...
Using OAuth 2 to authenticate with Synapse has many benefits over using older authentication mechanisms. We can try to consolidate authentication services to using OAuth flows wherever possible. The next step is to permit This document focuses on enabling the creation of OAuth 2 public clients, which would allow OAuth 2 based authentication in Synapse command line apps and third-party browser-based SPAs like the GWT web client.
To securely support OAuth 2 public clients, these actions are needed (backfill this list with Jira tickets when confirmed):
Support PKCE in the authorization code flow
Add field to label OAuth 2 clients as either “public” or “confidential”; backfill existing clients as “confidential”
Public clients should not be issued secrets
Public clients should be required to use PKCE
Revoke the current iteration of a refresh token when an old refresh token is used
Don’t save consent records for public OAuth clients
To fully support Synapse command line clients, in addition to the above, we must:
Add a bootstrapped OAuth client for Synapse command line apps
Add service to issue a (user-authenticated) refresh token for the Synapse command line apps OAuth client
Background
There are currently many ways to authenticate a request to Synapse, including
...
Leased/timed expiration - if a particular session isn’t used for 180 days, the token expires
The ability to grant/revoke access on the machine-level, instead of having one key used everywhere
Scoped access, in cases where the command line app is running a job that only needs a subset of access/functionality
Using either the authorization code (currently supported) or device code (not currently supported) OAuth 2 flows require users to login on Synapse.org via a browser, instead of entering credentials at the command line, or pasting an API key into a config file.
While currently a lower priority, we can also consider the ability to replace the usage of session tokens with access tokens in the GWT client as the backend requirements are very similar.
The caveat to using OAuth 2.0 for native apps and JavaScript-based SPAs is that in these circumstances, the OAuth client cannot securely and confidentially store credentials. This scenario is a use case that is outlined in the OAuth 2.0 specification, but we currently expect all Synapse OAuth clients to be confidential clients. Therefore, we must consider the security and access implications of creating/allowing public OAuth clients.
...
Best practices suggest that authorization servers are required to support PKCE [2][3]. PKCE only provides benefits to public OAuth clients, since a malicious app must have knowledge of an OAuth client’s credentials to execute the attack.
PKCE would require clients to generate a code and send a hash of the code to Synapse when initiating the authorization code OAuth flow. When redeeming the authorization code, the client must send the original code. Synapse would then hash the code and verify that the hash matches the initial hash sent by the client. For more details, expand the following section:
| Expand | ||
|---|---|---|
| ||
PKCE prevents a malicious application from hijacking an authorization code. This can occur when using a native app’s OAuth client, and the redirect URI is compromised by a malicious application observing traffic to the user agent (mobile app or web browser). In this scenario, the malicious app has the authorization code, and is in possession of the client’s credentials (if they exist), because it is a public client. Thus the malicious app now has access to the user’s account. With PKCE, the client generates a high-entropy random string, and calculates the SHA256 hash of the string. When making the initial authorization request, the hash is sent to the authorization server. The authorization server associates the hash with the authorization code, before sending it to the client (the hash may be embedded) When a client uses the authorization code, they must provide the initial random string. The authorization server computes the SHA256 hash of the string, and will only issue token(s) to the client if the hashes match. |
Designating new clients as either public or confidential
...
In OAuth 2.0, the authorization server SHOULD NOT assume client type [1]. We can require client creators to specify their client type upon creation. This is a breaking API change, but it is acceptable because OAuth clients aren’t created programmatically (it wouldn’t make sense if they are, since they must be manually verified to be functional).
...
Currently, the only grant types implemented are the more secure possible grant types, likely meaning no restrictions at this time.
Users may need to explicitly opt-in to allowing public client access to their resources (e.g. a simple checkbox and supplemental info modal on their profile settings page)
...
Use of an expired refresh token invalidates the active refresh token
The following would only apply to public OAuth clients.
OAuth 2.1 ([2]) Note on refresh tokens for public clients:
Authorization server MUST utilize one of these methods to detect refresh token replay by malicious actors for public clients:
* _Sender-constrained refresh tokens:_ the authorization server cryptographically binds the refresh token to a certain client instance by utilizing [I-D.ietf-oauth-token-binding] or [RFC8705]
* _Refresh token rotation:_ the authorization server issues a new refresh token with every access token refresh response. The previous refresh token is invalidated but information about the relationship is retained by the authorization server. If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach. The authorization server cannot determine which party submitted the invalid refresh token, but it will revoke the active refresh token. This stops the attack at the cost of forcing the legitimate client to obtain a fresh authorization grant.
We will use refresh token rotation because it is partially implemented (we currently do not revoke an active token if an expired token is used). We can extend our implementation to include this behavior. Token binding is also more complicated. For details, expand the following section.
| Expand | ||
|---|---|---|
| ||
Token binding introduces a public/private keypair and encrypts additional data to bind tokens to a TLS connection, thus a token could not be used if intercepted via MitM/token export. While powerful, implementing token binding can be complicated and has seen low adoption. We may choose to further investigate token binding as in [4] later. Token binding as described in [OAuth 2.0 Token Binding] does not seem to be an acceptable solution, because the draft has expired, and the mechanism to implement token binding on the client side does not seem to be supported by any major browsers (abandoned by Chrome, and only implemented in Edge before it became Chromium-based) |
...
. |
Risk in Saving Consent Records for Public OAuth Clients
...
A new attack mechanism is opened up if this same mechanism is applied to public OAuth clients, because the client credentials (if they exist) are not secrets--a malicious actor could use a public client that a user has already authorized to gain access to an account without a user’s consent.
To illustrate For an illustration of this scenario, consider:expand the following section:
| Expand | ||
|---|---|---|
| ||
Consider
Because Alice uses the Python client, she has authorized the related public OAuth client to access all of her account resources accessible via OAuth. Synapse has saved an authorization record, so that Alice can quickly log-in in the Python client with minimal friction. Mal has created an app that claims to save Alice a lot of time in her analyses, and has convinced Alice to attempt to log in to her app. Unbeknownst to Alice, the app uses the same OAuth client as the Synapse Python client, and requests the same set of scopes and claims. Because Synapse has saved her authorization record, Alice never has a chance to see that she is authorizing the “Synapse Python Client” OAuth client, which would tip off to Alice that Mal’s app is malicious. Mal now has unauthorized access Alice’s account. |
New Services
Enabling public clients in general is required before meeting requirements for browserless command line apps, so the first set of requirements below must be met before beginning the second set.
Public clients capable of using authorization code flow
Public clients are first- or third-party OAuth apps that cannot reliably store a client secret. Currently, the only way to acquire OAuth access is through the authorization code flow, which requires a web browser. The following services must be modified to support public OAuth clients that can use the authorization code flow
Endpoint | Request Body | Response Body | Notes/Modifications | New Service? |
|---|---|---|---|---|
OAuthClient extended to require a new field | ❌ | |||
None | Do not generate client secrets for public clients. | ❌ | ||
OIDCAuthorizationRequest extended to include | ❌ | |||
Multiple parameters | Additional request parameter | ❌ | ||
Multiple parameters | Client secret not required if the client ID is a public client. | ❌ | ||
Multiple parameters | If a public client uses an expired/rotated refresh token, revoke the current ‘version’ of the refresh token (e.g. refresh token is | ❌ | ||
- | - | Restrict client 0 from using these endpoints. | ❌ |
Support for first-party command line public clients
Command line apps (like the Synapse Python client) cannot always support the authorization code flow because some environments do not have a browser. To enable this, we must do at least one of the two:
Support another OAuth grant type (e.g.
device_code)Add a user-authenticated service to generate an OAuth refresh token
A user-authenticated service is similar to the OAuth token generation service in GitHub, and is similiar to how users retrieve API keys today, so we will move forward with that.
Endpoint | Request Body | Response Body | Notes | New Service? |
|---|---|---|---|---|
N/A | N/A | N/A | Add a bootstrapped ‘Synapse Command Line’ OAuth client. | ❌ |
POST /oauth2/userGeneratedToken | OIDCTokenGenerationRequest {
} | OIDCTokenResponse | Generates a token response that users can copy and paste into the command line client. Effectively replaces API keys. In most cases, the | ✅ |
...
References
...