feat: federated logout for SAML IdPs (#9931)

# Which Problems Are Solved

Currently if a user signs in using an IdP, once they sign out of
Zitadel, the corresponding IdP session is not terminated. This can be
the desired behavior. In some cases, e.g. when using a shared computer
it results in a potential security risk, since a follower user might be
able to sign in as the previous using the still open IdP session.

# How the Problems Are Solved

- Admins can enabled a federated logout option on SAML IdPs through the
Admin and Management APIs.
- During the termination of a login V1 session using OIDC end_session
endpoint, Zitadel will check if an IdP was used to authenticate that
session.
- In case there was a SAML IdP used with Federated Logout enabled, it
will intercept the logout process, store the information into the shared
cache and redirect to the federated logout endpoint in the V1 login.
- The V1 login federated logout endpoint checks every request on an
existing cache entry. On success it will create a SAML logout request
for the used IdP and either redirect or POST to the configured SLO
endpoint. The cache entry is updated with a `redirected` state.
- A SLO endpoint is added to the `/idp` handlers, which will handle the
SAML logout responses. At the moment it will check again for an existing
federated logout entry (with state `redirected`) in the cache. On
success, the user is redirected to the initially provided
`post_logout_redirect_uri` from the end_session request.

# Additional Changes

None

# Additional Context

- This PR merges the https://github.com/zitadel/zitadel/pull/9841 and
https://github.com/zitadel/zitadel/pull/9854 to main, additionally
updating the docs on Entra ID SAML.
- closes #9228
- backport to 3.x

---------

Co-authored-by: Silvan <27845747+adlerhurst@users.noreply.github.com>
Co-authored-by: Zach Hirschtritt <zachary.hirschtritt@klaviyo.com>
(cherry picked from commit 2cf3ef4de4)

internal/api/idp/idp.go

@@ -15,10 +15,13 @@ import (
 	"github.com/muhlemmer/gu"
 	"github.com/zitadel/logging"
 
+	"github.com/zitadel/zitadel/internal/api/authz"
 	http_utils "github.com/zitadel/zitadel/internal/api/http"
 	"github.com/zitadel/zitadel/internal/api/ui/login"
+	"github.com/zitadel/zitadel/internal/cache"
 	"github.com/zitadel/zitadel/internal/command"
 	"github.com/zitadel/zitadel/internal/crypto"
+	"github.com/zitadel/zitadel/internal/domain/federatedlogout"
 	"github.com/zitadel/zitadel/internal/form"
 	"github.com/zitadel/zitadel/internal/idp"
 	"github.com/zitadel/zitadel/internal/idp/providers/apple"
@@ -44,6 +47,7 @@ const (
 	metadataPath    = idpPrefix + "/saml/metadata"
 	acsPath         = idpPrefix + "/saml/acs"
 	certificatePath = idpPrefix + "/saml/certificate"
+	sloPath         = idpPrefix + "/saml/slo"
 
 	paramIntentID         = "id"
 	paramToken            = "token"
@@ -62,6 +66,7 @@ type Handler struct {
 	callbackURL         func(ctx context.Context) string
 	samlRootURL         func(ctx context.Context, idpID string) string
 	loginSAMLRootURL    func(ctx context.Context) string
+	caches              *Caches
 }
 
 type externalIDPCallbackData struct {
@@ -104,6 +109,7 @@ func NewHandler(
 	queries *query.Queries,
 	encryptionAlgorithm crypto.EncryptionAlgorithm,
 	instanceInterceptor func(next http.Handler) http.Handler,
+	federatedLogoutCache cache.Cache[federatedlogout.Index, string, *federatedlogout.FederatedLogout],
 ) http.Handler {
 	h := &Handler{
 		commands:            commands,
@@ -113,6 +119,7 @@ func NewHandler(
 		callbackURL:         CallbackURL(),
 		samlRootURL:         SAMLRootURL(),
 		loginSAMLRootURL:    LoginSAMLRootURL(),
+		caches:              &Caches{federatedLogouts: federatedLogoutCache},
 	}
 
 	router := mux.NewRouter()
@@ -121,9 +128,14 @@ func NewHandler(
 	router.HandleFunc(metadataPath, h.handleMetadata)
 	router.HandleFunc(certificatePath, h.handleCertificate)
 	router.HandleFunc(acsPath, h.handleACS)
+	router.HandleFunc(sloPath, h.handleSLO)
 	return router
 }
 
+type Caches struct {
+	federatedLogouts cache.Cache[federatedlogout.Index, string, *federatedlogout.FederatedLogout]
+}
+
 func parseSAMLRequest(r *http.Request) *externalSAMLIDPCallbackData {
 	vars := mux.Vars(r)
 	return &externalSAMLIDPCallbackData{
@@ -351,6 +363,38 @@ func (h *Handler) handleCallback(w http.ResponseWriter, r *http.Request) {
 	redirectToSuccessURL(w, r, intent, token, userID)
 }
 
+func (h *Handler) handleSLO(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	data := parseSAMLRequest(r)
+
+	logoutState, ok := h.caches.federatedLogouts.Get(ctx, federatedlogout.IndexRequestID, federatedlogout.Key(authz.GetInstance(ctx).InstanceID(), data.RelayState))
+	if !ok || logoutState.State != federatedlogout.StateRedirected {
+		err := zerrors.ThrowNotFound(nil, "SAML-3uor2", "Errors.Intent.NotFound")
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	// For the moment we just make sure the callback matches the IDP it was started on / intended for.
+
+	provider, err := h.getProvider(ctx, data.IDPID)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+	if _, ok = provider.(*saml2.Provider); !ok {
+		err := zerrors.ThrowInvalidArgument(nil, "SAML-ui9wyux0hp", "Errors.Intent.IDPInvalid")
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	// We could also parse and validate the response here, but for example Azure does not sign it and thus would already fail.
+	// Also we can't really act on it if it fails.
+
+	err = h.caches.federatedLogouts.Delete(ctx, federatedlogout.IndexRequestID, federatedlogout.Key(logoutState.InstanceID, logoutState.SessionID))
+	logging.WithFields("instanceID", logoutState.InstanceID, "sessionID", logoutState.SessionID).OnError(err).Error("could not delete federated logout")
+	http.Redirect(w, r, logoutState.PostLogoutRedirectURI, http.StatusFound)
+}
+
 func (h *Handler) tryMigrateExternalUser(ctx context.Context, idpID string, idpUser idp.User, idpSession idp.Session) (userID string, err error) {
 	migration, ok := idpSession.(idp.SessionSupportsMigration)
 	if !ok {