam Barth wrote:
>> On Sun, Sep 2, 2012 at 4:40 AM, Tobias Gondrom 
>> <> wrote:
>>> Hello all,
>>> thank you for your feedback and input.
>>> <hat="individual">
>>> NIH (not invented here) should definitely never be reason for a 
>>> decision
>>> -
>>> in either way.
>>> And I would also be open to assist the WebAppSec WG in writing this 
>>> tiny bit into CSP.
>>> I have two main reasons why I think we should keep FO separate to 
>>> CSP and would like to hear your thoughts about it before we should 
>>> make a decision.
>>> (Please note that in the case of this topic I will obviously not be 
>>> acting as WG chair.)
>>> 1. Model and Semantic Reason:
>>> Until now, I always understood the CSP model to be about "describe 
>>> the security policy for a loaded resource and say which parts of 
>>> that content you can execute and which references in that content 
>>> you shall follow and execute".
>>> While the XFO/FO model is the reverse: describing for a resource, 
>>> defining by whom your resource may be framed/loaded from. In my view 
>>> that was not a natural part of the CSP model. And in my 
>>> understanding that semantic difference was also one of the reasons 
>>> why it was not done in CSP1.0 in the first place, but at the time 
>>> agreed to be done in websec separately.
>> Now that we're done with CSP 1.0, I think it make sense to take a 
>> more expansive view of the sorts of security policies that can be 
>> expressed in a Content-Security-Policy.  My view is that a security 
>> policy expressed in CSP ought to have the following properties:
>> 1) The policy should apply only to an individual resource 
>> representation.  That means that the security policy is scoped to the 
>> individual HTTP response and doesn't have broader-reaching effects 
>> (e.g., about future HTTP responses).
>> 2) The policy should only restrict privileges---not grant any 
>> privileges.  That means that the security policy is useful for 
>> implementing "least privilege": you can use it to drop privileges 
>> you're not using (e.g., the ability to execute inline script) so that 
>> an attacker can't trick you into using this privileges to your 
>> detriment.
>> X-Frame-Options / Frame-Options fits nicely into this rubric.
> Well. Hm. I am not sure that I would agree that extending the CSP 
> semantic model to the superset that then includes "defining by whom 
> your resource may be framed/loaded from" instead of the current CSP1.0 
> model is necessarily a good thing. Maybe one question: If we extend 
> the semantic of a system, it is always good if such extension is not 
> for a group size of "1" (aka only one individual directive). Therefore 
> my question: Are there other use cases (directives planned for 2.0) 
> that require the same type of semantic extension or would FO be unique 
> in that regard? If so, which semantic extensions would that be?

I'm not sure I understood your question, but I'll try to answer it anyway.  We're considering a number of new directives for CSP 1.1 that aren't related to loading subresources.  For example, the form-action directive is about restricting form submissions:

We've designed CSP to be extensible, as requested by <>.
 Frame-options falls well inside the scope we envision.

>>> Or as someone else from W3C WebAppSec wrote it more clearly to me 
>>> about a year ago:
>>> "... removing frame-ancestors from CSP altogether if a better, 
>>> standardized Frame-Options is available to sites. In fact, it 
>>> simplifies the model in some ways since frame-ancestors is currently 
>>> the only directive that restricts content from the embedded site's 
>>> perspective."
>> Having a simplified model has been very helpful to us over the past 
>> year because it has let us finish CSP 1.0.  Now we're done with CSP
>> 1.0 and looking at what the next step should be in CSP's evolution.
>>> 2. Technical Implementation:
>>> The current FO spec allows only one "Allow-From" URI. Which means 
>>> that for complex framing relationships, the FO header needs to be 
>>> written/sent on the fly on a per request basis.
>>> My question is, what happens to only one "ALLOW-FROM" if we 
>>> integrate it into CSP?
>>> Can we generate individual CSPs on the fly as well (including if a 
>>> CSP header references a file), or would this then implicitely mean 
>>> we have to allow a list of "ALLOW-FROM"?
>> Integrating with CSP imposes no technical restrictions in this regard.
>>   If you want to have only a single source, you can define the syntax
>> as:
>> directive-name = "frame-options"
>> directive-value = source-expression
>> Most other directives use source-list (which is just a list of 
>> source-expressions), but there's no reason we can't do something 
>> different here if that makes sense.  In fact, the only hard technical 
>> restriction on the directive-value is that it conform to the 
>> following
>> ABNF:
>> directive-value   = *( WSP / <VCHAR except ";" and ","> )
> Adam, I am sorry. I may not have been clear enough with my question.
> My concern is not with regard of whether we can define such a 
> directive (in my view this is trivial).
> But the question is whether we can practically implement such, or 
> whether there are implementation problems that would forbid (or 
> seriously
> discourage) dynamic generation of a CSP on a per request basis?

There are none that I'm aware of.

> Because that
> would be the consequence of only one single "Allow-From" URI (instead 
> of list).
> Or to put it differently:
> - Have there been successful and scaling implementations of CSP that 
> have generated the CSP header on the fly for every request?

I'm not aware of any in the public world, but I have seen (non-public) implementations that construct the script-src whitelist dynamically based on statically analyzing the HTML templates used to generate a particular page.  You might imagine that this design would be attractive in a system like Google Web Toolkit <>, where the framework has enough insight into how the app works to build a CSP policy.

> - If not, what would be the pitfalls/problems if we would do so?

There's nothing magical going on.  It's just as easy to generate a Frame-Options header dynamically as it is to generate a frame-options directive in a Content-Security-Policy header dynamically.

> - What are possible performance issues with that?

The performance considerations are the same regardless of whether you're using a Frame-Options header or a frame-options directive in a Content-Security-Policy header.  The two are isomorphic.

> - And last but not least, is there a caching of CSP use case? (which 
> could break if we generate the CSP file on the fly for every single 
> request...)

The Content-Security-Policy header is cached in exactly the same way as the Frame-Options header.  There is nothing magical going on.

> If we move FO to CSP, I would like to know whether this will break (or 
> due to implementation/scaling problems basically forbids) the current 
> design of "Allow-From" _before_ we do so.

It will not.

>>> (please note, that the initial version did allow a list for 
>>> "Allow-From", but there were serious concerns for performance in 
>>> implementation for large lists and privacy matters. The change to 
>>> only one "Allow-From" is not "written in stone", still I would like 
>>> to understand if we limit ourselves back to the "Allow-From list" 
>>> implicitly by putting it into CSP? I had a couple of private 
>>> conversations on this problem in the last months, but they could not 
>>> definitively answer to that question...)
>> I'm a bit surprised that you'd want to limit frame-options to having 
>> only one source-expression, but we can discuss that point regardless 
>> of whether we decide to integrate it with CSP.
> Actually, this was not my idea, but from Dave, who explained to me the 
> performance and privacy implications when going with a (potentially 
> long) list of allowed URIs. Personally, I could still see both ("list" or "single"
> URI), though I can understand the very serious concerns with "list".
> We can discuss the FO design decision later, however, as explained 
> above, if the choice to move FO into CSP basically pre-decides that we 
> must then go with a "list" (for practical/implementation reasons), I 
> would like to know and spell out this limitation rather now than later.

Moving to CSP does not imply an pre-decision on this topic.