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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 Fri, 26 Aug 2011 04:46:29 GMT


Jason Smith commented on COUCHDB-431:

Benoit, love the patch. I originally developed _security object support but now I think it
has several problems.

CORS is fundamentally about which *origin* may query cross-site, and whether the query must
be anonymous, or may be authenticated. The best model is to specify which domains (origins)
Couch trusts, and to what degree (anonymous vs. authenticated).

A couch app lives within CouchDB but not completely within a database. Global handlers are
generally needed for couch apps (/, /_replicate, /_utils, _session, _uuids). For cross-domain
Couch to be useful, the global handlers must have a compatible CORS policy as the database.
_session is particularly important because if your session on couch has expired, the page
on needs a way to log you back in transparently.

Applications can use multiple databases. Once CORS ships, I envision a Cambrian explosion
of couch services and Javascript APIs to add couch features to any web page. Many apps will
provide one database per user. Keeping _security synchronized will be one extra hurdle for
developers to clear.

Finally, CORS is security-sensitive. I would hate to ship an XSS security vulnerability. In
your patch, set_default_cors_headers seems to set Access-Control-Allow-Origin: $your_origin;
Access-Control-Allow-Credentials: true. Those headers mean that any site on the Internet can
force any user to perform any query on the Couch, with their session cookie. That is probably
undesired. I have not audited your patch completely so perhaps I am wrong.

At this point, I wonder if you agree with me that DB _security settings could be omitted for
this time, and perhaps added later?

> 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, 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, test_cors2-1.tgz, test_cors2.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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