Yesterday marked the beta release of OAuth for the Heroku Platform API, which we hope will empower users to develop apps against the API by providing a simple and powerful authentication framework that’s consistent with other providers across the web.
One interesting discussion that developed while we were building this out was around OAuth scoping, the mechanism that allows OAuth clients to tell an authorization server what permissions they’ll need on resources they’re accessing. I thought this might be a good opportunity to talk a little about OAuth scoping, what the spec has to say about it, how it’s implemented elsewhere on the web, and our own design considerations.
RFC 6749 describes how scope should be implemented according to the proposed OAuth 2 standard. I’ve tried to summarize the main points presented in the document:
- Scope is specified on either the authorization or token endpoints using the
- Scope is expressed as a set of case-sensitive and space-delimited strings.
- The authorization server may override the scope request, in this case it must
scopein its response to inform a client of their actual scope.
- When a scope is not specified, the server may either fallback to a well-documented default, or fail the request.
The spec describes the format that a scope should have and how the server should handle it, but is open-ended in respect to what strings in a scope should actually look like. This decision allows providers to define their own strings, and gives them enough flexibility to ensure that OAuth 2 scoping is a good fit for accessing a wide variety of different resources.
The open-ended spec has resulted in all kinds of creative implementations across the web, with no two being exactly alike. I’ve compiled a few examples to demonstrate the range of ideas out there.
App.net allows developers to define a basic set of scopes in snake_case. This pretty standard scoping implementation is simple and effective.
basic stream update_profile
Facebook deviates from spec a bit by suggesting that scope strings be
comma-delimited. The two other interesting characteristics of Facebook scopes
are that more specific strings are namespaced under their broader category
user_actions.video), and that some strings are dynamic (e.g.
APP_NAMESPACE scopes to a particular app in
Facebook also offers a very extensive variety of available scopes so that apps
can be very precise about what powers they’ll require.
GitHub provides a concise set of scopes with some namespacing using the colon
character. For example,
user:email is a subset of the permissions allowed by
gist repo user user:email
Another interesting innovation here is that for any API requests, GitHub passes
back the response headers
indicate to the user what scope strings their token has, and what strings this
endpoint will accept. This makes their APIs self-documenting in that it
provides users an easy alternative to looking up documentation when choosing
scope for their apps and tokens.
Google mandates that scopes should start with the
openid string, then include
either or both of
profile. From there, scope is extended across
Google’s flourishing ecosystem by defining other strings as extensible URIs.
openid profile email https://www.googleapis.com/auth/drive.file
Another fairly simple implementation, with the notable use of plus signs rather than spaces for delimitation.
LinkedIn reserves the underscore to separate types of resources from the
read/write permissions to that type, with an
r specifying read privileges and
r_basicprofile r_emailaddress rw_groups w_messages
An uncommon trait here is that Salesforce requires a particular scope string for the privilege of being granted a refresh token.
api refresh_token web
Shopify also mixes read and write permissions into scope strings. Their system
is fairly intuitive in that
write_ also implies
read_ permission, so that
developers don’t need to specify both.
read_customers write_script_tags, write_shipping
Defines scope strings that are prefixed with
wl. (for Windows Live);
presumably so that scopes are unique across Microsoft’s entire product space.
wl.basic wl.offline_access wl.contacts_photos
The end product for Heroku OAuth scope was shaped by a few major product and engineering design goals:
- The set of scope strings should be minimal so that we still have the power to evolve scoping as we continue to build out our product. Even if we completely redesign our scope strings, all the old strings should be general enough to easily map to the new system. Adding things is easy, but deprecating is hard.
- Some app resources are sensitive enough that even if a scope grants almost universal permission to manage an app, they still need to be protected. A good example of this are an app’s config vars, which contain secrets like database connection strings. The scoping system must take this into account.
- We should provide a very minimal scope that provides basic user information and nothing else. This is useful in systems that will use OAuth to identify a user and little else like the Heroku Forums.
Taking these goals into account, along with the spec and the web’s other implementations, we came up with a starting point for our scope system which is what was released yesterday:
identity: Allows access to
GET /accountfor basic user info, but nothing else.
read: Read access to all a user’s apps and their subresources, except for protected subresources like config vars and releases.
write: Write access to apps and unprotected subresources. Superset of
read-protected: Read including protected subresources. Superset of
write-protected: Write including protected subresources. Superset of
global: Global access encompassing all other scope.
These strings map to more a much more granular set of permissions in the backend, which will allow us to continue evolving the public interface as need be.
Like a few other providers, we also elected for self-documenting API endpoints
that help developers along by specifying their accepted scope strings as
response headers (we tend to drop any
X- prefixes as they’re effectively
Oauth-Scope: global Oauth-Scope-Accepted: global read read-protected write write-protected
This isn’t a finalized design, and are looking forward to collecting requirements from the community and internal consumers and iterating on it, the hope being that we can provider a system which offers a powerful amount of flexibility and granularity, while staying simple to use and true to the original spec.