Synnefo

Serve untrusted user content

^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The current document describes how the pithos view accesses the protected

pithos resources and proposes a new design for granting access to

the specific resources. It sketches implementation details and configuration

concerns.

Current state and shortcomings

==============================

Pithos in order to identify the user who requests to view a pithos resource

uses the information stored by astakos in the cookie after a successful user

authentication login.

The main drawback of this design is that the pithos view which serves untrusted

user content has access to the sensitive information stored in the cookie.

Abstract

========

We want to serve untrusted user content in a domain which does not have access

to sensitive information. Therefore, we would like the pithos view to be

deployed in a domain outside the astakos cookie domain.

We propose a design for the pithos view to grant access to the

access-restricted resource without consulting the cookie.

Briefly the pithos view will request a short-term access token for a specific

resource from astakos. Before requesting the access token, the view should

obtain an authorization grant (authorization code) from astakos, which will be

presented by the view during the request for the access token.

The proposed scheme follows the guidelines of the OAuth 2.0 authentication

framework as described in http://tools.ietf.org/html/rfc6749/.

Proposed changes

================

We propose to alter the view to obtain authorization according to the

authorization code grant flow.

The general flow includes the following steps:

#. The user requests to view the content of the protected resource.

#. The view requests an authorisation code from astakos by providing its

   identifier, the requested scope, and a redirection URI.

#. Astakos authenticates the user and validates that the redirection URI

   matches with the registered redirect URIs of the view.

   As far as the pithos view is considered a trusted client, astakos grants the

   access request on behalf of the user.

#. Astakos redirects the user-agent back to the view using the redirection URI

   provided earlier. The redirection URI includes an authorisation code.

#. The view requests an access token from astakos by including the

   authorisation code in the request. The view also posts its client identifier

   and its client secret in order to authenticate itself with astakos. It also

   supplies the redirection URI used to obtain the authorisation code for

   verification.

#. Astakos authenticates the view, validates the authorization code,

   and ensures that the redirection URI received matches the URI

   used to redirect the client.

   If valid, astakos responds back with an short-term access token.

#. The view exchanges with astakos the access token for the information of the

   user to whom the authoritativeness was granted.

#. The view responds with the resource contents if the user has access to the

   specific resource.

Implementation details

======================

Pithos view registration to astakos

-----------------------------------

The pithos view has to authenticate itself with astakos since the latter has to

prevent serving requests by unknown/unauthorized clients.

Each oauth client is identified by a client identifier (client_id). Moreover,

the confidential clients are authenticated via a password (client_secret).

Then, each client has to declare at least a redirect URI so

that astakos will be able to validate the redirect URI provided during the

authorization code request. If a client is trusted (like a pithos view) astakos

grants access on behalf of the resource owner, otherwise the resource owner has

to be asked.

We can register an oauth 2.0 client with the following command::

    snf-manage oauth2-client-add <client_id> --secret=<secret> --is-trusted --url <redirect_uri>

For example::

    snf-manage oauth2-client-add pithos-view --secret=12345 --is-trusted --url https://pithos.synnefo.live/pithos/ui/view

Configure view credentials in pithos

------------------------------------

To set the credentials issued to pithos view in order to authenticate itself

with astakos during the resource access token generation procedure we have to

change the ``PITHOS_OAUTH2_CLIENT_CREDENTIALS`` setting.

The value should be a (<client_id>, <client_secret>) tuple.

For example::

    PITHOS_OAUTH2_CLIENT_CREDENTIALS = ('pithos-view', 12345)

Authorization code request

--------------------------

The view receives a request without either an access token or an authorization

code. In that case it redirects to astakos's authorization endpoint by adding

the following parameters to the query component using the

"application/x-www-form-urlencoded" format:

    response_type:

        'code'

    client_id:

        'pithos-view'

    redirect_uri:

        the absolute path of the view request

    scope:

        the user specific part of the view request path

For example, the client directs the user-agent to make the following HTTP

request using TLS (with extra line breaks for display purposes only)::

    GET /astakos/oauth2/auth?response_type=code&client_id=pithos-view

        &redirect_uri=https%3A//pithos.synnefo.live/pithos/ui/view/b0ee4760-9451-4b9a-85f0-605c48bebbdd/pithos/image.png

        &scope=/b0ee4760-9451-4b9a-85f0-605c48bebbdd/pithos/image.png HTTP/1.1

        Host: accounts.synnefo.live

Access token request

--------------------

Astakos's authorization endpoint responses to a valid authorization code

request by redirecting the user-agent back to the requested view

(redirect_uri parameter).

The view receives the request which includes the authorization code and

makes a POST request to the astakos's token endpoint by sending the following

parameters using the "application/x-www-form-urlencoded" format in the HTTP

request entity-body:

    grant_type:

        "authorization_code"

    code:

        the authorization code received from the astakos.

    redirect_uri:

        the "redirect_uri" parameter was included in the authorization request

Since the pithos view is registered as a confidential client it MUST

authenticate with astakos by providing an Authorization header including the

encoded client credentials as described in

http://tools.ietf.org/html/rfc2617#page-11.

For example, the view makes the following HTTP request using TLS (with extra

line breaks for display purposes only)::

     POST /astakos/oauth2/token HTTP/1.1

     Host: accounts.synnefo.live

     Authorization: Basic cGl0aG9zLXZpZXc6MTIzNDU=

     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA

     &redirect_uri=https%3A//pithos.synnefo.live/pithos/ui/view/b0ee4760-9451-4b9a-85f0-605c48bebbdd/pithos/image.png

Access to the protected resource

--------------------------------

Astakos's token endpoint replies to a valid token request with a (200 OK)

response::

     HTTP/1.1 200 OK

     Content-Type: application/json;charset=UTF-8

     Cache-Control: no-store

     Pragma: no-cache

     {

       "access_token":"2YotnFZFEjr1zCsicMWpAA",

       "token_type":"Bearer",

       "expires_in":20

     }

The view redirects the user-agent to itself by adding to the query component

the access token.

The view receives the request which includes an access token and requests

from astakos to validate the token by making a GET HTTP request to the

astakos's validation endpoint::

    GET /astakos/identity/v2.0/tokens/2YotnFZFEjr1zCsicMWpAA?belongsTo=/b0ee4760-9451-4b9a-85f0-605c48bebbdd/pithos/image.png HTTP/1.1

    Host: accounts.synnefo.live

The astakos's validation endpoint checks whether the token is valid, has not

expired and that the ``belongsTo`` parameter matches with the ``scope``

parameter that was included in the token request.

If not valid returns a 404 NOT FOUND response.

If valid, returns the information of the user to whom the token was assigned.

In the former case the view redirects to the requested path

(without the access token or the authorization code) in order to re-initiate

the procedure by requesting an new authorization code.

In the latter case the view proceeds with the request and if the user has access

to the requested resource the resource's data are returned, otherwise the

access to resource is forbidden.

Authorization code and access token invalidation

------------------------------------------------

Authorization codes can be used only once (they are deleted after a

successful token creation)

Token expiration can be set by changing the ``OAUTH2_TOKEN_EXPIRES`` setting.

By default it is set to 20 seconds.

Tokens granted to a user are deleted after user logout or authentication token

renewal.

Expired tokens presented to the validation endpoint are also deleted.

Authorization code and access token length

------------------------------------------

Authorization code length is adjustable by the

``OAUTH2_AUTHORIZATION_CODE_LENGTH`` setting. By default it is set to

60 characters.

Token length is adjustable by the ``OAUTH2_TOKEN_LENGTH`` setting.

By default it is set to 30 characters.

Restrict file serving endpoints to a specific host

--------------------------------------------------

A new setting ``PITHOS_UNSAFE_DOMAIN`` has been introduced. When set,

all api views that serve pithos file contents will be restricted to be served

only under the domain specified in the setting value.

If an invalid host is identified and request HTTP method is one

of ``GET``, ``HOST``, the server will redirect using a clone of the request

with host replaced to the one the restriction applies to.