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Re: New header for "Fragment-Scope"?

From: Brad Hill <hillbrad@fb.com>
Date: Sat, 10 Jan 2015 22:22:14 +0000
To: "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
Message-ID: <D0D6E582.33BD%hillbrad@fb.com>
Here's an example that I hope will clarify:

http://tools.ietf.org/html/rfc6749#section-4.2



In OAuth, an Access Token Request looks like the following:

GET 
/authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz&redirect_uri=
https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
Host: server.example.com

If the authorization server is willing to process this request, it
responds like so:

     HTTP/1.1 302 Found
     Location: 
https://client.example.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&state=xyz
&token_type=example&expires_in=3600


The fragment containing the access token is not sent with the subsequent
HTTP request to client.example.com, but if client.example.com responds
with text/html content, the user agent applies the fragment, which can be
read by e.g. script in the DOM.

The nature of OAuth and similar protocols is that the redirect location is
passed in with the request (in this example, the redirect_uri paremeter).
If that redirect_uri is maliciously modified, the server might send
credentials to an incorrect host.  The common mitigation to this is to
whitelist the set of uris that server.example.com is willing to redirect
to.

But even if server.example.com is only ever willing to redirect to
https://client.example.com/cb, what if there exists unknown processing
logic at that endpoint (the attacker may add query string parameters not
part of the whitelist, exploit additional client state like cookies,
exploit differences in url parsing between client, sever and browser,
etc., or perhaps the whitelist is only at the granularity of a server)
such that additional redirects are triggered from client.example.com to
evil.com?, 

The UA issues a request for https://client.example.com/cb, and in this
case gets a new 302 response with a redirect to evil.com, requests that,
and then applies the fragment to the resulting text/html resource.
Evil.com now has the access token.

What Andrey and I are suggesting is that the initial 302 being sent by the
OAuth authorization server could look like:

HTTP/1.1 302 Found
Location: 
https://client.example.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&state=xyz
&token_type=example&expires_in=3600
Fragment-Scope: same-origin


This would instruct the UA that the fragment is only intended for
client.example.com, and should be discarded if client.example.com replies
with another redirect to a different origin.

Note that server.example.com here is not returning a resource or
constructing a DOM, so there is no opportunity to apply parameters in the
fetch algorithm, nor to rely on media-type specific handling of fragments.
 There isn't really even an OAuth "protocol" here to fix - it's riding
entirely on top of HTTP semantics at this point.

For Facebook and other large OAuth providers, "client.example.com" is
really thousands or tens of thousands of different endpoints. These have
all kinds of various and sundry peculiarities, but one of the most common
is redirect behavior like this.  We would like to be able, in the role of
authorization server, enlist the assistance of the user agent to mitigate
many these vulnerabilities by providing additional information about how
to dispose of a sensitive fragment when chaining 3xx responses.

Brad Hill
Received on Saturday, 10 January 2015 22:23:28 UTC

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