Overview and Use Cases
This document will outline the design and implementation of OAuth 2.0 refresh tokens and token revocation in Synapse.
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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/permissionsto/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:
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Refresh Tokens:
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Token Revocation:
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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.
Use Cases for Refresh Tokens
Support for Workflows. Workflows that act on behalf of a user may have a long queue of jobs. If a job does not execute within the lifetime of the issued access token (currently 24 hours), then the job will fail. When a user triggers a workflow job via a submission queue, Synapse could issue a refresh token to grant access to a workflow engine beyond the current duration of an access token. When the job is finished, the refresh token can be revoked.
OAuth in the Synapse Python Client. With refresh tokens, users can use OAuth to authenticate in the Python client, eliminating the use of their username and password, or an API key. Similarly to the API key, a user is capable of revoking the token. A key advantage over the API key is that a user can issue multiple refresh tokens, allowing for granular, machine-level access tokens. Because access tokens are short-lived, a user authenticating into the Python client with our current implementation of OAuth would be forced to reauthenticate every 24 hours. A refresh token can be stored locally, so that users would not be required to reauthenticate.
Token Revocation
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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.
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Details about how evaluations and workflows could interact are intended as examples and are not the focus of this document |
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.
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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.
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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.
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Use Cases for user-centric token revocation
A An user may no longer trust or need to use an OAuth client that has been granted access to their Synapse identity and resources (revoke all tokens from one client)
A user may no longer have access to a machine with their Synapse credentials used for a command line application, but they do not wish to expire all machines on which they are authenticated (revoke one token from one client)
A user may forget about a machine on which they have authenticated, and would benefit from being signed out automatically (token expiration)
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
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.
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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 account resources at the conclusion of a job. They may revoke the token to ensure it is no longer valid.
Token Revocation
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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.
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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, the amount of access that the client has (via scopes), how long the client has had access, and how recently the client has accessed that user’s resources by requesting a new access token.
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Field | Type | Description |
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token | string | The token to revoke |
token_type_hint | enum | The type of token to revoke (must be |
New API Endpoints
Seven This section outlines new endpoints and an extension of implementation for one existing endpoint are proposed.
Viewing applications that have OAuth access to a user’s account
Endpoint: GET /oauth2/permissions/
Request body: 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/permissionsaudit/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/permissionsaudit/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/permissions/:client_idaudit/tokens/:token_id/metadata
Request Parameters:
client_id: the OAuth2 client that is associated with the tokentoken_id: the token to update
Request Body: OAuthRefreshTokenInformation
Response: On successful revocationupdate, 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
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token
Endpoint: POST /oauth2/permissions/:client_idaudit/tokens/:token_id/revoke
Request Parameters:
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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.
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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.
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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.
Additionally, we should require that a refresh token be passed in the request body and not as a request parameter. If the token is passed as a request parameter, it will be logged in the web application firewall and on the server, so it is insecure.
We should also consider enabling the device code flow (this is the “smart TV” flow: the client will request a short code from Synapse and display the code to the user. The user enters the code into Synapse on a separate device, and the authorization is approved), for cases where a user may be accessing a Synapse command line app without access to a browser.
Public vs. Private Clients - A Security Note
It is important to understand the distinction between confidential and public OAuth clients, and how they would interact with Synapse (RFC 6749 § 2.1).
A confidential client is capable of keeping their credentials confidential. The example given in the OAuth specification is a web application running on a web server. In another context, this could also be a workflow engine. Maintaining the confidentiality of the client credentials adds an additional layer of security because the credentials must be supplied when using a refresh token to request an access token.
A public client cannot ensure that their credentials are confidential. Examples given in the OAuth specification are user agent based applications (i.e. a single-page application where the client code runs in the browser) and native applications (where the client credentials may be extracted or decompiled, exposing a client secret). In our context, the Synapse CLI applications would act as public clients (i.e. the client secret is not a secret).
A major implication of this is that refresh tokens issued to public clients (e.g. the Synapse Python client) are no more secure than bearer tokens (any user may use it because the client secret is not confidential).
Despite the security flaw, this scheme is used in practice
For native apps integrating with Google, they simply suggest embedding your secret in the app, noting that it is not really a secret
GSUtil, the command line app for interfacing with Google Cloud, stores credentials in the app
MSAL, Microsoft’s current authorization library, indicates that Microsoft internally discerns public/confidential clients (public clients do not have a secret).
This doesn’t remove all of the benefits of using OAuth in Synapse CLIs. Using OAuth still accomplishes
Removing password from the authentication flow
Scoped access
Ability to issue multiple tokens and revoke them individually
Additional usage context for a user (e.g. for a particular refresh token or client you can say “Used for Synapse CLI 2 days ago”)
Backend Implementation Detail: Database Model
This section covers implementation details that will not be visible to users of the new services, and is not necessary to read to have an understanding of how to use the new services.
To support revoking refresh tokens, we will need to track them in the database. As mentioned earlier, we can trace access tokens to a refresh token by adding a refresh token ID to the access token as a JWT claim. By doing this, we only need to store records of refresh tokens in the database.
We hash the token to reduce the severity of tokens being exposed. It is proposed that tokens will be randomly generated, and then hashed using SHA256, but changing this is open to discussion.
New DB Table: OAUTH_REFRESH_TOKENS
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Name
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Type
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Notes
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ID
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INTEGER
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Primary key
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TOKEN_HASH
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CHAR(64)
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SHA256 hash of the refresh token passed to the client
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NAME
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VARCHAR(256)
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Human-readable identifier for the token
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USER_ID
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BIGINT
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Foreign key reference to the principal whose resources this token grants access to
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CLIENT_ID
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BIGINT
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The client that this token is issued to
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SCOPE
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VARCHAR(256)
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The scope/permissions that the refresh token allows. Stored as a serialized array of strings (is there a better way to do this? a binary column for each possible scope?)
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CREATED_ON
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TIMESTAMP
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When this refresh token was created
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LAST_USED
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TIMESTAMP
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The last time this refresh token was used to issue an access token
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MODIFIED_ON
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TIMESTAMP
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The last time this token was modified (i.e. the name was changed)
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ETAG
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CHAR(36)
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For OCC
Additional constraint: UNIQUE(USER_ID, NAME) – a user may not have duplicate names for their tokens
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.
Open Questions
Choice of a refresh token? Proposed: 256 bit random string using SecureRandom
(When) should a refresh token expire?
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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.
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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
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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.
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PKCE is an additional safeguard that specifically applies to native applications (e.g. a command line app) using the authorization code flow (i.e. the user authenticates in a browser, and is redirected back to the native app with an authorization code). The threat in this situation is that when the user is redirected back to the app through a custom URI or to a temporary local webserver, a malicious app may be the recipient of the redirect, rather than the intended client. The malicious app now has access to the authorization code. Because the client credentials are not confidential (due to the intended application being a native app), the authorization code gives the malicious app access to the user’s account. The solution is PKCE:
If the authorization code is hijacked, the malicious app cannot use it because it does not know the additional secret. |
Open Questions
Choice of a refresh token? Proposed: 256 bit random string using SecureRandom
(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)
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?)
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