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From "Randall Leeds (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-431) Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
Date Sun, 18 Sep 2011 02:27:09 GMT


Randall Leeds commented on COUCHDB-431:

I propose the following concrete actions:

1) Make default Access-Control-Allow-Credentials: false, which only allows unauthenticated
requests. This would allow the cross-origin browser client  to do no more than any other client
on the web. This seems safe AND useful. I'd even be fine with CORS enabled by default if this
is the case.

2) Punt on everything but the db-level _security object. No CORS for resources like /_session,
etc. As an example of why it seems like you need /_session but you don't, remember that a
couchapp could expose a well known URL for a login page popup, which is very much like browserid,
facebook connect, etc.

3) Have a separate, default-off toggle for enabling the _admin role on authenticated requests.
In other words, if the request is a CORS request AND Access-Control-Allow-Credentials is on
for the origin, couch should by default strip the _admin role (if the cookie'd user has it).

4) Does it make sense to configure Access-Control-Allow-Credentials information on the user
document? In other words, some couch trusts a set of origins for a particular set of dbs (apps),
but users still need to allow it as well. This behavior mirrors things like OAuth: api key
: origin enabled :: auth flow : allowing credential use

I think this gets us 80% of the use cases and feels pretty safe.
Concrete suggestions/recommendations/criticisms? If you want me to implement any of this let
me know. I'd be happy to do it. I'd rather see features stripped out of this and have a minimal
viable CORS in 1.2 than to leave it out completely because it's, uh... next level beats for
couchapps. :-D.

> 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: Benoit Chesneau
>            Priority: Minor
>             Fix For: 1.2
>         Attachments: 0001-cors-support.-should-fix-COUCHDB-431-2.patch, 0001-cors-support.-should-fix-COUCHDB-431.patch,
0001-cors-support.-should-fix-COUCHDB-431.patch, 0001-cors-support.-should-fix-COUCHDB-431.patch,
0001-cors-support.-should-fix-COUCHDB-431.patch, 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, cors_test.html, test_cors2-1.tgz,
> 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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