README.md

@@ -54,10 +54,12 @@ All configuration is via environment variables; every value has a default, so `.
 | `PANSY_OIDC_ISSUER`       | *(empty)*          | OIDC issuer/discovery URL (Authentik). Enables SSO when set.       |
 | `PANSY_OIDC_CLIENT_ID`    | *(empty)*          | OIDC client ID.                                                    |
 | `PANSY_OIDC_CLIENT_SECRET`| *(empty)*          | OIDC client secret.                                                |
-| `PANSY_OIDC_BUTTON_LABEL` | `Sign in with SSO` | Label for the OIDC button on the login page.                      |
+| `PANSY_OIDC_BUTTON_LABEL` | `Sign in with Authentik` | Label for the OIDC button on the login page.                |
 | `PANSY_TRUSTED_PROXIES`   | *(none)*           | Comma-separated proxy CIDRs/IPs to trust for client-IP resolution. |
 
-Local email/password auth is live (`POST /api/v1/auth/register`, `/auth/login`, `/auth/logout`, `GET /auth/me`, `GET /auth/providers`); the session is an HttpOnly cookie (`Secure` when `PANSY_BASE_URL` is https). The first account registered becomes admin, and it may register even when `PANSY_REGISTRATION=closed` to bootstrap the instance. OIDC (`PANSY_OIDC_*`) lands in #5.
+Local email/password auth is live (`POST /api/v1/auth/register`, `/auth/login`, `/auth/logout`, `GET /auth/me`, `GET /auth/providers`); the session is an HttpOnly cookie (`Secure` when `PANSY_BASE_URL` is https). The first account registered becomes admin, and it may register even when `PANSY_REGISTRATION=closed` to bootstrap the instance.
+
+OIDC (Authentik-first) is live too: set `PANSY_OIDC_ISSUER`, `PANSY_OIDC_CLIENT_ID`, `PANSY_OIDC_CLIENT_SECRET`, and `PANSY_BASE_URL` (needed for the redirect URI). Register `PANSY_BASE_URL` + `/api/v1/auth/oidc/callback` as the redirect URI in your IdP. `GET /auth/oidc/login` starts an authorization-code + PKCE flow; first login provisions a user just-in-time (a matching *verified* email links to an existing local account instead of duplicating it). Provider discovery is lazy, so a briefly-unreachable IdP never blocks startup or local auth. Set `PANSY_LOCAL_AUTH=false` for pure-Authentik deployments (local register/login are then rejected and hidden from `/auth/providers`).
 
 ## Docker & deployment
 

go.mod

@@ -3,9 +3,11 @@ module gitea.stevedudenhoeffer.com/steve/pansy
 go 1.26.2
 
 require (
+	github.com/coreos/go-oidc/v3 v3.20.0
 	github.com/gin-gonic/gin v1.10.1
 	github.com/samber/slog-gin v1.15.0
 	golang.org/x/crypto v0.31.0
+	golang.org/x/oauth2 v0.36.0
 	modernc.org/sqlite v1.34.4
 )
 
@@ -17,6 +19,7 @@ require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.4 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
+	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.22.0 // indirect

go.sum

@@ -6,6 +6,8 @@ github.com/cloudwego/base64x v0.1.4 h1:jwCgWpFanWmN8xoIUHa2rtzmkd5J2plF/dnLS6Xd/
 github.com/cloudwego/base64x v0.1.4/go.mod h1:0zlkT4Wn5C6NdauXdJRhSKRlJvmclQ1hhJgA0rcu/8w=
 github.com/cloudwego/iasm v0.2.0 h1:1KNIy1I1H9hNNFEEH3DVnI4UujN+1zjpuk6gwHLTssg=
 github.com/cloudwego/iasm v0.2.0/go.mod h1:8rXZaNYT2n95jn+zTI1sDr+IgcD2GVs0nlbbQPiEFhY=
+github.com/coreos/go-oidc/v3 v3.20.0 h1:EtE0WIBHk03N+DqGkY4+UONzzZHk7amKt6IyNd7OsZE=
+github.com/coreos/go-oidc/v3 v3.20.0/go.mod h1:DYCf24+ncYi+XkIH97GY1+dqoRlbaSI26KVTCI9SrY4=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -17,6 +19,8 @@ github.com/gin-contrib/sse v0.1.0 h1:Y/yl/+YNO8GZSjAhjMsSuLt29uWRFHdHYUb5lYOV9qE
 github.com/gin-contrib/sse v0.1.0/go.mod h1:RHrZQHXnP2xjPF+u1gW/2HnVO7nvIa9PG3Gm+fLHvGI=
 github.com/gin-gonic/gin v1.10.1 h1:T0ujvqyCSqRopADpgPgiTT63DUQVSfojyME59Ei63pQ=
 github.com/gin-gonic/gin v1.10.1/go.mod h1:4PMNQiOhvDRa013RKVbsiNwoyezlm2rm0uX/T7kzp5Y=
+github.com/go-jose/go-jose/v4 v4.1.4 h1:moDMcTHmvE6Groj34emNPLs/qtYXRVcd6S7NHbHz3kA=
+github.com/go-jose/go-jose/v4 v4.1.4/go.mod h1:x4oUasVrzR7071A4TnHLGSPpNOm2a21K9Kf04k1rs08=
 github.com/go-playground/assert/v2 v2.2.0 h1:JvknZsQTYeFEAhQwI4qEt9cyV5ONwRHC+lYKSsYSR8s=
 github.com/go-playground/assert/v2 v2.2.0/go.mod h1:VDjEfimB/XKnb+ZQfWdccd7VUvScMdVu0Titje2rxJ4=
 github.com/go-playground/locales v0.14.1 h1:EWaQ/wswjilfKLTECiXz7Rh+3BjFhfDFKv/oXslEjJA=
@@ -94,6 +98,8 @@ golang.org/x/mod v0.17.0 h1:zY54UmvipHiNd+pm+m0x9KhZ9hl1/7QNMyxXbc6ICqA=
 golang.org/x/mod v0.17.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
 golang.org/x/net v0.33.0 h1:74SYHlV8BIgHIFC/LrYkOGIwL19eTYXQ5wc6TBuO36I=
 golang.org/x/net v0.33.0/go.mod h1:HXLR5J+9DxmrqMwG9qjGCxZ+zKXxBru04zlTvWlWuN4=
+golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
+golang.org/x/oauth2 v0.36.0/go.mod h1:YDBUJMTkDnJS+A4BP4eZBjCqtokkg1hODuPjwiGPO7Q=
 golang.org/x/sync v0.10.0 h1:3NQrjDixjgGwUOCaF8w2+VYHv0Ve/vGYSbdkTa98gmQ=
 golang.org/x/sync v0.10.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
 golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

internal/api/api.go

@@ -19,8 +19,9 @@ import (
 // handlers carries the dependencies shared by every HTTP handler. Handlers stay
 // thin: decode the request, call a service method, encode the result.
 type handlers struct {
-	cfg *config.Config
-	svc *service.Service
+	cfg  *config.Config
+	svc  *service.Service
+	oidc *oidcClient // nil unless OIDC is configured (see config.OIDCReady)
 }
 
 // New builds the gin engine with the standard middleware stack and registers the
@@ -59,6 +60,19 @@ func New(cfg *config.Config, svc *service.Service) *gin.Engine {
 	auth.GET("/providers", h.providers)
 	auth.GET("/me", h.requireAuth(), h.me)
 
+	// OIDC routes exist only when OIDC can actually be offered, so an unconfigured
+	// instance 404s them (matching what /auth/providers advertises). Provider
+	// discovery is lazy (first request), so a briefly-unreachable IdP doesn't stop
+	// the server — or local auth — from starting.
+	switch {
+	case cfg.OIDCReady():
+		h.oidc = newOIDCClient(cfg)
+		auth.GET("/oidc/login", h.oidcLogin)
+		auth.GET("/oidc/callback", h.oidcCallback)
+	case cfg.OIDC.Enabled():
+		slog.Warn("api: OIDC is configured but PANSY_BASE_URL is unset; OIDC disabled (an absolute redirect URI is required)")
+	}
+
 	return r
 }
 

internal/api/auth.go

@@ -160,18 +160,26 @@ func (h *handlers) csrfGuard() gin.HandlerFunc {
 	}
 }
 
+// startSession issues a session for the user and writes the session cookie.
+func (h *handlers) startSession(c *gin.Context, userID int64) error {
+	token, expiresAt, err := h.svc.CreateSession(c.Request.Context(), userID)
+	if err != nil {
+		return err
+	}
+	h.setSessionCookie(c, token, expiresAt)
+	return nil
+}
+
 // startSessionAndRespond issues a session, sets the cookie, and returns the user
 // as JSON — the shared tail of register and login.
 func (h *handlers) startSessionAndRespond(c *gin.Context, user *domain.User) {
-	token, expiresAt, err := h.svc.CreateSession(c.Request.Context(), user.ID)
-	if err != nil {
+	if err := h.startSession(c, user.ID); err != nil {
 		// The account exists and is usable via login; only the auto-login cookie
 		// failed. Log it so the operator can see the underlying DB problem.
 		slog.Error("api: could not start session", "user_id", user.ID, "error", err)
 		writeAPIError(c, http.StatusInternalServerError, "INTERNAL", "could not start session")
 		return
 	}
-	h.setSessionCookie(c, token, expiresAt)
 	c.JSON(http.StatusOK, user)
 }
 
@@ -215,6 +223,12 @@ func writeServiceError(c *gin.Context, err error) {
 		writeAPIError(c, http.StatusForbidden, "REGISTRATION_CLOSED", "registration is closed")
 	case errors.Is(err, domain.ErrLocalAuthDisabled):
 		writeAPIError(c, http.StatusForbidden, "LOCAL_AUTH_DISABLED", "local authentication is disabled")
+	case errors.Is(err, domain.ErrOIDCNoEmail):
+		writeAPIError(c, http.StatusBadRequest, "OIDC_NO_EMAIL", "the identity provider returned no email")
+	case errors.Is(err, domain.ErrOIDCEmailUnverified):
+		writeAPIError(c, http.StatusForbidden, "OIDC_EMAIL_UNVERIFIED", "the identity provider's email is not verified")
+	case errors.Is(err, domain.ErrOIDCIdentityConflict):
+		writeAPIError(c, http.StatusConflict, "OIDC_IDENTITY_CONFLICT", "this identity conflicts with an existing account")
 	case errors.Is(err, domain.ErrInvalidInput):
 		writeAPIError(c, http.StatusBadRequest, "INVALID_INPUT", "invalid input")
 	default:

internal/api/oidc.go

@@ -0,0 +1,319 @@
+package api
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"log/slog"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/coreos/go-oidc/v3/oidc"
+	"github.com/gin-gonic/gin"
+	"golang.org/x/oauth2"
+
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/config"
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/service"
+)
+
+const (
+	// oidcCallbackPath is appended to PANSY_BASE_URL to form the redirect URI
+	// registered with the IdP.
+	oidcCallbackPath = "/api/v1/auth/oidc/callback"
+	// oidcTxCookie holds the short-lived per-login transaction state (CSRF state,
+	// PKCE verifier, nonce) between /oidc/login and /oidc/callback.
+	oidcTxCookie = "pansy_oidc_tx"
+	oidcTxMaxAge = 10 * time.Minute
+	// oidcDiscoveryTimeout bounds a single lazy discovery attempt so a hung IdP
+	// can't wedge a request goroutine.
+	oidcDiscoveryTimeout = 10 * time.Second
+	// oidcExchangeTimeout bounds the callback's token exchange + ID-token
+	// verification (which also fetches JWKS) so a slow IdP can't outlast the
+	// server's write timeout and leave the user on a blank page.
+	oidcExchangeTimeout = 15 * time.Second
+)
+
+// oidcClient lazily performs OIDC discovery and holds the derived verifier and
+// oauth2 config. Discovery is deferred to the first login/callback (and retried
+// on the next request if it fails) so an IdP that's momentarily unreachable at
+// boot doesn't stop the server — or local auth — from starting.
+type oidcClient struct {
+	issuer       string
+	clientID     string
+	clientSecret string
+	redirectURL  string
+
+	mu       sync.Mutex
+	provider *oidc.Provider
+	verifier *oidc.IDTokenVerifier
+	oauth    *oauth2.Config
+}
+
+func newOIDCClient(cfg *config.Config) *oidcClient {
+	return &oidcClient{
+		issuer:       cfg.OIDC.Issuer,
+		clientID:     cfg.OIDC.ClientID,
+		clientSecret: cfg.OIDC.ClientSecret,
+		redirectURL:  cfg.BaseURL + oidcCallbackPath,
+	}
+}
+
+// ensure performs discovery once (idempotent) and builds the verifier + oauth2
+// config. Safe for concurrent callers; a failure leaves the client uninitialized
+// so the next request retries. The network discovery runs OUTSIDE the mutex, so
+// concurrent cold-start (or IdP-outage) requests don't serialize behind one
+// another's full timeout; the first to finish installs the result.
+func (o *oidcClient) ensure(ctx context.Context) error {
+	o.mu.Lock()
+	ready := o.provider != nil
+	o.mu.Unlock()
+	if ready {
+		return nil
+	}
+
+	dctx, cancel := context.WithTimeout(ctx, oidcDiscoveryTimeout)
+	defer cancel()
+
+	provider, err := oidc.NewProvider(dctx, o.issuer)
+	if err != nil {
+		return err
+	}
+
+	o.mu.Lock()
+	defer o.mu.Unlock()
+	if o.provider != nil {
+		return nil // another goroutine won the race; keep its result.
+	}
+	o.provider = provider
+	o.verifier = provider.Verifier(&oidc.Config{ClientID: o.clientID})
+	o.oauth = &oauth2.Config{
+		ClientID:     o.clientID,
+		ClientSecret: o.clientSecret,
+		Endpoint:     provider.Endpoint(),
+		RedirectURL:  o.redirectURL,
+		Scopes:       []string{oidc.ScopeOpenID, "email", "profile"},
+	}
+	return nil
+}
+
+// oidcTx is the per-login transaction stashed in the tx cookie. It never leaves
+// the browser as anything an attacker can forge (HttpOnly, our domain), and the
+// callback checks the returned state against it (CSRF), uses the PKCE verifier
+// for the code exchange, and checks the ID-token nonce.
+type oidcTx struct {
+	State    string `json:"s"`
+	Verifier string `json:"v"`
+	Nonce    string `json:"n"`
+}
+
+// redirectAuthError sends the browser back to the login page with an error code
+// the UI (#6) can render. Codes: oidc_unavailable, state, no_email,
+// email_unverified, oidc_conflict, oidc (generic).
+func redirectAuthError(c *gin.Context, code string) {
+	c.Redirect(http.StatusFound, "/login?error="+code)
+}
+
+// oidcLogin starts the authorization-code + PKCE flow: it stashes fresh state,
+// PKCE verifier, and nonce in a short-lived cookie, then redirects to the IdP.
+func (h *handlers) oidcLogin(c *gin.Context) {
+	if err := h.oidc.ensure(c.Request.Context()); err != nil {
+		slog.Error("api: oidc discovery failed", "error", err)
+		redirectAuthError(c, "oidc_unavailable")
+		return
+	}
+
+	state, err1 := randToken()
+	nonce, err2 := randToken()
+	if err1 != nil || err2 != nil {
+		slog.Error("api: oidc token generation failed", "state_err", err1, "nonce_err", err2)
+		redirectAuthError(c, "oidc")
+		return
+	}
+	verifier := oauth2.GenerateVerifier()
+
+	if err := h.setOIDCTxCookie(c, oidcTx{State: state, Verifier: verifier, Nonce: nonce}); err != nil {
+		slog.Error("api: set oidc tx cookie", "error", err)
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	authURL := h.oidc.oauth.AuthCodeURL(state,
+		oauth2.S256ChallengeOption(verifier),
+		oidc.Nonce(nonce),
+	)
+	c.Redirect(http.StatusFound, authURL)
+}
+
+// oidcCallback completes the flow: verify state, exchange the code (with the PKCE
+// verifier), verify the ID token and nonce, provision/link the user, and start a
+// pansy session. Every failure clears the tx cookie and redirects to the login
+// page with an error code rather than leaking details to the browser.
+func (h *handlers) oidcCallback(c *gin.Context) {
+	tx, haveTx := h.readOIDCTxCookie(c)
+	h.clearOIDCTxCookie(c)
+
+	// A provider-side error (e.g. user denied consent) comes back as ?error=.
+	if e := c.Query("error"); e != "" {
+		slog.Warn("api: oidc provider returned error", "error", e)
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	// State must be present and match the cookie (CSRF protection). Constant-time
+	// compare avoids leaking via timing.
+	state := c.Query("state")
+	if !haveTx || state == "" || subtle.ConstantTimeCompare([]byte(state), []byte(tx.State)) != 1 {
+		slog.Warn("api: oidc state mismatch or missing transaction")
+		redirectAuthError(c, "state")
+		return
+	}
+
+	// Bound the network work (token exchange + JWKS fetch during verify) so a slow
+	// IdP can't outlast the server's write timeout.
+	ctx, cancel := context.WithTimeout(c.Request.Context(), oidcExchangeTimeout)
+	defer cancel()
+
+	if err := h.oidc.ensure(ctx); err != nil {
+		slog.Error("api: oidc discovery failed on callback", "error", err)
+		redirectAuthError(c, "oidc_unavailable")
+		return
+	}
+
+	code := c.Query("code")
+	if code == "" {
+		slog.Warn("api: oidc callback missing code")
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	token, err := h.oidc.oauth.Exchange(ctx, code, oauth2.VerifierOption(tx.Verifier))
+	if err != nil {
+		slog.Error("api: oidc code exchange failed", "error", err)
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	rawIDToken, ok := token.Extra("id_token").(string)
+	if !ok || rawIDToken == "" {
+		slog.Error("api: oidc token response missing id_token")
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	idToken, err := h.oidc.verifier.Verify(ctx, rawIDToken)
+	if err != nil {
+		slog.Error("api: oidc id_token verification failed", "error", err)
+		redirectAuthError(c, "oidc")
+		return
+	}
+	if subtle.ConstantTimeCompare([]byte(idToken.Nonce), []byte(tx.Nonce)) != 1 {
+		slog.Warn("api: oidc nonce mismatch")
+		redirectAuthError(c, "oidc")
+		return
+	}
+
+	var claims struct {
+		Email             string `json:"email"`
+		EmailVerified     bool   `json:"email_verified"`
+		Name              string `json:"name"`
+		PreferredUsername string `json:"preferred_username"`
+	}
+	if err := idToken.Claims(&claims); err != nil {
+		slog.Error("api: oidc claims decode failed", "error", err)
+		redirectAuthError(c, "oidc")
+		return
+	}
+	name := claims.Name
+	if name == "" {
+		name = claims.PreferredUsername
+	}
+
+	// Issuer/Subject come from the *verified* token, not raw claims.
+	user, err := h.svc.LoginOIDC(ctx, service.OIDCIdentity{
+		Issuer:        idToken.Issuer,
+		Subject:       idToken.Subject,
+		Email:         claims.Email,
+		EmailVerified: claims.EmailVerified,
+		Name:          name,
+	})
+	if err != nil {
+		slog.Warn("api: oidc provisioning failed", "error", err)
+		redirectAuthError(c, oidcProvisionErrorCode(err))
+		return
+	}
+
+	if err := h.startSession(c, user.ID); err != nil {
+		slog.Error("api: oidc session start failed", "user_id", user.ID, "error", err)
+		redirectAuthError(c, "oidc")
+		return
+	}
+	c.Redirect(http.StatusFound, "/gardens")
+}
+
+// oidcProvisionErrorCode maps a LoginOIDC failure to a login-page error code so
+// the UI can explain what went wrong instead of showing a generic message.
+func oidcProvisionErrorCode(err error) string {
+	switch {
+	case errors.Is(err, domain.ErrOIDCNoEmail):
+		return "no_email"
+	case errors.Is(err, domain.ErrOIDCEmailUnverified):
+		return "email_unverified"
+	case errors.Is(err, domain.ErrOIDCIdentityConflict):
+		return "oidc_conflict"
+	default:
+		return "oidc"
+	}
+}
+
+// setOIDCTxCookie writes the login transaction as a compact base64 JSON cookie.
+// It returns an error rather than swallowing a marshal failure, so oidcLogin
+// never redirects to the IdP with no way to complete the callback.
+func (h *handlers) setOIDCTxCookie(c *gin.Context, tx oidcTx) error {
+	b, err := json.Marshal(tx)
+	if err != nil {
+		return err // tx holds only our own strings, so this can't happen in practice.
+	}
+	value := base64.RawURLEncoding.EncodeToString(b)
+	// SameSite=Lax so the cookie survives the IdP's top-level redirect back to the
+	// callback (Strict would drop it on that cross-site navigation).
+	c.SetSameSite(http.SameSiteLaxMode)
+	c.SetCookie(oidcTxCookie, value, int(oidcTxMaxAge.Seconds()), "/", "", h.cookieSecure(), true)
+	return nil
+}
+
+func (h *handlers) readOIDCTxCookie(c *gin.Context) (oidcTx, bool) {
+	value, err := c.Cookie(oidcTxCookie)
+	if err != nil || value == "" {
+		return oidcTx{}, false
+	}
+	raw, err := base64.RawURLEncoding.DecodeString(value)
+	if err != nil {
+		return oidcTx{}, false
+	}
+	var tx oidcTx
+	// All three fields must be present: an empty nonce would make the callback's
+	// nonce check pass vacuously against an ID token that omitted the claim.
+	if err := json.Unmarshal(raw, &tx); err != nil || tx.State == "" || tx.Verifier == "" || tx.Nonce == "" {
+		return oidcTx{}, false
+	}
+	return tx, true
+}
+
+func (h *handlers) clearOIDCTxCookie(c *gin.Context) {
+	c.SetSameSite(http.SameSiteLaxMode)
+	c.SetCookie(oidcTxCookie, "", -1, "/", "", h.cookieSecure(), true)
+}
+
+// randToken returns 32 bytes of URL-safe randomness for state/nonce values.
+func randToken() (string, error) {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		return "", err
+	}
+	return base64.RawURLEncoding.EncodeToString(b), nil
+}

internal/api/oidc_test.go

@@ -0,0 +1,114 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/config"
+)
+
+// fakeIssuer stands up a minimal OIDC discovery endpoint so oidcClient.ensure
+// succeeds without a real IdP. It only needs to serve the discovery document for
+// the login-initiation and route tests; token exchange is covered by the
+// service-layer provisioning tests and manual Authentik verification.
+func fakeIssuer(t *testing.T) string {
+	t.Helper()
+	mux := http.NewServeMux()
+	var issuer string
+	mux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                 issuer,
+			"authorization_endpoint": issuer + "/authorize",
+			"token_endpoint":         issuer + "/token",
+			"jwks_uri":               issuer + "/jwks",
+		})
+	})
+	ts := httptest.NewServer(mux)
+	t.Cleanup(ts.Close)
+	issuer = ts.URL
+	return issuer
+}
+
+func oidcCfg(t *testing.T) *config.Config {
+	cfg := localCfg()
+	cfg.BaseURL = "https://pansy.example.com"
+	cfg.OIDC = config.OIDCConfig{Issuer: fakeIssuer(t), ClientID: "pansy-client", ButtonLabel: "Sign in with Authentik"}
+	return cfg
+}
+
+func TestOIDCRoutesAbsentWhenUnconfigured(t *testing.T) {
+	r := authEngine(t, localCfg()) // no OIDC
+	for _, path := range []string{"/api/v1/auth/oidc/login", "/api/v1/auth/oidc/callback"} {
+		w := doJSON(t, r, http.MethodGet, path, nil, nil)
+		if w.Code != http.StatusNotFound {
+			t.Errorf("GET %s status = %d, want 404 when OIDC unconfigured", path, w.Code)
+		}
+	}
+}
+
+func TestProvidersReportsOIDCWhenConfigured(t *testing.T) {
+	r := authEngine(t, oidcCfg(t))
+	w := doJSON(t, r, http.MethodGet, "/api/v1/auth/providers", nil, nil)
+	var got struct {
+		Local     bool   `json:"local"`
+		OIDC      bool   `json:"oidc"`
+		OIDCLabel string `json:"oidcLabel"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decode providers: %v", err)
+	}
+	if !got.OIDC || got.OIDCLabel != "Sign in with Authentik" {
+		t.Errorf("providers = %+v, want oidc=true with Authentik label", got)
+	}
+}
+
+func TestOIDCLoginRedirectsToProviderWithPKCE(t *testing.T) {
+	r := authEngine(t, oidcCfg(t))
+	w := doJSON(t, r, http.MethodGet, "/api/v1/auth/oidc/login", nil, nil)
+	if w.Code != http.StatusFound {
+		t.Fatalf("oidc login status = %d, want 302 (body %s)", w.Code, w.Body.String())
+	}
+	loc := w.Header().Get("Location")
+	for _, want := range []string{"/authorize?", "code_challenge=", "code_challenge_method=S256", "state=", "nonce="} {
+		if !strings.Contains(loc, want) {
+			t.Errorf("auth redirect %q missing %q", loc, want)
+		}
+	}
+	// The transaction cookie must be set so the callback can validate state.
+	var haveTx bool
+	for _, ck := range w.Result().Cookies() {
+		if ck.Name == oidcTxCookie {
+			haveTx = ck.HttpOnly && ck.Value != ""
+		}
+	}
+	if !haveTx {
+		t.Error("expected an HttpOnly pansy_oidc_tx cookie to be set")
+	}
+}
+
+func TestOIDCCallbackWithoutTransactionRejected(t *testing.T) {
+	r := authEngine(t, oidcCfg(t))
+	// No tx cookie → state can't be validated → redirect to login with error=state.
+	w := doJSON(t, r, http.MethodGet, "/api/v1/auth/oidc/callback?state=abc&code=xyz", nil, nil)
+	if w.Code != http.StatusFound {
+		t.Fatalf("callback status = %d, want 302", w.Code)
+	}
+	if loc := w.Header().Get("Location"); !strings.Contains(loc, "error=state") {
+		t.Errorf("callback redirect = %q, want error=state", loc)
+	}
+}
+
+func TestOIDCCallbackProviderErrorRedirects(t *testing.T) {
+	r := authEngine(t, oidcCfg(t))
+	w := doJSON(t, r, http.MethodGet, "/api/v1/auth/oidc/callback?error=access_denied", nil, nil)
+	if w.Code != http.StatusFound {
+		t.Fatalf("callback status = %d, want 302", w.Code)
+	}
+	if loc := w.Header().Get("Location"); !strings.Contains(loc, "error=oidc") {
+		t.Errorf("callback redirect = %q, want error=oidc", loc)
+	}
+}

internal/config/config.go

@@ -56,6 +56,14 @@ func (o OIDCConfig) Enabled() bool {
 	return o.Issuer != "" && o.ClientID != ""
 }
 
+// OIDCReady reports whether OIDC login can actually be offered: it needs an
+// issuer + client ID and a BaseURL to build the absolute redirect URI that
+// providers require. Both the login page (via /auth/providers) and route
+// registration gate on this, so the advertised methods match the live routes.
+func (c *Config) OIDCReady() bool {
+	return c.OIDC.Enabled() && c.BaseURL != ""
+}
+
 // RegistrationOpen reports whether local self-service signup is allowed.
 func (c *Config) RegistrationOpen() bool {
 	return c.Registration == RegistrationOpen
@@ -74,7 +82,7 @@ func Load() *Config {
 			Issuer:       envStr("PANSY_OIDC_ISSUER", ""),
 			ClientID:     envStr("PANSY_OIDC_CLIENT_ID", ""),
 			ClientSecret: envStr("PANSY_OIDC_CLIENT_SECRET", ""),
-			ButtonLabel:  envStr("PANSY_OIDC_BUTTON_LABEL", "Sign in with SSO"),
+			ButtonLabel:  envStr("PANSY_OIDC_BUTTON_LABEL", "Sign in with Authentik"),
 		},
 		TrustedProxies: envList("PANSY_TRUSTED_PROXIES"),
 	}

internal/domain/domain.go

@@ -35,6 +35,19 @@ var (
 	// ErrLocalAuthDisabled means PANSY_LOCAL_AUTH=false, so local register/login
 	// are rejected in favor of OIDC. Mapped to 403.
 	ErrLocalAuthDisabled = errors.New("local authentication disabled")
+
+	// ErrOIDCNoEmail means the IdP returned no email claim, so no account can be
+	// provisioned (email is the account's unique key). The email scope is required.
+	ErrOIDCNoEmail = errors.New("oidc identity has no email")
+	// ErrOIDCEmailUnverified means the IdP asserted an email it hasn't verified;
+	// pansy won't provision or link on an unverified email (it would enable
+	// account takeover / squatting).
+	ErrOIDCEmailUnverified = errors.New("oidc email not verified")
+	// ErrOIDCIdentityConflict means the OIDC identity can't be attached: either
+	// the target account already carries a different identity (refusing to
+	// overwrite it prevents lockout/takeover) or the (issuer, subject) pair is
+	// already bound to another account. Mapped to 409.
+	ErrOIDCIdentityConflict = errors.New("oidc identity conflict")
 )
 
 // Enumerated string values mirrored from the schema CHECK constraints.

internal/service/auth.go

@@ -30,17 +30,90 @@ type Providers struct {
 	OIDCLabel string `json:"oidcLabel"`
 }
 
-// Providers returns the enabled auth methods. OIDC is always false until #5
-// wires the endpoints; advertising it before then would point the UI at routes
-// that don't exist.
+// OIDCIdentity is the set of claims the API layer extracts from a verified ID
+// token and hands to LoginOIDC for provisioning. Issuer and Subject come from
+// the verified token (not raw claims); Email/Name come from claims.
+type OIDCIdentity struct {
+	Issuer        string
+	Subject       string
+	Email         string
+	EmailVerified bool
+	Name          string
+}
+
+// Providers returns the enabled auth methods, so the login page renders the
+// right controls. OIDC is reported only when it can actually be offered (issuer,
+// client ID, and a BaseURL for the redirect URI), matching the routes that
+// api.New registers.
 func (s *Service) Providers() Providers {
 	return Providers{
 		Local:     s.cfg.LocalAuth,
-		OIDC:      false,
+		OIDC:      s.cfg.OIDCReady(),
 		OIDCLabel: s.cfg.OIDC.ButtonLabel,
 	}
 }
 
+// LoginOIDC provisions and returns the pansy user for a verified OIDC identity,
+// issuing no session itself (the caller does). The IdP has already gated access,
+// so PANSY_REGISTRATION does not apply. Resolution order:
+//  1. an existing user with the same (issuer, subject) — a returning OIDC user;
+//  2. else an existing user with the same email — linked to this identity (so
+//     one person isn't split across a local and an OIDC account);
+//  3. else a new just-in-time account stamped with the identity.
+//
+// Steps 2 and 3 require a verified email: linking on an unverified email would
+// let anyone who can assert that email at an IdP take over an account, and
+// JIT-creating on one would let them squat an email a real user later owns
+// (then the real user's different identity would be refused by LinkOIDC). An
+// identity with no email can't be provisioned at all (email is the account key).
+// Returning users (step 1) skip the email check — their identity is already
+// proven.
+func (s *Service) LoginOIDC(ctx context.Context, id OIDCIdentity) (*domain.User, error) {
+	if id.Issuer == "" || id.Subject == "" {
+		return nil, domain.ErrInvalidInput
+	}
+
+	// 1. Returning OIDC user (identity already proven; email state irrelevant).
+	u, err := s.store.GetUserByOIDC(ctx, id.Issuer, id.Subject)
+	if err == nil {
+		return u, nil
+	}
+	if !errors.Is(err, domain.ErrNotFound) {
+		return nil, err
+	}
+
+	// Provisioning (link or create) requires a verified email.
+	email := normalizeEmail(id.Email)
+	if email == "" {
+		return nil, domain.ErrOIDCNoEmail
+	}
+	if !id.EmailVerified {
+		return nil, domain.ErrOIDCEmailUnverified
+	}
+
+	// 2. Link to an existing account by email. LinkOIDC refuses to overwrite a
+	// different stored identity (returns ErrOIDCIdentityConflict).
+	existing, err := s.store.GetUserByEmail(ctx, email)
+	switch {
+	case err == nil:
+		return s.store.LinkOIDC(ctx, existing.ID, id.Issuer, id.Subject)
+	case !errors.Is(err, domain.ErrNotFound):
+		return nil, err
+	}
+
+	// 3. Just-in-time provisioning.
+	name := strings.TrimSpace(id.Name)
+	if name == "" {
+		name = emailLocalPart(email)
+	}
+	return s.store.CreateUser(ctx, &domain.User{
+		Email:       email,
+		DisplayName: name,
+		OIDCIssuer:  &id.Issuer,
+		OIDCSubject: &id.Subject,
+	}, true) // allowSignup: the IdP gates access, so registration policy is bypassed.
+}
+
 // Register creates a local (password) account and returns it. The first user on
 // a fresh instance becomes admin and may always register (bootstrap), even when
 // PANSY_REGISTRATION=closed; afterward, closed registration is enforced.
@@ -208,3 +281,12 @@ func (s *Service) CleanupExpiredSessions(ctx context.Context) (int64, error) {
 func normalizeEmail(email string) string {
 	return strings.ToLower(strings.TrimSpace(email))
 }
+
+// emailLocalPart returns the portion of an email before '@', used as a fallback
+// display name for JIT-provisioned OIDC users whose token carried no name.
+func emailLocalPart(email string) string {
+	if i := strings.IndexByte(email, '@'); i > 0 {
+		return email[:i]
+	}
+	return email
+}

internal/service/auth_test.go

@@ -280,15 +280,148 @@ func TestCleanupExpiredSessions(t *testing.T) {
 }
 
 func TestProvidersReflectsConfig(t *testing.T) {
-	cfg := openConfig()
-	cfg.OIDC.ButtonLabel = "Sign in with Authentik"
-	s := newTestService(t, cfg)
-
+	// No OIDC configured → local only.
+	s := newTestService(t, openConfig())
 	p := s.Providers()
-	if !p.Local {
-		t.Error("expected local=true")
+	if !p.Local || p.OIDC {
+		t.Errorf("providers = %+v, want local=true oidc=false", p)
 	}
-	if p.OIDC {
-		t.Error("OIDC must be false until #5 wires it")
+
+	// OIDC configured with a base URL → reported ready.
+	ready := openConfig()
+	ready.BaseURL = "https://pansy.example.com"
+	ready.OIDC = config.OIDCConfig{Issuer: testOIDCIssuer, ClientID: "cid", ButtonLabel: "Sign in with Authentik"}
+	if got := newTestService(t, ready).Providers(); !got.OIDC || got.OIDCLabel != "Sign in with Authentik" {
+		t.Errorf("providers = %+v, want oidc=true with Authentik label", got)
+	}
+
+	// OIDC configured but no base URL → not ready (can't build a redirect URI).
+	noBase := openConfig()
+	noBase.OIDC = config.OIDCConfig{Issuer: "https://idp.example", ClientID: "cid"}
+	if got := newTestService(t, noBase).Providers(); got.OIDC {
+		t.Error("OIDC should be false without a base URL")
+	}
+}
+
+// testOIDCIssuer is the issuer URL used across the OIDC service tests.
+const testOIDCIssuer = "https://idp.example"
+
+func oidcIdentity(sub, email, name string, verified bool) OIDCIdentity {
+	return OIDCIdentity{Issuer: testOIDCIssuer, Subject: sub, Email: email, EmailVerified: verified, Name: name}
+}
+
+func TestLoginOIDCJITProvisionsThenReturnsSameUser(t *testing.T) {
+	s := newTestService(t, openConfig())
+
+	u, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-1", "[email protected]", "Alice", true))
+	if err != nil {
+		t.Fatalf("LoginOIDC create: %v", err)
+	}
+	if !u.IsAdmin {
+		t.Error("first provisioned OIDC user should be admin")
+	}
+	if u.OIDCSubject == nil || *u.OIDCSubject != "sub-1" {
+		t.Errorf("oidc subject not stamped: %+v", u.OIDCSubject)
+	}
+
+	// A second login with the same identity returns the same user (no duplicate).
+	again, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-1", "[email protected]", "Alice", true))
+	if err != nil {
+		t.Fatalf("LoginOIDC repeat: %v", err)
+	}
+	if again.ID != u.ID {
+		t.Errorf("repeat login made a new user: %d vs %d", again.ID, u.ID)
+	}
+}
+
+func TestLoginOIDCLinksExistingLocalAccount(t *testing.T) {
+	s := newTestService(t, openConfig())
+	local := mustRegister(t, s, "[email protected]", "Bob", "password123")
+
+	linked, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-2", "[email protected]", "Bob", true))
+	if err != nil {
+		t.Fatalf("LoginOIDC link: %v", err)
+	}
+	if linked.ID != local.ID {
+		t.Errorf("linked to a new user %d, want existing %d", linked.ID, local.ID)
+	}
+	if linked.OIDCSubject == nil || *linked.OIDCSubject != "sub-2" {
+		t.Error("existing account was not stamped with the oidc identity")
+	}
+	// Local password still works after linking (one account, two methods).
+	if _, err := s.Login(context.Background(), "[email protected]", "password123"); err != nil {
+		t.Errorf("local login after linking failed: %v", err)
+	}
+}
+
+func TestLoginOIDCRefusesUnverifiedEmail(t *testing.T) {
+	s := newTestService(t, openConfig())
+
+	// Unverified email colliding with an existing account is refused (takeover).
+	mustRegister(t, s, "[email protected]", "Carol", "password123")
+	if _, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-3", "[email protected]", "Carol", false)); !errors.Is(err, domain.ErrOIDCEmailUnverified) {
+		t.Errorf("unverified collision err = %v, want ErrOIDCEmailUnverified", err)
+	}
+
+	// Unverified email with NO collision is also refused (can't JIT-provision on
+	// an unverified email — it could squat an address a real user later owns, and
+	// could make an unverified identity the first admin).
+	if _, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-3b", "[email protected]", "Fresh", false)); !errors.Is(err, domain.ErrOIDCEmailUnverified) {
+		t.Errorf("unverified JIT err = %v, want ErrOIDCEmailUnverified", err)
+	}
+}
+
+func TestLoginOIDCRefusesOverwritingDifferentIdentity(t *testing.T) {
+	s := newTestService(t, openConfig())
+
+	// A user links identity A.
+	first, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-A", "[email protected]", "Dana", true))
+	if err != nil {
+		t.Fatalf("initial link: %v", err)
+	}
+
+	// A different identity asserting the same verified email must NOT overwrite
+	// the stored identity (that would hijack/lock out the account).
+	_, err = s.LoginOIDC(context.Background(), oidcIdentity("sub-B", "[email protected]", "Dana", true))
+	if !errors.Is(err, domain.ErrOIDCIdentityConflict) {
+		t.Fatalf("overwrite attempt err = %v, want ErrOIDCIdentityConflict", err)
+	}
+
+	// The original identity still works and still points at the same account.
+	again, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-A", "[email protected]", "Dana", true))
+	if err != nil || again.ID != first.ID {
+		t.Errorf("original identity broken: user=%v err=%v", again, err)
+	}
+	if again.OIDCSubject == nil || *again.OIDCSubject != "sub-A" {
+		t.Errorf("stored identity was overwritten: %v", again.OIDCSubject)
+	}
+}
+
+func TestLoginOIDCRequiresEmail(t *testing.T) {
+	s := newTestService(t, openConfig())
+	_, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-4", "", "No Email", true))
+	if !errors.Is(err, domain.ErrOIDCNoEmail) {
+		t.Errorf("no-email err = %v, want ErrOIDCNoEmail", err)
+	}
+}
+
+func TestLoginOIDCFallsBackToEmailLocalPartForName(t *testing.T) {
+	s := newTestService(t, openConfig())
+	u, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-5", "[email protected]", "", true))
+	if err != nil {
+		t.Fatalf("LoginOIDC: %v", err)
+	}
+	if u.DisplayName != "dave" {
+		t.Errorf("display name = %q, want %q", u.DisplayName, "dave")
+	}
+}
+
+func TestLocalAuthDisabledDoesNotBlockOIDC(t *testing.T) {
+	cfg := openConfig()
+	cfg.LocalAuth = false
+	s := newTestService(t, cfg)
+	// OIDC provisioning must work even when local auth is off (pure-Authentik).
+	if _, err := s.LoginOIDC(context.Background(), oidcIdentity("sub-6", "[email protected]", "Erin", true)); err != nil {
+		t.Errorf("LoginOIDC with local auth disabled: %v", err)
 	}
 }

internal/store/users.go

@@ -105,6 +105,54 @@ func (d *DB) GetUserByEmail(ctx context.Context, email string) (*domain.User, er
 	return u, nil
 }
 
+// GetUserByOIDC returns the user with the given (issuer, subject) identity pair,
+// or domain.ErrNotFound. Both arguments must be non-empty.
+func (d *DB) GetUserByOIDC(ctx context.Context, issuer, subject string) (*domain.User, error) {
+	u, err := scanUser(d.sql.QueryRowContext(ctx,
+		`SELECT `+userColumns+` FROM users WHERE oidc_issuer = ? AND oidc_subject = ?`, issuer, subject))
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, domain.ErrNotFound
+	}
+	if err != nil {
+		return nil, fmt.Errorf("store: get user by oidc: %w", err)
+	}
+	return u, nil
+}
+
+// LinkOIDC stamps an OIDC identity onto an existing user (first OIDC login for a
+// pre-existing local account) and returns the updated row.
+//
+// The UPDATE only matches when the account carries no identity yet, or already
+// carries this exact one (idempotent) — it will NOT overwrite a different stored
+// identity, which would let anyone asserting the same email at a second IdP
+// hijack or lock out the account. A no-match (different identity, or the row is
+// gone) and a UNIQUE (issuer, subject) collision with another account both
+// surface as domain.ErrOIDCIdentityConflict. RETURNING folds the read-back into
+// the same statement.
+func (d *DB) LinkOIDC(ctx context.Context, userID int64, issuer, subject string) (*domain.User, error) {
+	u, err := scanUser(d.sql.QueryRowContext(ctx,
+		`UPDATE users
+		    SET oidc_issuer = ?, oidc_subject = ?,
+		        version = version + 1,
+		        updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
+		  WHERE id = ?
+		    AND (oidc_subject IS NULL OR (oidc_issuer = ? AND oidc_subject = ?))
+		  RETURNING `+userColumns,
+		issuer, subject, userID, issuer, subject,
+	))
+	if errors.Is(err, sql.ErrNoRows) {
+		// The account already has a different identity (or no longer exists).
+		return nil, domain.ErrOIDCIdentityConflict
+	}
+	if err != nil {
+		if isUniqueViolation(err) {
+			return nil, domain.ErrOIDCIdentityConflict
+		}
+		return nil, fmt.Errorf("store: link oidc: %w", err)
+	}
+	return u, nil
+}
+
 // CountUsers returns the number of user rows. Used to decide first-user-is-admin
 // and to allow bootstrap registration when signup is otherwise closed.
 func (d *DB) CountUsers(ctx context.Context) (int, error) {