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OAuth Details

Last updated: April 17, 2023

Configuring OAuth clients​

How to register a client manually​

Usually, when you connect applications to their OAuth resource server, they generate a client id and secret for you. In our case, we are the resource server.

The id and secret keys, in this context, can be seen as a username and password. They do not need to be generated in relation between one and another. In other words, they are not public and private keys.

With this procedure you will generate both client id and secret tokens to provide to your other applications and also partners who wants to leverage your identity provider service. Once you have the client id and secret, paste them in both the fxa-oauth-server AND your client service you want to bind using OAuth.

Difference between same site and external consumers​

While other applications within your infrastructure would ideally be pre-approved at the user point of view, external consumers shouldn't be. This is why when we develop a service leveraging another site, the user gets a confirmation window.

If you want to pre-approve your own web applications and prevent users in your accounts userbase to have a confirmation window, set the trusted flag to true.

Installing a new consumer​

Creating the client id and secret keys​

Use the fxa-oauth-client CLI tool for registering new clients with your server.

FxA OAuth development environments support localhost and localhost as valid redirectUri values to ease development.

OAuth resource server (a.k.a. fxa-oauth-server)​

Let's assume that the client in this example is the 123done web application that you deployed at https://clientapp.example.com and that the OAuth route is available at api/oauth (this is specific to each client application, beware (!))

Add a new object literal within the clients array, that would look like:

{
  "clients": [
    {
      "id": "<8-byte client id in hex>",
      "hashedSecret": "<32-byte sha256 of the client secret in hex>",
      "name": "123done",
      "imageUri": "https://clientapp.example.com/static/img/logo100.png",
      "redirectUri": "https://clientapp.example.com/api/oauth",
      "trusted": true
    }
  ]
}

NOTE: the trusted, this would be for an internal application that you manage.

OAuth clients​

This can be very different depending on your installed and supported version, you should have a look at the profile server and client application what are the required callbacks.

{
  "client_id": "<8-byte client id in hex>",
  "client_secret": "<32-byte client secret in hex>",
  "name": "123done",
  "redirect_uri": "https://clientapp.example.com/api/oauth",
  "signin_uri": "https://accounts.firefox.com/oauth/signin",
  "oauth_uri": "https://oauth.accounts.firefox.com/v1",
  "profile_uri": "https://profile.firefox.com/v1",
  "scopes": "profile"
}

OAuth Scopes​

Each authorization grant in OAuth has an associated "scope", a list containing one or more "scope values" that indicate what capabilities the granted token will have. Each individual scope value indicates a particular capability, such as the ability to read or write profile data, or to access the user's data in a particular service.

As defined in RFC6749 Section 3.3, the scope of a token is expressed as a list of space-delimited, case-sensitive strings, and it is left up to the service to define the format and semantics of the individual scope values that make up this string.

This document defines the scope values accepted in the Mozilla accounts ecosystem, and the rules for parsing and validating them.

Short-name scope values​

FxA supports a small set of "short-name" scope values that are identified by a short English word. These correspond either to scope values defined by external specifications (e.g. OpenID Connect), or to legacy scope values introduced during early development.

No new short-name scope values should be added. Instead we prefer to use URLs for new scope values, both to ensure uniqueness and to simplify parsing rules.

Short-name scope values imply read-only access by default, with write access indicated by the suffix ":write". The may also have "sub-scopes" to indicate finer-grained access control. Each name component may contain only ascii alphanumeric characters and the underscore.

For example:

  • profile indicates read-only access to the user's profile data.
  • profile:write indicates read/write access to the user's profile data.
  • profile:display_name indicates read-only access to the user's display name, but not any other profile data.
  • profile:email:write indicates read/write access to the user's email address.

The following short-name scope values are recognized in the FxA ecosystem.

Profile data​

  • profile: access the user's profile data.
  • profile:uid: access the user's opaque user id.
  • profile:email: access the user's email address.
  • profile:locale: access the user's locale.
  • profile:avatar: access the user's avatar picture.
  • profile:display_name: access the user's human-readable display name.
  • profile:amr: access information about the user's authentication methods and 2FA status.

OpenID Connect​

  • openid: used to request an OpenID Connect id_token.
  • email: a synonym for profile:email, defined by the OIDC spec.

OAuth Client Management​

  • clients: access the list of OAuth clients connected to a user's account.
  • oauth: register a new OAuth client record.

Basket​

  • basket: access the user's subscription data in basket

URL Scopes​

For new capabilities, scope values are represented as URLs. This helps to ensure uniqueness and reduces ambiguity in parsing. URL-format scope value imply read/write access by default, are compared as heirarchical resource references, and use the hash fragment for permission qualifiers. For example:

  • https://identity.mozilla.com/apps/oldsync indicates full access to the user's data in Firefox Sync.
  • https://identity.mozilla.com/apps/oldsync/bookmarks indicates full access to the user's bookmark data in Firefox Sync, but not to other data types.
  • https://identity.mozilla.com/apps/oldsync#read indicates read-only access to the user's data in Firefox Sync.
  • https://identity.mozilla.com/apps/oldsync/history#write indicates write-only access to the user's history data in Firefox Sync.

To be a valid scope value, the URL must:

  • Be an absolute https:// URL.
  • Have no username, password, or query component.
  • If present, have a fragment component consisting only of alphanumeric ascii characters and underscore.
  • Remain unchanged when parsed and serialized following the rules in the WhatWG URL Spec.

The following URL scope values are currently recognized by FxA:

  • https://identity.mozilla.com/apps/oldsync: access to data in Firefox Sync.
  • https://identity.mozilla.com/apps/notes: access to data in Firefox Notes.
  • https://identity.mozilla.com/account/subscriptions: access to subscription data from Subscription Platform.

Scope Matching and Implication​

We say that a scope value A implies another scope value B if they are exactly equal, or if A represents a more general capability than B. Similarly, a scope A implies scope value B if there is some scope value in A that implies B. This is the basic operation used to check permissions when processing an OAuth token.

Consumers of OAuth tokens should avoid directly parsing and comparing scopes where possible, and instead use the existing implementation in the fxa-shared node module.

For consumers that must implement their own scope checking, the rules for implication can be summarized as:

  • For URL scope values, A implies B if A is a parent resource of B.
  • For short-name scope values, split on the ":" character, and A implies B if either:
    • B[-1] is not "write" and A is a prefix of B, or.
    • A[-1] is "write", and:
      • A[:-1] is a prefix of B, or
      • B[-1] is "write" and A[:-1] is a prefix of B[:-1]

More precisely, the algoritm for checking implication is:

  • If A is a https:// URL, then:
    • If B is not a https:// URL, then fail.
    • If the origin of B is different than that of A, then fail.
    • If the path component list of A is not a prefix of the path component list of B, then fail.
    • If A has a fragment, then:
      • If B does not have a fragment, then fail.
      • If B has a fragment that differs from A, then fail.
    • Otherwise, succeed.
  • Otherwise:
    • If B is a https:// URL, then fail.
    • Split A and B into components based on : delimiter.
    • If the last component of B is write, then:
      • If the last component of A is not write, then fail.
    • If the last component of A is write, remove it.
    • If A is not a prefix of B, then fail.
    • Otherwise, succeed.

Below are some testcases against which scope-checking code can be validated.

Valid implications:

  • profile:write implies profile.
  • profile implies profile:email.
  • profile:write implies profile:email.
  • profile:write implies profile:email:write.
  • profile:email:write implies profile:email.
  • profile profile:email:write implies profile:email.
  • profile profile:email:write implies profile:display_name.
  • profile https://identity.mozilla.com/apps/oldsync implies profile.
  • profile https://identity.mozilla.com/apps/oldsync implies https://identity.mozilla.com/apps/oldsync.
  • https://identity.mozilla.com/apps/oldsync implies https://identity.mozilla.com/apps/oldsync#read.
  • https://identity.mozilla.com/apps/oldsync implies https://identity.mozilla.com/apps/oldsync/bookmarks.
  • https://identity.mozilla.com/apps/oldsync implies https://identity.mozilla.com/apps/oldsync/bookmarks#read.
  • https://identity.mozilla.com/apps/oldsync#read implies https://identity.mozilla.com/apps/oldsync/bookmarks#read.
  • https://identity.mozilla.com/apps/oldsync#read profile implies https://identity.mozilla.com/apps/oldsync/bookmarks#read.

Invalid implications:

  • profile:email:write does not imply profile.
  • profile:email:write does not imply profile:write.
  • profile:email does not imply profile:display_name.
  • profilebogey does not imply profile.
  • profile:write does not imply https://identity.mozilla.com/apps/oldsync.
  • profile profile:email:write does not imply profile:write.
  • https does not imply https://identity.mozilla.com/apps/oldsync.
  • https://identity.mozilla.com/apps/oldsync does not imply profile.
  • https://identity.mozilla.com/apps/oldsync#read does not imply https://identity.mozilla.com/apps/oldsync/bookmarks.
  • https://identity.mozilla.com/apps/oldsync#write does not imply https://identity.mozilla.com/apps/oldsync/bookmarks#read.
  • https://identity.mozilla.com/apps/oldsync/bookmarks does not imply https://identity.mozilla.com/apps/oldsync.
  • https://identity.mozilla.com/apps/oldsync/bookmarks does not imply https://identity.mozilla.com/apps/oldsync/passwords.
  • https://identity.mozilla.com/apps/oldsyncer does not imply https://identity.mozilla.com/apps/oldsync.
  • https://identity.mozilla.com/apps/oldsync does not imply https://identity.mozilla.com/apps/oldsyncer.
  • https://identity.mozilla.org/apps/oldsync does not imply https://identity.mozilla.com/apps/oldsync.

Pairwise Pseudonymous Identifiers​

A Pairwise Pseudonymous Identifier (PPID) is defined in the OpenID Connect Spec as:

Identifier that identifies the Entity to a Relying Party that cannot be correlated with the Entity's PPID at another Relying Party.

Put another way, PPIDs provide a single user with distinct userids across distinct Relying Parties (RPs), enhancing user privacy by preventing cross-site correlation based on userid.

By default, Mozilla accounts provides the same userid to all Mozilla-internal RPs to facilitate cross service correlation. As Mozilla expands its product offering to include white labeled 3rd party services, we want to uphold Mozilla's principles regarding user privacy by providing external services with only the minimal data necessary.

PPIDs allow Mozilla to enforce our privacy stance by providing each PPID enabled RP with a distinct userid for a given user. For example, a user that signs into 2 PPID enabled services would have 3 distinct userids:

  • FxA userid, only seen by Mozilla properties
  • PPID for service 1, known by Mozilla and service 1
  • PPID for service 2, known by Mozilla and service 2

PPIDs prevents service 1 and service 2 from correlating user data with each other based on userid alone. PPIDs only prevent correlation based on userid, transient information such as IP address, shared 3rd party cookies, and user-agent information can still be used by RPs to correlate users. This also assumes that RPs are unable to fetch user profile information such as email address and display name that could be used as additional data points.

PPIDs are returned as the sub claim within an OIDC ID Token or OAuth JWT Access Token.

Further enhancing privacy through PPID rotation​

User privacy can be further enhanced by preventing long term profiles from being built for a given user through PPID rotation. Mozilla accounts supports two methods of PPID rotation, RP initiated, and server initiated.

RP initiated PPID rotation​

Any PPID enabled RP that receives JWT access tokens or OIDC ID tokens can initiate PPID rotation by specifying an additional parameter, ppid_seed, when requesting tokens from /token endpoints on the OAuth server. ppid_seed must be an integer between 0 and 1024, and can be rotated at any time.

Server initiated PPID rotation​

By default, Mozilla accounts does not enforce sub rotation, but for sensitive RPs where long term user profiles are undesirable, Mozilla accounts can enforce periodic rotation without depending on the RP to pass in a ppid_seed. The default period is 6 hours.

Ensuring 3rd parties are unable to access user information​

An OAuth access token is a bearer token that can be presented by anyone to an OAuth service provider to access a protected resource. If a JWT access token contains a PPID sub claim that is meant to protect a user's privacy, but is granted the profile scope allows the service provider to present the same access token to the FxA profile server and learn the user's true identity. As such, tokens MUST NOT be requested with the profile scope or any of its implicants such as profile:uid or profile:email.

PPIDs and logging​

PPIDs are not used by internal Mozilla properties and are not logged.

PPID generation​

PPIDs are generated using HKDF with the following inputs:

  • km: client_id.userid_hex.ppid_seed||0.serverEnforcedRotationInput
  • info: oidc ppid sub
  • salt: A secret configured by Ops
  • len: 16 bytes, which leads to a 32 hex character result

Server enforced rotation input​

The server enforced rotation input defaults to the string 0. If server enforced rotation is enabled for the client, it is calculated as follows:

serverEnforcedRotationInput = Math.floor(Date.now() / ROTATION_PERIOD)

If an RP requests two tokens in short succession, their sub claims could be different if the first token was generated previous to the rotation epoch and the second after.

The future​

Mozilla accounts does not currently support sector identifiers. Sector identifiers allow multiple client_ids to receive the same PPID, this would be useful if the RP has applications on multiple platforms, e.g., an app on Android, iOS, and an addon in Firefox.

PKCE Support​

Proof Key for Code Exchange by OAuth Public Clients

Mozilla accounts OAuth flow supports the PKCE RFC7636. This feature helps us authenticate clients such as WebExtensions and Native apps and any other clients that do not have a server component or a secure way to store a client_secret.

To better understand this protocol please read the Proof Key for Code Exchange (RFC 7636) by Authlete Inc..

You'll need to include support parameters code_challenge_method, code_challenge and code_verifier.

At this time Mozilla accounts requires you to use the S256 flow, we do not support the plain code challenge method.

prompt=none support​

prompt=none enables authorized RPs to check a user's authentication state and receive an OAuth grant or access token without requiring user interaction.

More formally, prompt=none is a flow described by the OpenID Connect spec as:

The Authorization Server MUST NOT display any authentication or consent user interface pages. An error is returned if an End-User is not already authenticated or the Client does not have pre-configured consent for the requested Claims or does not fulfill other conditions for processing the request. The error code will typically be login_required, interaction_required, or another code defined in Section 3.1.2.6. This can be used as a method to check for existing authentication and/or consent.

prompt=none usage is controlled​

Usage of prompt=none with login_hint / email is controlled to an authorized list of RPs, since it bypasses the normal authorization flow and use could easily fall afoul of user expectations.

However, all RPs can check the user's FxA login state using id_token_hint: since RPs not on the authorized list can only obtain an id_token by going through the normal authorization flow, it is safe to assume an RP presenting the id_token as part of a prompt=none flow has already been granted authorization.

Requesting prompt=none during authorization​

prompt=none and either login_hint=<email> or id_token_hint=<ID Token> are appended onto the query parameters when opening the /authorization endpoint.

GET https://accounts.firefox.com/authorization?client_id=ea3ca969f8c6bb0d&state=2sfas415FSSF@A5f&scope=profile&prompt=none&login_hint=conscious.chooser%40mozilla.com

If a different user is currently signed in to the email specified by the login_hint or id_token_hint, an account_selection_required error will be returned.

Handling errors​

Most prompt=none failures cause the user to be redirected back to the RP's redirectURI with state and error query parameters.

The following is a table of OIDC compliant error codes → reasons

errorreason
invalid_requestprompt=none is not enabled
 OR login_hint is missing
 OR scoped keys are requested
 OR the id_token_hint token is invalid
unauthorized_clientprompt=none is not enabled for the client
* interaction_requiredaccount or session is not verified
* login_requireduser is not signed in
* account_selection_requireda different user is signed in

* indicates the user must take some form of action to recover

As an example, suppose the authorization request from the previous section failed because the user is not signed in and assume the redirectURI for client ea3ca969f8c6bb0d is https://service.firefox.com/oauth/complete. The user would be redirected to:

GET https://service.firefox.com/oauth/complete?state=2sfas415FSSF@A5f&error=login_required

Recovering from errors​

At this point, the RP knows the user must authenticate to Mozilla accounts before OAuth codes or access tokens are returned. The RP can generate a new state and try again without the prompt=none query parameter:

GET https://accounts.firefox.com/authorization?client_id=ea3ca969f8c6bb0d&state=gASDF-3df@A5f&scope=profile&login_hint=conscious.chooser%40mozilla.com
warning

When prompt=none is not specified, FxA handles login_hint as a suggestion, users are still able to authenticate/authorize using a different email. If the user specified in login_hint MUST be used, specify action=force_auth when redirecting back to FxA.

If a different user is allowed to sign in, it may be necessary to clear locally held session state before redirecting back to FxA.

Handling user logout​

Whenever the user terminates a session at the RP, any access and refresh tokens held by the RP for that session should be destroyed. Destroying the access and refresh tokens will ensure an entry for the session no longer appears in the Devices and Apps list.

Destroying an access token​

POST https://oauth.accounts.firefox.com/v1/destroy
{
  access_token: <access_token>
}

Destroying a refresh token​

POST https://oauth.accounts.firefox.com/v1/destroy
{
  refresh_token: <refresh_token>
}

More information on destroying tokens can be found on the OAuth server's API docs.

SSO logout​

Several concerns are noted, most importantly:

...there's potentially confusing behaviour around logging out of an FxA relier. If I use FxA to sign in to AMO, and then I sign out in AMO, I could reasonably expect to have to enter my password again if I want to sign back in. But we don't have a way for AMO to signal back to FxA that the user signed out.

This concern is not currently addressed as none of the OIDC logout flows are currently supported.

RPs can protect themselves a little bit here by not using prompt=none unless they still have a valid local login session for that user. If the users signs out of the RP, they will always see some UI when trying to sign back in, even if it's just the "Continue as X" button rather than a password prompt.

prompt=none and security​

For users that are already authenticated to Mozilla accounts, prompt=none bypasses the FxA authorization screen and redirects back to the RP without any user interaction. This behavior applies equally to users with 2FA enabled and could easily cause confusion with users. The Mozilla accounts team only enables the use of prompt=none for a service if both a good use case exists and the integration has been audited.

Future directions​

Support for and OIDC RP Initiated Logout may go some way to reducing user confusion on what it means to sign out of an RP and whether they should have to enter their password again. Upon signing out of the RP, RPs that support the OIDC RP Initiated Logout protocol will redirect the user to FxA where they are given the option to destroy their FxA session as well.

Management of JWT Signing Keys​

This server uses signed JWTs to represent various kinds of security token, including:

RPs are expected to verify the signatures on these JWTs by fetching our public keys via the OpenID Connect Discovery Protocol, which involves:

  1. Fetching our main OpenID metadata document at https://accounts.firefox.com/.well-known/openid-configuration
  2. Extracting the contained "jwks_uri" entry
  3. Fetching a JWK Set document from that URI
  4. Respecting standard HTTP cache-control headers on those resources

We thus have a coordination problem between this server and its RPs when it comes to changing our JWT signing keys. Let T_cache be the max age in the cache-control headers of our metadata documents, and T_tokens be the maximum lifetime of any token issued by this service. Then:

  • We must advertise a new signing key in our JWK Set for at least T_cache seconds before we start using it to sign tokens. If we don't, RPs with a cached copy of our JWK Set may not know to trust the new key, resulting in spurious verification failures and/or fragile logic to refresh the cached copy on-demand when verification fails.
  • We must continue to advertise an old signing key in our JWK Set for at least T_tokens seconds after we stop using it to sign tokens. If we don't, RPs who refresh their cache of our JWK Set may treat tokens signed with that key as invalid before they expire, resulting in spurious verification failures.

To help manage this, the server config has slots for three different keys, one required and the others optional:

  • openid.key: The private key that is actively being used to sign tokens. This key is required.
  • openid.newKey: A new private key that we intend to start using to sign tokens in the future. This key is optional, but can be specified in order to start advertising it before use.
  • openid.oldKey: The public component of a key we previously used to sign tokens. This key is optional, but can be specified in order to continue advertising it while tokens bearing its signature may be active.

The procedure for rotating the active signing key is:

  • Create the new signing key and configure it as openid.newKey, while leaving the existing openid.key intact.
    • This can be performed using ../scripts/prepare-new-signing-key.js.
  • Deploy to all server instances, then wait for at least T_cache seconds plus some margin for clock error.
  • Take the public component of openid.key and configure it as openid.oldKey, then move openid.newKey to openid.key.
    • This can be performed using ../scripts/activate-new-signing-key.js.
  • Deploy to all server instances, then wait for at least T_tokens seconds plus some margin for clock error.
  • Remove openid.oldKey from the configuration.
    • This can be performed using ../scripts/retire-old-signing-key.js.
  • Deploy to all server instances, then celebrate a job well done.

This scheme keeps things fairly simple, but it also means that each key rotation event must be at least T_cache + T_tokens seconds apart. If this proves to be a problem in practice, we could reduce the limit to T_cache by keeping a list of old signing keys rather than a single key.

For our main production environment, secret keys are managed via SOPS. If you're working in such an environment you can do this to extract the keys from SOPS into local files:

./scripts/sops-key-config.js extract path/to/encrypted/config.yaml

Then, after operating on them locally using the above key-rotation scripts, you can insert the modified values back into SOPS via:

./scripts/sops-key-config.js insert path/to/encrypted/config.yaml