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Authentication flow

Source: vignettes/authentication-flow.Rmd
authentication-flow.Rmd

Overview

This vignette provides a step-by-step description of what happens during an authentication flow when using the oauth_module_server() Shiny module. It maps protocol concepts (OAuth 2.0 Authorization Code + PKCE, OpenID Connect) to the concrete implementation details in the package.

For a concise quick-start (minimal and manual button examples, options, and security checklist) see: vignette("usage", package = "shinyOAuth").

For an explanation of logging key events during the flow, see: vignette("audit-logging", package = "shinyOAuth").

What happens during the authentication flow?

The package implements the OAuth 2.0 ‘Authorization Code’ flow and optional ‘OpenID Connect’ (OIDC) checks end‑to‑end. Below is the sequence of operations and the rationale behind each step.

1. First page load: set a browser token

On the first load of your app, the module sets a small random cookie in the user’s browser (SameSite=Strict; Secure when over HTTPS). This browser token is mirrored to Shiny as an input. Its purpose is to ensure that the same browser that starts the OAuth 2.0 flow is the one that finishes it (a “double-submit” style CSRF defense).

2. Decide whether to start login

If oauth_module_server(auto_redirect = TRUE), an unauthenticated session triggers immediate redirection to the provider authorization endpoint.

If oauth_module_server(auto_redirect = FALSE), you manually call $request_login() (e.g., via a button) to do so.

3. Build the authorization URL (prepare_call())

To redirect the user to the provider, the module constructs an authorization request URL. The URL is built from the provider’s authorization endpoint and includes various query parameters to ensure security and proper context tracking:

  • State: this is a high-entropy random string to prevent CSRF; this package seals the state to enhance security (see below)
  • PKCE: a code_verifier (random) and code_challenge (S256 hash) proving the same party finishes the flow
  • Nonce (OIDC): random string echoed back in the ID token, mitigating replay attacks

This package seals the state, meaning it encrypts and authenticates (AES-GCM AEAD) a payload containing:

  • state, client_id, redirect_uri
  • requested scopes
  • provider fingerprint (issuer/auth/token URLs)
  • issued_at timestamp

Sealing the state prevents tampering, stale callbacks, and mix-ups with other providers/clients.

On the server side, the package will store the sealed state (as a cache-safe hash key) in the state store (e.g., a ‘cachem’ backend) along with the following data:

  • browser token
  • code_verifier
  • nonce (OIDC)

All this data will be used for validation during the callback processing.

4. App redirects to the provider

The browser of the app user will be redirected to the provider’s authorization endpoint with the following parameters: response_type=code, client_id, redirect_uri, state=<sealed state>, PKCE parameters, nonce (OIDC), scope, plus any configured extra parameters.

5. User authenticates and authorizes

Once at the provider’s authorization page, the user is prompted to log in and authorize the app to access the requested scopes.

6. Provider redirects user back to the app

The provider redirects the user’s browser back to your Shiny app (your redirect_uri), including the code and state parameters (and optionally error and error_description on failure).

7. Callback processing & state verification (handle_callback())

Once the user is redirected back to the app, the module processes the callback. This consists of the following steps:

  • Wait for the browser token input if not yet visible
  • Decrypt and verify the sealed state, ensuring integrity, authenticity, and freshness (using the issued_at window)
  • Check that embedded context matches expected client/provider (defends against misconfiguration/multi-tenant mix-ups)
  • Fetch and immediately delete the one-time state entry from the configured state store
    • If the entry is missing, malformed, or deletion fails, the flow aborts with a shinyOAuth_state_error
    • Audit events are emitted on failures (e.g., state_store_lookup_failed, state_store_removal_failed)
  • Verify that user’s browser token matches the previously stored browser token
  • Ensure PKCE components are available when required

Note: in asynchronous token exchange mode, the module may pre‑decrypt the sealed state and prefetch plus remove the state store entry on the main thread before handing work to the async worker, preserving the same single‑use and strict failure behavior.

8. Exchange authorization code for tokens

Once the callback is verified, the module proceeds to exchange the authorization code for tokens.

A POST request is made to the token endpoint with grant_type=authorization_code, the code, the redirect_uri, and the code_verifier (PKCE). Client authentication method depends on provider style: HTTP Basic header (client_secret_basic), body params (client_secret_post), or JWT-based assertions (client_secret_jwt, private_key_jwt) when configured. The response must include at least access_token. Malformed or error responses abort the flow.

When successful, the package also applies two safety rails:

  • If the token response includes scope, shinyOAuth can reconcile it against the requested scopes (defaults to strict enforcement; configurable via the client scope_validation setting)
  • If the provider was configured with a non-empty allowed_token_types, the token response must include token_type and its value must be one of the allowed types (case-insensitive, e.g., Bearer)

9. Fetch userinfo (optional)

If userinfo is requested via oauth_provider(userinfo_required = TRUE) (for which you should have a userinfo_url configured), the module calls the userinfo endpoint with the access token and stores returned claims. If this request fails, the flow aborts with an error.

10. Validate ID token (OIDC only)

When using oauth_provider(id_token_validation = TRUE), the following verifications are performed:

  • Signature: verified against provider JWKS (with optional pinning) for standard asymmetric algorithms (RSA-PKCS1, RSA-PSS, ECDSA, EdDSA). HMAC algorithms (HS256/384/512) are only allowed with explicit opt-in (options(shinyOAuth.allow_hs = TRUE)) and a sufficiently strong server-held secret
  • Claims: iss matches expected issuer; aud vector contains client_id; sub present; iat is required and must be a single finite numeric; time-based claims (exp is required, nbf optional) are evaluated with a small configurable leeway; tokens issued in the future are rejected
  • Header typ (when present): must indicate a JWT (JWT, case-insensitive). Other values (e.g., at+jwt) are rejected for ID tokens
  • Nonce: must match the previously stored value (if configured)
  • Subject match: if oauth_provider(userinfo_id_token_match = TRUE), it is checked that sub in userinfo equals sub in the ID token

11. Token introspection (optional)

Some providers support RFC 7662 token introspection (an additional endpoint where the server can ask the provider whether an access token is currently active and retrieve related metadata).

If you enable introspect = TRUE when creating your oauth_client(), the module calls the provider’s introspection endpoint during callback processing and requires the response to indicate active = TRUE. If introspection is unsupported by the provider or the introspection request fails, the login is aborted and $authenticated is not set to TRUE.

You can optionally enforce additional provider-dependent fields via oauth_client(introspect_elements = ...):

  • "sub" – require introspection sub to match the session subject
  • "client_id" – require introspection client_id to match your OAuth client id
  • "scope" – validate introspection scope against requested scopes (respects the client’s scope_validation mode)

(Note that not all providers may return each of these fields in introspection responses.)

12. Build the OAuthToken object

Now that all verifications have passed, the module builds the final token object. This is an S7 OAuthToken object which contains:

  • access_token (string)
  • refresh_token (optional string)
  • expires_at (numeric timestamp, seconds since epoch; Inf for non-expiring tokens)
  • id_token (optional string)
  • userinfo (optional list)

The $authenticated value as returned by oauth_module_server() now becomes TRUE, meaning all requested verifications have passed.

13. Clean URL & tidy UI; clear browser token

The user’s browser was redirected to your app with OAuth 2.0 query parameters (code, state, etc.). To improve UX and avoid leaking sensitive data, these values are removed from the address bar with JavaScript. Optionally, the page title may also be adjusted (see the tab_title_ arguments in oauth_module_server()).

The browser token cookie is also cleared and immediately re-issued with a fresh value, so a future flow can start with a new per-session token.

14. Post-flow session management

Now that the flow is complete, the module will manage the token lifetime during the active session. This may consist of:

  • Proactive refresh: if enabled and a refresh token exists, the access token is refreshed before expiry
  • Expiration: expired tokens are cleared automatically, setting the $authenticated flag to FALSE
  • Re-authentication: optionally, oauth_module_server(reauth_after_seconds = ...) can force periodic re-authentication

Refresh behavior (refresh_token())

When the module refreshes a session (or when you call refresh_token() directly), it performs an OAuth 2.0 refresh token grant against the provider’s token endpoint and updates the OAuthToken object. This works as follows:

  • A token request is sent with grant_type=refresh_token and the current refresh_token
  • The response must include a new access_token. expires_at is updated from expires_in when present; otherwise it is set to Inf
  • If the provider rotates the refresh token (returns a new refresh_token), it is stored; otherwise the original is preserved
  • If oauth_provider(userinfo_required = TRUE), userinfo is re-fetched using the fresh access token

With respect to OIDC ID token handling:

  • Per OIDC Core Section 12.2, refresh responses may omit id_token. When omitted, the original id_token from the initial login is preserved. Thus, a refresh does not necessarily revalidate identity
  • If the provider does return an id_token during refresh, shinyOAuth enforces OIDC 12.2 subject continuity: the refresh-returned id_token must have the same sub as the original id_token from login
    • If an original id_token did not exist in the session, and the refresh does return one, the refresh fails (cannot establish subject claim match with no baseline)
    • If id_token_validation = TRUE, the refresh-returned id_token is fully validated (signature + claims); the sub claim match is enforced as part of validation
    • If id_token_validation = FALSE, shinyOAuth still enforces the sub match by parsing the JWT payload (ensuring that the sub claim still matches but without full validation)

If refresh fails inside oauth_module_server(), the module exposes the failure via its reactive state (for example, token_refresh_error). By default it also clears the current session token; if oauth_module_server(indefinite_session = TRUE), the token is kept but marked stale. In all cases, the $authenticated flag becomes FALSE while the error is present.

15. Logout and token revocation

When auth$logout() is called, the module:

  1. Attempts to revoke both refresh and access tokens at the provider (RFC 7009) if a revocation_url is configured. This runs asynchronously only when oauth_module_server(async = TRUE)
  2. Clears the local session (OAuthToken, browser cookie)
  3. Emits a "logout" audit event
  4. Re-issues a fresh browser token for subsequent logins

You can also revoke tokens directly via revoke_token(client, token, which = "refresh").

To automatically attempt revocation when a Shiny session ends (for example, a tab close or session timeout), set revoke_on_session_end = TRUE:

auth <- oauth_module_server(
  "auth",
  client = client,
  revoke_on_session_end = TRUE
)

This is best-effort: the session may end while the provider is unavailable, and revocation failures do not block session cleanup.