Oauth -- shall we start using this?

Xavier Lamien laxathom at fedoraproject.org
Sat Mar 9 14:03:59 UTC 2013


On Fri, Mar 8, 2013 at 11:07 PM, Toshio Kuratomi <a.badger at gmail.com> wrote:

> So in the past week a bunch of us have been talking about API Keys, OAuth,
> passwords, and other means of managing authn and authz in the web apps that
> are up and coming (specifically mentioned were copr and datagrepper).
> Puiterwijk has put in some time reading the OAuth specifications and on
> Friday he walked me through how OAuth is supposed to work.  I'll give a
> summary of his talkl here and then we can kick off some discussion.
>
> OAuth is a standardized method for a user to grant access to resources that
> they own to people and things that are not themselves.  Currently this is
> being
> used to allow a user to control the access to data and actions that may be
> performed on one web service by another web service.  The concepts and
> mechanisms can be used in any situation where the user wants to limit what
> the
> software they are using can do on their behalf.
>
> = Part I: What is OAuth? =
>
> <snip>

>
> == Flow of a basic request from start to finish ==
>
> * The client program needs to get access to a protected resource.
> * The client asks the authorization server for a client-id and tells the
> server
>   which permissions it needs
> * The authrization server gives a url to the client
> * The client program redirects the user to that url so the user can grant
>   permissions to the client
> * The authorization server authenticates the user (ie: they login to the
>   authorization server).
> * The authorization server asks the user to confirm they want to grant the
>   requested permissions to the client.
> * If the answer is no, the protocol ends.
> * If he answer is yes, the user is redirected to the client with an
>   `authorization code` in the request
> * The client sends the `authorization code` to the authorization server.
> * The authorization server generates an `access token` with the specific
>   permissions that the client requested, expires the `authorization code`,
> and
>   returns the `access token` to the client,
> * The client requests the protected resource from the resource server
> using the
>   `access token`.
> * The resource server verifes that the `access token` is valid.  If it is,
> it
>   allows access
>
> .. note:: the `authorization code` is only good for retrieving a single
>     `access token` for the particular set of permissions that the user
>     confirmed.
>
> .. note:: A client can request access to multiple resources at once.
>  Assuming
>     the resource owner accepted all of them, the access token the client
>     receives at the end will allow access to all of those.  A client
> typically
>     has one access token from an authorization server that grants it all
> needed
>     permissions on all of the resource servers that the authorization
> server
>     can give out permissions for.  It is possible for a client to have
> multiple
>     access tokens with different permissions from the same authorization
> server
>     but the client would have to keep track of which permissions were
> granted
>     by which token (and the user would have had to confirm that the client
>     should be granted each set of permissions).
>
> .. question:: an access token can contain permissions for multiple resource
>     servers.  How do we secure the token from being used maliciously by a
>     different resource server?  ie: I get an access token which grants some
>     permissions on both fas and bodhi.  I send that access token to fas to
>     retrieve some information.  What prevents fas from hanging onto that
> token
>     and using it to access the protected resources on bodhi that it grants
>     without my knowledge?
>
> Every request send to auth server or resources server have to be signed
with a consumer secret related to the token/access_token which mean that
any other program than
the one which get that access token can't get through

== But wait, there's more! ==
>
> We've now seen one authorization via oauth.  But Oauth is flexible.
>  There's a
> few different ways this can work to be aware of:
>
> * Other ways to request the access token.  The example above is what works
> best
>   for third-party web clients.  However, there's other flows that might
> work
>   better for CLI apps or "trusted" web clients
>   - Implicit: user gets the access token directly from the authorization
> server
>     rather than through a authorization code.  This sortcut is useful when
> the
>     client is entirely in the browser (no third-party server involved).
>  With a
>     third party server, the authorization code makes it so the user never
> sees
>     the actual access token, only the authorization code.  if the client is
>     running on the user's machine anyhow, there's no sense in that step.
>   - Resource owner password credentials: The resource owner provides their
>     credentials (username and password) to the client.  The client
> retrieves
>     the access token from the authorization server using the credentials.
>  Then
>     it discards the credentials and only keeps the access token for further
>     requests.
>   - Client credentials: Just defines that if the client is the resource
> server,
>     it can authenticate itself to access its own resources... I'm a little
>     unclear on this but I think one use would be for a resource server to
> use
>     its externally available functions (which are protected by oauth)
> rather
>     than having to write an equivalent function that is usable internally.
>     puiterwijk mentions a different use: having a strict separation between
>     tenants in the resource server's model and then having to prove you
> have
>     permission to access the resource from a different tenant (not
> something
>     we're likely to do).
> * Verification of the access token can take many forms.
>   - The authorization server could notify the resource server whenever a
> new
>     access token is issued/revoked
>   - The resource server could ask the authorization server to verify the
> token
>     each time it receives one
>   - The token could be signed by the auth server and thus be verifiable in
> and
>     of itself.  The token could then contain the list of permissions so
> that
>     the resource server would just consult the token to know what was
>     available.  This should not be preferred as it makes revoking a token
>     harder.
> * The authorization server may or may not know about the range of
> permissions
>   that it can grant.  The resource server needs to interpret what the
>   permissions the access token grants mean so if the authorization server
>   grants a made-up permission the application should just ignore it.
>
> .. question:: Is it possible for the user to grant some of the requested
>     permissions and deny others?  Or is it all or nothing?
>

It's all or nothing.
It's obvious, if you deny access to requested resources, the related token
get revoked.

We have a case at work where we have 20 tokens for one resources server.
it's just a matter of security level/choice.


> == Refreshing a token and its caveats ==
>
> An access token can have an expire time.  The expire time can be coupled
> with a
> second token called a refresh token.  Usually the refresh token would
> expire
> sometime after the access token would expire.  When the access token
> expires,
> the refresh token could be used by the client to request a new access token
> without prompting the user.  This is indended to protect against an
> attacker
> who is sniffing packets from amassing enough ciphertext from multiple uses
> of a
> single access token to be able to brute force that token.
>
> This sort of automatic expiration and refresh **is not** meant to protect
> the
> user in case the access token is copied without their knowledge (because
> the
> refresh token can be copied at the same time).
>
> = Part II: How do we use this? =
>
> This section is less about OAuth itself but some proposals about how we can
> best code OAuth usage in our web applications to be secure and featureful.
>
> == Session vs token ==
>
> Currently we have a concept of a session in all of our web apps.  You
> login.
> Once you're logged in, the web app knows that future connections from your
> web
> browser/CLI/etc are being made by you.  At some point the session expires
> or
> you explicitly log out.  At that point, the session is over.  The
> expiration
> time for most of our apps is currently 20 minuts of idle time but we've
> talked
> about increasing this in the past.  Sessions in my mind should last tens of
> minutes to hours.  Certainly no more than a day.  A session conceptually
> tells
> the server that the user is present and interacting with the website (by
> saying
> that the user has "recently" authenticated).
>
> Tokens are more akin to passwords coupled with a restricted set of
> permissions.
> They're intended to be valid for days to weeks.  Refresh tokens can (but
> don't
> necessarily) be used to keep a low amount of ciphertext in the system while
> still making authentication via access token transparent to the client.
> Conceptually, they tell the server that the **client** (not user) is the
> same
> one that was granted the permissions.
>
> === Using tokens to implement sessions ===
>
> * Sessions need to be short term -- expiration would need to be low
> (perhaps an
>   hour).  No possibility to refresh the token.  If you need to continue,
> you
>   have to re-send your username + password (+ otp?)
> * We want this specific token to represent that the user is present, not
> just
>   that the client has been delegated permissions.
> * It would make sense for the token to give out all permissions that the
> user
>   has (at least, on this resource server) because the user is present.
> Example
>   token permission: "*@*" permissions token
> * If possible, saving this type of session token into a wallet/keyring
> would
>   make sense as that would encrypt the on-disk representation.  However,
> we'd
>   also have to account for the fact that these services might not be
> present.
> * Suggested to have access tokens with validity of 5 minutes.  refresh
> tokens
>   of 20 minutes.  This would approximate our current cookie-based idle
> timeout.
>
> .. question:: Can we also have a maximum number of refreshes or maximum
> time
>     before the user has to reenter their credentials (username + password
>     (+otp?))
>
> == Some proposed best practices ==
>
> * Oauth allows for very granular permissions.  You could put a separate
>   permission on each resource that a client can request.  However, it
> doesn't
>   require that you are granular or not because the application interprets
> the
>   meaning of the permission.  A lazy resource server could have a single
>   permission that covered anything that can be performed on the server but
> this
>   means that a stolen token can be used to do anything that that user
> could do
>   on that resource server.  We should attempt to identify common use cases
> and
>   code separate permissions for them.  ie: "building a package in a copr"
> would
>   belong in a separate permission from "creating a new copr".
>

I'm definitively +1 on this one.


>
> * An access token should not be taken to represent the presence of the
> user.
>   It means the user has delegated permission to perform this action to some
>   "client".  It is possible that the client is a command line app or an
> api and
>   the user is interacting with it directly but it cannot be assunmed that
> this
>   is the case.
>

+1 Also, all allowed access should be revoke-able by the user at any time.


>
> * Following from that, changing authentication methods, password, yubikey,
>   security questions, etc should never be allowed via an access token.  We
> want
>   the user to be present to change these settings.
>
> +1

* Tokens and sessions should not contain information about the
> authentication
>   status.  They should not contain what permissions are held or when the
>   session expires.  These are for the resource server and authorization
> server
>   to determine.
>

Yeah, that's the resource sever which actually defines what third-parties
are allowed to
to get from it even if token is granted.


> * Also following from that -- we should write things to allow for a
> session to
>   be sufficient for allowing users to perform actions.  access tokens
> describe
>   a subset of the functions that the user themselves is allowed to perform.
>
> * Client side -- we want to have different permissions if the user is
> running
>   the cli from the command line vs running the cli from a cron job.  A user
>   running from the cli could be said to have a session.
>
> Hmm... I'm not sure we can really prevent user from running the exact same
cmd-line from a terminal
to a cron tab.
Unless having strong policies on user/admin's operation. SOP!

IRC log (since this is all paraphrased and I could have misunderstood what
> puiterwijk meant):
>
> http://toshio.fedorapeople.org/puiterwijk-oauth.html
>
>

-- 
Xavier
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