Google and Apple sign-in

Status: Accepted (2026-09-15) · Amends: ADR-0024, ADR-0043, ADR-0045

Context#

ADR-0024 plans Google and Apple sign-in for v0.3: OIDC authorization code with PKCE, state and nonce on the web; ID-token verification for native apps; Apple's client secret signed from the .p8 key; account linking only on a verified email match; golang.org/x/oauth2 and github.com/coreos/go-oidc. ADR-0045 fixed the environment variables, the callback URLs (/v1/auth/{google,apple}/callback) and how developers are told what to provide.

Open questions left: how the web flow returns to a separate frontend, how native apps prove freshness, what happens to an existing account with the same email, whether a second factor still applies, how accounts without a password confirm sensitive changes, and what Apple requires on account deletion.

The maintainer decided (2026-09-15): link automatically on a provider-verified email match, and keep asking for the second factor on accounts that have one.

Decision#

1. Library: apistock.dev/modules/auth/social#

Dependencies: golang.org/x/oauth2 (v0.37), github.com/coreos/go-oidc/v3 (v3.21), and github.com/golang-jwt/jwt/v5 (already in the graph through passkeys). Separate package, so apps that don't import it don't compile it.

2. Configuration#

ADR-0045's variables, plus one:

• Google is on when GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET are set; GOOGLE_IOS_CLIENT_ID and GOOGLE_ANDROID_CLIENT_ID add native audiences. A native ID without the web client is refused at start.
• Apple web is on with APPLE_TEAM_ID, APPLE_SERVICES_ID, APPLE_KEY_ID and the key (APPLE_PRIVATE_KEY_FILE or APPLE_PRIVATE_KEY); APPLE_BUNDLE_IDS adds native audiences and needs the team, key ID and key.
• Partial configuration, an unreadable or non-P-256 key, and APP_PUBLIC_URL without https in production stop the app at start, naming the variable (ADR-0045).
• The status block, auth-providers and GET /ops/auth/providers gain google, google_ios, google_android, apple and apple_ios.

3. Web flow#

Browser → GET /v1/auth/{provider}/start?return_to=https://app.example.com/after-login
        ← 302 to Google or Apple, with a __Host-oauth cookie
Provider → GET /v1/auth/google/callback?code&state   or   POST /v1/auth/apple/callback (form_post)
        ← 303 to return_to: session cookie set, or #mfa_challenge_token=…&methods=… , or #error=…

4. Native flow#

Both return 200 with a session, or 202 with a second-factor challenge, exactly like POST /v1/auth/login.

5. Accounts#

identity (provider, subject) known       → its user
else provider-verified email matches user → link to that user
else provider-verified email              → new user, email verified, no password
else                                      → 403 social_email_unverified

6. Second factor and step-up#

• A Google or Apple sign-in to an account with two-factor authentication returns a challenge, like a password sign-in; the session is verified only after the second factor.
• Accounts without a password confirm sensitive changes (starting authenticator app setup, deleting the account, turning 2FA off, adding or removing a passkey after the 10-minute window) by signing in with their provider again: the session records authenticated_at, and a sign-in less than 10 minutes old stands in for the password. Setting a password through the reset flow is always possible.

7. Managing identities#

Account deletion revokes stored Apple tokens, as App Store rules require.

8. Security and limits#

• The per-IP auth limit extends to GET /v1/auth/{provider}/start and the callbacks; token endpoints count toward it like login.
• ID tokens older than 10 minutes (iat) are refused; nonces and states are single use and deleted by auth_cleanup once expired.
• Provider errors never leak tokens or codes to logs; audit records provider and subject hash only.

Audit, emails and errors#

Audit: auth.login.succeeded (method: google|apple), auth.login.failed (reasons invalid_social_token, social_email_unverified, invalid_state), auth.identity.linked, auth.identity.unlinked, auth.identity.apple_notification. Emails: provider sign-in added, provider sign-in removed.

Errors: invalid_social_token (401), invalid_state (401), social_email_unverified (403), last_sign_in_method (409), invalid_return_to (422), social_unavailable (503).

Why#

• Standard OIDC with a maintained verifier avoids hand-written JWT validation and vendor SDKs on the server.
• Server-side state bound to a cookie defeats login CSRF; fragments keep challenge tokens out of server logs and Referer.
• Automatic linking matches what users expect, and removing an unverified account's password closes the pre-hijacking hole automatic linking otherwise opens.
• Keeping the second factor makes 2FA policy independent of each provider's own security.
• Revoking Apple tokens and handling Apple's notifications meets App Store rules without a separate job.

Trade-offs#

• SameSite=None on the short-lived state cookie is needed for Apple's form post.
• Apple's web flow can't be tried on localhost; development uses a tunnel or the native flow.
• Relying on provider email verification means a provider mistake could link the wrong account; unverified Google emails are refused.
• One more table, a library package, and seven endpoints.

Consequences#

• modules/auth/social and socialtest (public API, ADR-0015).
• The Full preset gains a migration, repository files, use cases, endpoints, emails, .env.example blocks, status lines and tests for both flows, linking (including the unverified-account case), the second factor, unlinking, deletion revocation and Apple notifications.
• ADR-0045 gains APP_PUBLIC_URL; AUTH_PROVIDERS.md loses "coming in a later version".
• Threat model rows 14–15 are reviewed when this ships.

Implementation notes (2026-09-15)#

• modules/auth/social verifies tokens with go-oidc's remote key set and checks the issuer list, audience list, nonce (constant time) and age itself; Apple's ES256 client secret is signed with go-jose, already in the graph through go-oidc. New modules: golang.org/x/oauth2 v0.37, github.com/coreos/go-oidc/v3 v3.21, github.com/go-jose/go-jose/v4.
• "A recent sign-in" is the session's start: auth.Principal.SignedInAt, from auth_sessions.created_at, within auth.RecentVerification (10 minutes). No new column. It replaces the password for accounts without one in authenticator app setup, turning it off, account deletion, passkey changes and unlinking.
• A consent-revoked or account-delete notification unlinks the Apple identity and ends the account's sessions only when no password, passkey or other identity remains: sessions don't record which method started them.
• Cross-origin protection skips POST /v1/auth/apple/callback and POST /v1/auth/apple/notifications; the per-IP auth limit covers GET …/start and …/callback.
• A web sign-in without the __Host-oauth cookie still uses up its state and returns to its return_to with #error=invalid_state; a person cancelling at the provider returns #error=access_denied.
• Account deletion unlinks identities in its transaction and revokes Apple tokens afterwards (failures logged). rotate-auth-keys re-encrypts Apple refresh tokens with the authenticator secrets.
• User responses gain has_password, so clients can hide "change password" for accounts created with Google or Apple.
• Two first sign-ins of one person at the same moment race to create the account or identity; the loser's transaction rolls back on the unique constraint (ErrEmailTaken, ErrIdentityTaken) and retries once, finding the winner's account. Tested with concurrent requests under the race detector.
• Follow-up: Apple token revocation runs in the account-deletion request (up to 10 seconds per token, failures logged, no retry). A background job with retries would be more reliable; planned with the jobs-based cleanup work.
• The status block, auth-providers and GET /ops/auth/providers report google, google_ios, google_android, apple and apple_ios, with the callback URL.