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Re: CORS Header Filtering?

From: Nathan <nathan@webr3.org>
Date: Wed, 12 May 2010 21:21:46 +0100
Message-ID: <4BEB0DDA.7030201@webr3.org>
To: Tyler Close <tyler.close@gmail.com>
CC: Devdatta <dev.akhawe@gmail.com>, public-webapps <public-webapps@w3.org>
Tyler Close wrote:
> On Wed, May 12, 2010 at 1:05 PM, Nathan <nathan@webr3.org> wrote:
>> Tyler Close wrote:
>>> On Wed, May 12, 2010 at 12:33 PM, Nathan <nathan@webr3.org> wrote:
>>>> Yes,
>>>>
>>>> The simplest argument I can give is that we (server admins) are trusted
>>>> to
>>>> set the CORS headers, but not to remove any headers we don't want an XHR
>>>> request to see - this is frankly ridiculous.
>>> The problem is there might not be a single server admin but many.
>>> Quoting from the UMP spec:
>>>
>>> """
>>> Some HTTP servers construct an HTTP response in multiple stages. In
>>> such a deployment, an earlier stage might produce a uniform response
>>> which is augmented with additional response headers by a later stage
>>> that does not understand a uniform response header. This later stage
>>> might add response headers with the expectation they will be protected
>>> by the Same Origin Policy. The developer of the earlier stage might be
>>> unable to update the program logic of the later stage. To accommodate
>>> this deployment scenario, user-agents can filter out response headers
>>> on behalf of the server before exposing a uniform response to the
>>> requesting content.
>>> """
>>>
>>> http://dev.w3.org/2006/waf/UMP/#response-header-filtering
>>>
>>> I believe the design presented in UMP for response header filtering
>>> addresses all use-cases, including your "Location" header example
>>> below.
>> Yes that pretty much covers it, can you confirm if "Uniform-Headers" would
>> include the Link header as white-listed? That's the last remaining crucial
>> one not covered. (Link header is standards track now).
> 
> The response would have to also include the header "Uniform-Headers: Link"
> 
>> BTW: I will point out that I hadn't reviewed the UMP spec yet so thisn't
>> isn't any political or preference thing.
>>
>> I still stand by my statement though, CORS cannot possible go through to REC
>> status without the headers whitelisted in UMP + the Link header.
>>
>> Although my preference for both specs would be a Blacklist..
> 
> We can't know the names of all the possibly dangerous headers. A
> dynamic whitelist defined by the server is the best we can do.

Ahh now that I understand what Uniform Headers does that's a perfectly 
fine solution.

That was somewhat less painful than I expected!

Although if CORS goes through to REC and is implemented widely then it 
really doesn't matter what's in UMP does it? - or maybe i misunderstand 
the rather complication situation.

Best,

Nathan
Received on Wednesday, 12 May 2010 20:23:13 GMT

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