Review Changes

For those viewing this document after the initial review meeting, here are the core changes made since then:

• Added detail to workflow use case

  • Sequence diagrams

• Removed CLI use case and related requirements

  • Requirements for workflows use case is a subset of the CLI requirements. Let’s tackle workflows first and talk about CLI later.

• Renamed endpoints for auditing tokens from /oauth2/permissions to /oauth2/grantedClients

Overview and Selected Use Case

This document will outline the design and implementation of OAuth 2.0 refresh tokens and token revocation in Synapse.

Jira Epic:

Refresh Tokens:

Token Revocation:

Use Case: Workflows

Users are interested in running workflows (which require access to a user’s Synapse resources) on workflow engines operated by a third party. Because the workflow engine is operated by a third party, it is unacceptable for a user to authorize access using their username and password, or their API key. The short-lived OAuth access tokens currently in Synapse are not sufficient for this use case either, because a submission to a workflow engine may sit in a queue and/or execute for a duration exceeding the lifetime of an access token (currently 24 hours).

These issues can be overcome by issuing a refresh token to the workflow engine.

Refresh Tokens

Refresh tokens would allow a 3rd party client to request a new access token from Synapse. This enables OAuth clients to have long-lived access to a user’s identity and resources. Refresh tokens are an optional component defined in the OAuth 2.0 specification.

In this example scenario, a Synapse Evaluation queue could be associated an OAuth client and a running instance of a workflow engine. The workflow engine has access to the OAuth client credentials.

Open

When a user creates a Submission and submits it to the Evaluation, Synapse will submit the job to the workflow engine with a refresh token intended to be used for the duration of the job. Synapse may revoke the token once the job is completed.

Open

Note that in this example, there is no authorization/consent flow. This is because consent to third party access is granted by submitting a job to the evaluation queue. There may be other scenarios involving workflows that do not utilize evaluations, and a typical authorization code flow, including granting consent, may be used

Token Revocation

Because refresh tokens allow a user to issue long-lived access to a 3rd party client, we should allow users and clients to revoke this access. This gives a user more control over their data, and additional services allow a user to audit their granted permissions so they may re-evaluate the services with access to their data. In cases where a client no longer needs access to a Synapse user’s resources, they may revoke the token in order to prevent future unauthorized access.

User-centric token revocation is not defined in any of the OAuth Specifications. The OIDC Specification § 16.18 simply suggests it:

The Authorization Server SHOULD provide a mechanism for the End-User to revoke Access Tokens and Refresh Tokens granted to a Client.

Thus the design for this feature is influenced by other services, such as those showcased by Okta/OAuth.com on their page Listing Authorizations.

Use Cases for user-centric token revocation

• An user may no longer trust or need to use an OAuth client that has been granted access to their Synapse identity and resources

Client-centric token revocation is defined in RFC 7009. From the RFC:

From an end-user's perspective, OAuth is often used to log into a certain site or application. This revocation mechanism allows a client to invalidate its tokens if the end-user logs out, changes identity, or uninstalls the respective application. Notifying the authorization server that the token is no longer needed allows the authorization server to clean up data associated with that token (e.g., session data) and the underlying authorization grant. This behavior prevents a situation in which there is still a valid authorization grant for a particular client of which the end-user is not aware. This way, token revocation prevents abuse of abandoned tokens and facilitates a better end-user experience since invalidated authorization grants will no longer turn up in a list of authorization grants the authorization server might present to the end-user.

Use Case for client-centric token revocation in context

• Resource access is no longer needed by the client. As an example, a job executed by an OAuth client may no longer require access to a user’s Synapse resources at the conclusion of a job. They may revoke the token to ensure it is no longer valid.

Token Revocation Behavior

Currently, only short-lived access tokens are minted in Synapse, and these tokens cannot be revoked because they are not stored in the database.

Since we will be issuing long-lived refresh tokens, we will need a mechanism to revoke refresh tokens. While not necessary, it would be ideal to also revoke the access tokens themselves. (RFC 7009 § 2)

To accomplish this, we can store refresh tokens in the database, which can be revoked by a user or client with a REST API call. Once the refresh token is revoked, it may no longer be used to generate access tokens.

By linking an access token to its associated refresh token, we are able to invalidate access tokens without storing them in the database.

Linking Access Tokens to Refresh Tokens

Access tokens are JWTs, so they can securely transmit information. Thus we can link any particular access token to the refresh token that permits it to be issued by adding a claim to the access token that contains its corresponding refresh token ID. The JWT specification § 4.2 suggests we use a namespace for this claim, (e.g. Auth0 recommends a URL like https://synapse.org/refresh_token_id or https://sagebionetworks.org/refresh_token_id, but we should be able to use a reverse domain name like org.sagebionetworks.repo.model.oauth.claims.refresh_token_id). As a side note, I think we are already in violation of this specification, since we currently use nonstandard, non-namespaced claims such as orcid, is_certified, etc. We should determine if we should get back “in-spec” and add namespaces to the existing claims (breaking API change).

When we verify the access token, we can deserialize the JWT and make a database call to see if the corresponding refresh token is revoked. If the refresh token is revoked, we reject the request.

Similarly, when an access token is revoked, we just revoke the refresh token (RFC 7009 § 2.1: “If the token passed to the [revocation] request is an access token, the server MAY revoke the respective refresh token as well.”)

Since we are now storing refresh tokens in the database, we can link them to the user and show them when the user wants to see them. Since access tokens are directly linked to refresh tokens, we need not show them (under this design, we couldn’t because we don’t even store them)

Additional Scope: offline_access

Per OIDC Core 1.0 § 11, we should only permit the use of a refresh token when a client requests the offline_access scope.

When offline_access is not requested, we can issue only a short-lived access token that is not associated with a refresh token (this is the current behavior of the system).

(Aside: the specification only requires that without the offline_access scope, a refresh token may not be used to issue tokens that access identity. The specification does not dictate that we could not otherwise use a refresh token. For the sake of simplicity, we can apply this to all permissions and only issue refresh tokens when offline_access is approved.)

REST API

This section will identify three new objects used in the REST API, seven new endpoints, and an extended implementation for an existing endpoint.

New objects

There are three new objects proposed in this document.

OAuthClientAuthorization

This object can be used to show a user the OAuth clients that have access to the requesting user’s resources and identity. Using this information, the user can identify the client that has access, how long the client has had access, and how recently the client has accessed that user’s resources by requesting a new access token.

OAuthRefreshTokenInformation

This object captures information about an active refresh token, intended to be seen by the user whose resources can be accessed by the token. Note that the token itself is not shown.

OAuthTokenRevocationRequest

This object is used when a client makes a request to revoke a refresh/access token. It is defined by RFC 7009 § 2.1.

New API Endpoints

This section outlines new endpoints and the behavior of each.

As a note, all new methods that can be accessed on behalf of a user (i.e. with a session token or access token) are organized under /oauth2/audit. All client actions (that require client credentials) are organized under /oauth2/token.

Viewing applications that have OAuth access to a user’s account

Endpoint: GET /oauth2/audit/grantedClients
Request body: none
Return body: PaginatedList<OAuthClientAuthorization>
Returns a paginated list of the clients and permissions that the user has granted. Allows a user to audit which parties have access to their resources.

Viewing tokens for an application that has OAuth access to a user’s account

Endpoint: GET /oauth2/audit/grantedClients/:client_id/tokens
Path Parameter: client_id: returned tokens will be associated with this OAuth2 client
Request body: none
Return body: PaginatedList<OAuthRefreshTokenInformation>
Returns a paginated list of the clients and permissions that the user has granted. Allows a user to audit which parties have access to their resources.

User revocation of a client’s access

Endpoint: POST /oauth2/audit/grantedClients/:client_id/revoke
Path Parameter: client_id: the OAuth2 client that will no longer have access to the user’s resources and/or identity
Response: On successful revocation, return HTTP 200. No body.
Upon calling this method, the refresh token and access tokens held by the specified client for the authenticated user making the API call will be revoked.

Update metadata for a token

Endpoint: PUT /oauth2/audit/tokens/:token_id/metadata
Request Parameters:

• client_id: the OAuth2 client that is associated with the token

• token_id: the token to update

Request Body: OAuthRefreshTokenInformation

Response: On successful update, return HTTP 200. No body
Upon calling this method, the token identifier will be updated

In practice, only the token name can be updated.

User revocation of a particular token

Endpoint: POST /oauth2/audit/tokens/:token_id/revoke
Request Parameters:

• client_id: the OAuth2 client that is associated with the token to revoke

• token_id: the token to revoke

Response: On successful revocation, return HTTP 200. No body.

Upon calling this method, the refresh token and access tokens held by the specified client for the authenticated user making the API call will be revoked.

Client revocation of a token

Endpoint: POST /oauth2/revoke
Request Body: OAuthTokenRevocationRequest
Response: By RFC 7009 § 2.2, on successful revocation, HTTP 200. No body.
Upon calling this method, the refresh/access token and associated tokens held by this client and associated with the user are revoked. Note: a specific path for this endpoint is not named by OAuth 2.0/OIDC specifications.

Retrieval of token metadata

Endpoint (users): GET /oauth2/audit/tokens/:token_id/metadata
Endpoint (clients): GET /oauth2/token/:token_id/metadata
Request Parameter: token_id - the ID of the token to gather metadata about
Response: OAuthRefreshTokenInformation
The client can call this endpoint to get token metadata name. This metadata can be displayed to the user so that they may more easily identify the token in use when auditing/revoking tokens.

Additional grant types to issue tokens

Endpoint: POST /oauth2/token
This method exists. This feature proposal would add support for grant_type=refresh_token, and return a refresh token for grant_type=code. For details, see OIDC Core 1.0 § 12.1, 12.2.

Public vs. Confidential Clients

We will initially require that all of our clients are confidential. Public clients (where credentials are not secrets, e.g. a native application or single page app), are permitted by the OAuth specification, but require additional security consideration that we do not support at this time.

FAQ/Anticipated Concerns

Does adding refresh tokens break any existing behavior?

It should not because we are merely extending the access token with a reference to its refresh token ID. Current clients would not see the refresh token without requesting the offline_access scope, and if they did receive it, they may ignore it. The duration of access tokens is unchanged.

PKCE - What does it protect against?

This design document has been revised to only support confidential OAuth clients at this time. The threat mitigated by PKCE only applies to certain types of public clients, so it is not necessary for this feature. We should consider adding it when we support public/native OAuth clients.

Open Questions

Choice of a refresh token? Proposed: 256 bit random string using SecureRandom

• “The refresh token is usually opaque”

(When) should a refresh token expire?

• Per sections 1.5 and 10.4 of the OAuth 2.0 spec (by way of this StackOverflow post), it seems we have some liberty in terms of the lifecycle of a refresh token.

• Some options we have include

  • Refresh tokens last until they are revoked

  • Refresh tokens are leased (i.e. they expire X days after last use)

  • Rotate the refresh token after each use

  • Refresh tokens expire after a duration

    • User will be required to reauthorize the application after a certain period of time (e.g. one year)

    • This would break things like cron jobs

• Proposal: refresh tokens are leased and expire 6 months after their last use

• External examples

  • Google

    • 6 month lease

  • GitHub

    • GitHub apps (newer/preferred)

      • Never

      • “Expiring access/refresh tokens” is in beta and was just released at the end of April 2020

        • Refresh tokens last for 6 months

        • Every time a refresh token is used, a new refresh token is issued (it is unclear if the old refresh token expires)

    • OAuth apps (older)

      • Refresh tokens are not issued. Only long-lived access tokens

      • Support forums indicate they have a 1 year lease

Is there a compelling use case for a user to be able to see the access they have given in the past but have revoked? (i.e. do we keep a record of revoked refresh tokens?)

Should we limit the number of active refresh tokens per (user, client) pair?

• Google limits this to 50 and expires the least recently used token when a new token is issued

• Implications

  • Simplifying an interface where a user is trying to audit/revoke tokens

  • Limits number of jobs that can be submitted to a workflow queue