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From "Jason Smith (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-431) Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
Date Tue, 31 May 2011 04:39:47 GMT


Jason Smith commented on COUCHDB-431:

Alex, thanks for your thoughts. Some feedback:

I disagree that CORS is not hard security. It is hard security because same-origin is the
primary protector of all data on the web. Without same-origin restrictions, any site you visit
could reach your data on any other site you visit. It is also hard security in that it is
both difficult and important to get right.

I am hoping for a fail-safe system.

It seems reasonable that the database admin should control the CORS permissions of that database.
For a /_config change, that requires the server admin.

The _config imposes practical problems too. It is not a general system registry: all the data
must go section-key-value. There is no way to have a per-database config except to use an
entire new section as a namespace, with each DB name inside it. IMO I don't like that at all.

The only place I know of for per-database config is the _security object. Since CORS is about
security, it's logical to place it there (where the db admin can modify it).

Finally, the original idea that I heard about CORS was to allow people to specify any header.
I don't think that is fail-safe. I have no idea what headers people use out there. It seems
impossible to evaluate the security of permitting any header for any response.

Finally, CORS headers are generally generated dynamically. The only exception is the wildcard
header which for CouchDB would be a very dangerous setting. That means any code from any site
on any browser can access your couch without the user knowing but (potentially) with the user's
couch credentials. Therefore, I am hoping for a whitelist to specify, "yes,
is my own website and I trust all its code to access this couch".

> Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
> -----------------------------------------------------------------------------------
>                 Key: COUCHDB-431
>                 URL:
>             Project: CouchDB
>          Issue Type: New Feature
>          Components: HTTP Interface
>    Affects Versions: 0.9
>            Reporter: James Burke
>            Assignee: Randall Leeds
>            Priority: Minor
>         Attachments: A_0001-Generalize-computing-the-appropriate-headers-for-any.patch,
A_0002-Send-server-headers-for-externals-responses.patch, A_0003-Usably-correct-w3c-CORS-headers-for-valid-requests.patch,
A_0004-Respond-to-CORS-preflight-checks-HTTP-OPTIONS.patch, cors.html
> Historically, browsers have been restricted to making XMLHttpRequests (XHRs) to the same
origin (domain) as the web page making the request. However, the latest browsers now support
cross-domain requests by implementing the Access Control spec from the W3C:
> In order to keep older servers safe that assume browsers only do same-domain requests,
the Access Control spec requires the server to opt-in to allow cross domain requests by the
use of special HTTP headers and supporting some "pre-flight" HTTP calls.
> Why should CouchDB support this: in larger, high traffic site, it is common to serve
the static UI files from a separate, differently scaled server complex than the data access/API
server layer. Also, there are some API services that are meant to be centrally hosted, but
allow API consumers to use the API from different domains. In these cases, the UI in the browser
would need to do cross domain requests to access CouchDB servers that act as the API/data
access server layer.
> JSONP is not enough in these cases since it is limited to GET requests, so no POSTing
or PUTing of documents.
> Some information from Firefox's perspective (functionality available as of Firefox 3.5):
> And information on Safari/Webkit (functionality in latest WebKit and Safari 4):
> IE 8 also uses the Access Control spec, but the requests have to go through their XDomainRequest
object (XDR):
> and I thought IE8 only allowed GET or POST requests through their XDR.
> But as far as CouchDB is concerned, implementing the Access Control headers should be
enough, and hopefully IE 9 will allow normal xdomain requests via XHR.

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