[management] Refuse services on unvalidated custom domains

A custom domain row was bound to a live service whether or not its CNAME
validation ever succeeded: extractClusterFromCustomDomains matched on the name
alone, so an account that never proved DNS control still had its hostname
routed. Cluster derivation now matches only validated rows and reports "domain
is not validated" instead of the generic no-cluster message.

Service updates no longer fall back to the previously derived cluster when
derivation fails, which was a way around the same check.

The unique index on the domain column already prevents two accounts holding the
same name, but the violation surfaced as an internal error. Creation now
pre-checks and returns AlreadyExists so the caller gets a 409, and the message
does not say which account holds the domain.

management/internals/modules/reverseproxy/domain/manager/domain_test.go

@@ -66,8 +66,8 @@ func TestExtractClusterFromFreeDomain(t *testing.T) {
 
 func TestExtractClusterFromCustomDomains(t *testing.T) {
 	customDomains := []*domain.Domain{
-		{Domain: "example.com", TargetCluster: "eu1.proxy.netbird.io"},
-		{Domain: "proxy.corp.io", TargetCluster: "us1.proxy.netbird.io"},
+		{Domain: "example.com", TargetCluster: "eu1.proxy.netbird.io", Validated: true},
+		{Domain: "proxy.corp.io", TargetCluster: "us1.proxy.netbird.io", Validated: true},
 	}
 
 	tests := []struct {
@@ -120,19 +120,49 @@ func TestExtractClusterFromCustomDomains(t *testing.T) {
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			cluster, ok := extractClusterFromCustomDomains(tc.domain, customDomains)
-			assert.Equal(t, tc.wantOK, ok)
-			if ok {
-				assert.Equal(t, tc.wantVal, cluster)
+			cluster, match := extractClusterFromCustomDomains(tc.domain, customDomains)
+			if !tc.wantOK {
+				assert.Equal(t, customDomainNoMatch, match, "unrelated domain should not match any custom domain")
+				return
 			}
+			assert.Equal(t, customDomainValidated, match, "validated custom domain should resolve a cluster")
+			assert.Equal(t, tc.wantVal, cluster)
 		})
 	}
 }
 
+// An unvalidated row must never yield a cluster: the account has not shown it
+// controls the name, so no service may be bound to it.
+func TestExtractClusterFromCustomDomains_UnvalidatedDomainRefused(t *testing.T) {
+	customDomains := []*domain.Domain{
+		{Domain: "example.com", TargetCluster: "eu1.proxy.netbird.io", Validated: false},
+	}
+
+	for _, serviceDomain := range []string{"example.com", "app.example.com"} {
+		t.Run(serviceDomain, func(t *testing.T) {
+			cluster, match := extractClusterFromCustomDomains(serviceDomain, customDomains)
+			assert.Equal(t, customDomainUnvalidated, match, "unvalidated row must be reported as such")
+			assert.Empty(t, cluster, "unvalidated row must not resolve a cluster")
+		})
+	}
+}
+
+// A more specific unvalidated row must not shadow a validated parent domain.
+func TestExtractClusterFromCustomDomains_ValidatedParentWinsOverUnvalidatedChild(t *testing.T) {
+	customDomains := []*domain.Domain{
+		{Domain: "example.com", TargetCluster: "cluster-generic", Validated: true},
+		{Domain: "app.example.com", TargetCluster: "cluster-app", Validated: false},
+	}
+
+	cluster, match := extractClusterFromCustomDomains("app.example.com", customDomains)
+	assert.Equal(t, customDomainValidated, match)
+	assert.Equal(t, "cluster-generic", cluster, "validated parent domain should provide the cluster")
+}
+
 func TestExtractClusterFromCustomDomains_OverlappingDomains(t *testing.T) {
 	customDomains := []*domain.Domain{
-		{Domain: "example.com", TargetCluster: "cluster-generic"},
-		{Domain: "app.example.com", TargetCluster: "cluster-app"},
+		{Domain: "example.com", TargetCluster: "cluster-generic", Validated: true},
+		{Domain: "app.example.com", TargetCluster: "cluster-app", Validated: true},
 	}
 
 	tests := []struct {
@@ -164,8 +194,8 @@ func TestExtractClusterFromCustomDomains_OverlappingDomains(t *testing.T) {
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			cluster, ok := extractClusterFromCustomDomains(tc.domain, customDomains)
-			assert.True(t, ok)
+			cluster, match := extractClusterFromCustomDomains(tc.domain, customDomains)
+			assert.Equal(t, customDomainValidated, match)
 			assert.Equal(t, tc.wantVal, cluster)
 		})
 	}

management/internals/modules/reverseproxy/domain/manager/manager.go

@@ -22,6 +22,7 @@ type store interface {
 	GetAccount(ctx context.Context, accountID string) (*types.Account, error)
 
 	GetCustomDomain(ctx context.Context, accountID string, domainID string) (*domain.Domain, error)
+	GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error)
 	ListFreeDomains(ctx context.Context, accountID string) ([]string, error)
 	ListCustomDomains(ctx context.Context, accountID string) ([]*domain.Domain, error)
 	CreateCustomDomain(ctx context.Context, accountID string, domainName string, targetCluster string, validated bool) (*domain.Domain, error)
@@ -146,6 +147,10 @@ func (m Manager) CreateDomain(ctx context.Context, accountID, userID, domainName
 		return nil, fmt.Errorf("target cluster %s is not available", targetCluster)
 	}
 
+	if err := m.checkDomainAvailable(ctx, domainName); err != nil {
+		return nil, err
+	}
+
 	// Attempt an initial validation against the specified cluster only
 	var validated bool
 	if m.validator.IsValid(ctx, domainName, []string{targetCluster}) {
@@ -162,6 +167,23 @@ func (m Manager) CreateDomain(ctx context.Context, accountID, userID, domainName
 	return d, nil
 }
 
+// checkDomainAvailable reports whether the domain is free to claim. The unique
+// index on the column is the real guard; this turns the violation into a
+// conflict the caller can act on instead of a database error, and says nothing
+// about which account holds the domain.
+func (m Manager) checkDomainAvailable(ctx context.Context, domainName string) error {
+	_, err := m.store.GetCustomDomainByName(ctx, domainName)
+	if err == nil {
+		return status.Errorf(status.AlreadyExists, "domain %s is already registered", domainName)
+	}
+
+	if sErr, ok := status.FromError(err); ok && sErr.Type() == status.NotFound {
+		return nil
+	}
+
+	return fmt.Errorf("look up domain: %w", err)
+}
+
 func (m Manager) DeleteDomain(ctx context.Context, accountID, userID, domainID string) error {
 	ok, ctx, err := m.permissionsManager.ValidateUserPermissions(ctx, accountID, userID, modules.Services, operations.Delete)
 	if err != nil {
@@ -294,9 +316,12 @@ func (m Manager) DeriveClusterFromDomain(ctx context.Context, accountID, domain
 		return "", fmt.Errorf("list custom domains: %w", err)
 	}
 
-	targetCluster, valid := extractClusterFromCustomDomains(domain, customDomains)
-	if valid {
+	targetCluster, match := extractClusterFromCustomDomains(domain, customDomains)
+	switch match {
+	case customDomainValidated:
 		return targetCluster, nil
+	case customDomainUnvalidated:
+		return "", status.Errorf(status.PreconditionFailed, "domain %s is not validated", domain)
 	}
 
 	return "", fmt.Errorf("domain %s does not match any available proxy cluster", domain)
@@ -330,19 +355,46 @@ func (m Manager) getClusterAllowList(ctx context.Context, accountID string) ([]s
 	return merged, nil
 }
 
-func extractClusterFromCustomDomains(serviceDomain string, customDomains []*domain.Domain) (string, bool) {
+// customDomainMatch describes how a service domain relates to the account's
+// custom domain rows.
+type customDomainMatch int
+
+const (
+	customDomainNoMatch customDomainMatch = iota
+	customDomainUnvalidated
+	customDomainValidated
+)
+
+// extractClusterFromCustomDomains finds the longest custom domain covering the
+// service domain and reports its target cluster. Only a validated row yields a
+// cluster: until the CNAME check has passed the account has not shown it
+// controls the name, so no traffic may be routed for it.
+func extractClusterFromCustomDomains(serviceDomain string, customDomains []*domain.Domain) (string, customDomainMatch) {
 	bestCluster := ""
 	bestLen := -1
+	matched := false
 	for _, cd := range customDomains {
 		if serviceDomain != cd.Domain && !strings.HasSuffix(serviceDomain, "."+cd.Domain) {
 			continue
 		}
+		matched = true
+		if !cd.Validated {
+			continue
+		}
 		if l := len(cd.Domain); l > bestLen {
 			bestLen = l
 			bestCluster = cd.TargetCluster
 		}
 	}
-	return bestCluster, bestLen >= 0
+
+	switch {
+	case bestLen >= 0:
+		return bestCluster, customDomainValidated
+	case matched:
+		return "", customDomainUnvalidated
+	default:
+		return "", customDomainNoMatch
+	}
 }
 
 // ExtractClusterFromFreeDomain extracts the cluster address from a free domain.

management/internals/modules/reverseproxy/domain/manager/manager_realstore_test.go

@@ -0,0 +1,249 @@
+package manager
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/otel/metric/noop"
+
+	"github.com/netbirdio/netbird/management/internals/modules/reverseproxy/domain"
+	proxymanager "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/proxy/manager"
+	"github.com/netbirdio/netbird/management/server/activity"
+	"github.com/netbirdio/netbird/management/server/mock_server"
+	"github.com/netbirdio/netbird/management/server/permissions"
+	nbstore "github.com/netbirdio/netbird/management/server/store"
+	"github.com/netbirdio/netbird/management/server/types"
+	"github.com/netbirdio/netbird/shared/management/status"
+)
+
+const (
+	testCluster  = "eu.proxy.test"
+	accountA     = "account-a"
+	accountAUser = "account-a-admin"
+	accountB     = "account-b"
+	accountBUser = "account-b-admin"
+)
+
+// stubResolver answers CNAME lookups from a table the test controls, so a
+// domain can point at the cluster or nowhere without touching a real resolver.
+type stubResolver struct {
+	mu     sync.Mutex
+	cnames map[string]string
+}
+
+func (r *stubResolver) LookupCNAME(_ context.Context, host string) (string, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	cname, ok := r.cnames[host]
+	if !ok {
+		return "", fmt.Errorf("lookup %s: no such host", host)
+	}
+	return cname + ".", nil
+}
+
+func (r *stubResolver) set(host, cname string) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.cnames[host] = cname
+}
+
+type domainTestEnv struct {
+	manager  Manager
+	store    nbstore.Store
+	resolver *stubResolver
+}
+
+// setupDomainTest builds the domain manager on a real SQLite store with two
+// accounts and one active public proxy cluster.
+func setupDomainTest(t *testing.T) *domainTestEnv {
+	t.Helper()
+
+	ctx := context.Background()
+	testStore, cleanup, err := nbstore.NewTestStoreFromSQL(ctx, "", t.TempDir())
+	require.NoError(t, err)
+	t.Cleanup(cleanup)
+
+	for accountID, userID := range map[string]string{accountA: accountAUser, accountB: accountBUser} {
+		require.NoError(t, testStore.SaveAccount(ctx, &types.Account{
+			Id:        accountID,
+			CreatedBy: userID,
+			Settings:  &types.Settings{},
+			Users: map[string]*types.User{
+				userID: {
+					Id:        userID,
+					AccountID: accountID,
+					Role:      types.UserRoleAdmin,
+				},
+			},
+		}))
+	}
+
+	proxyMgr, err := proxymanager.NewManager(testStore, noop.NewMeterProvider().Meter(""))
+	require.NoError(t, err)
+
+	_, err = proxyMgr.Connect(ctx, "proxy-1", "session-1", testCluster, "127.0.0.1", nil, nil)
+	require.NoError(t, err)
+
+	resolver := &stubResolver{cnames: make(map[string]string)}
+
+	mgr := Manager{
+		store:              testStore,
+		proxyManager:       proxyMgr,
+		validator:          domain.Validator{Resolver: resolver},
+		permissionsManager: permissions.NewManager(testStore),
+		accountManager: &mock_server.MockAccountManager{
+			StoreEventFunc: func(context.Context, string, string, string, activity.ActivityDescriber, map[string]any) {},
+		},
+	}
+
+	return &domainTestEnv{manager: mgr, store: testStore, resolver: resolver}
+}
+
+// storedDomain reads a domain row back through the store so assertions are made
+// on what was persisted rather than on the value the manager returned.
+func storedDomain(t *testing.T, s nbstore.Store, accountID, domainName string) *domain.Domain {
+	t.Helper()
+
+	domains, err := s.ListCustomDomains(context.Background(), accountID)
+	require.NoError(t, err)
+	for _, d := range domains {
+		if d.Domain == domainName {
+			return d
+		}
+	}
+	return nil
+}
+
+// A domain whose CNAME check fails is stored unvalidated and must not resolve a
+// cluster, which is what service creation gates on.
+func TestCreateDomain_FailedLookupIsNotServable(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	created, err := env.manager.CreateDomain(ctx, accountA, accountAUser, "apps.example.com", testCluster)
+	require.NoError(t, err)
+	assert.False(t, created.Validated, "a domain whose CNAME lookup fails must not be created validated")
+
+	stored := storedDomain(t, env.store, accountA, "apps.example.com")
+	require.NotNil(t, stored, "domain row should exist")
+	assert.False(t, stored.Validated, "persisted row must be unvalidated")
+
+	cluster, err := env.manager.DeriveClusterFromDomain(ctx, accountA, "apps.example.com")
+	require.Error(t, err, "an unvalidated domain must not resolve a cluster")
+	assert.Empty(t, cluster)
+	assert.Contains(t, err.Error(), "not validated", "error should tell the caller what to fix")
+
+	sErr, ok := status.FromError(err)
+	require.True(t, ok, "error should be a typed status error")
+	assert.Equal(t, status.PreconditionFailed, sErr.Type())
+
+	_, err = env.manager.DeriveClusterFromDomain(ctx, accountA, "sub.apps.example.com")
+	assert.Error(t, err, "subdomains of an unvalidated custom domain are not servable either")
+}
+
+// A second account claiming a registered domain gets a clean conflict, not a
+// database error surfaced as a 500.
+func TestCreateDomain_DuplicateIsAConflict(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	_, err := env.manager.CreateDomain(ctx, accountA, accountAUser, "shared.example.com", testCluster)
+	require.NoError(t, err)
+
+	_, err = env.manager.CreateDomain(ctx, accountB, accountBUser, "shared.example.com", testCluster)
+	require.Error(t, err)
+
+	sErr, ok := status.FromError(err)
+	require.True(t, ok, "conflict must be a typed status error, not a raw database error")
+	assert.Equal(t, status.AlreadyExists, sErr.Type(), "conflict should map to 409, not 500")
+	assert.NotContains(t, sErr.Message, accountA, "the response must not reveal the holding account")
+
+	assert.Nil(t, storedDomain(t, env.store, accountB, "shared.example.com"), "no row should be written on conflict")
+}
+
+// The same account re-adding one of its own domains is a conflict too.
+func TestCreateDomain_SameAccountDuplicateIsAConflict(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	_, err := env.manager.CreateDomain(ctx, accountA, accountAUser, "dup.example.com", testCluster)
+	require.NoError(t, err)
+
+	_, err = env.manager.CreateDomain(ctx, accountA, accountAUser, "dup.example.com", testCluster)
+	require.Error(t, err)
+
+	sErr, ok := status.FromError(err)
+	require.True(t, ok)
+	assert.Equal(t, status.AlreadyExists, sErr.Type())
+}
+
+// The negative control: a validated domain still derives its cluster, for the
+// bare name and for subdomains, exactly as before.
+func TestCreateDomain_ValidatedDomainDerivesCluster(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+	env.resolver.set("validation.valid.example.com", testCluster)
+
+	created, err := env.manager.CreateDomain(ctx, accountA, accountAUser, "valid.example.com", testCluster)
+	require.NoError(t, err)
+	require.True(t, created.Validated, "a matching CNAME should validate on create")
+
+	cluster, err := env.manager.DeriveClusterFromDomain(ctx, accountA, "valid.example.com")
+	require.NoError(t, err)
+	assert.Equal(t, testCluster, cluster)
+
+	cluster, err = env.manager.DeriveClusterFromDomain(ctx, accountA, "app.valid.example.com")
+	require.NoError(t, err)
+	assert.Equal(t, testCluster, cluster, "subdomains of a validated custom domain resolve too")
+}
+
+// Validating a domain flips the gate: the same lookup that failed before now
+// resolves a cluster.
+func TestValidateDomain_UnlocksClusterDerivation(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	created, err := env.manager.CreateDomain(ctx, accountA, accountAUser, "later.example.com", testCluster)
+	require.NoError(t, err)
+	require.False(t, created.Validated)
+
+	_, err = env.manager.DeriveClusterFromDomain(ctx, accountA, "later.example.com")
+	require.Error(t, err)
+
+	env.resolver.set("validation.later.example.com", testCluster)
+	env.manager.ValidateDomain(ctx, accountA, accountAUser, created.ID)
+
+	require.True(t, storedDomain(t, env.store, accountA, "later.example.com").Validated)
+
+	cluster, err := env.manager.DeriveClusterFromDomain(ctx, accountA, "later.example.com")
+	require.NoError(t, err)
+	assert.Equal(t, testCluster, cluster)
+}
+
+// Free cluster domains are unaffected by the custom domain gate.
+func TestDeriveClusterFromDomain_FreeDomainUnaffected(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	cluster, err := env.manager.DeriveClusterFromDomain(ctx, accountA, "myapp.abc123."+testCluster)
+	require.NoError(t, err)
+	assert.Equal(t, testCluster, cluster)
+}
+
+// The manager pre-check exists to turn a conflict into a 409, but the unique
+// index on the column is what actually guarantees the domain is claimed once.
+func TestStore_DuplicateDomainRejectedByIndex(t *testing.T) {
+	ctx := context.Background()
+	env := setupDomainTest(t)
+
+	_, err := env.store.CreateCustomDomain(ctx, accountA, "indexed.example.com", testCluster, false)
+	require.NoError(t, err)
+
+	_, err = env.store.CreateCustomDomain(ctx, accountB, "indexed.example.com", testCluster, false)
+	assert.Error(t, err, "the unique index must reject the same domain in a second account")
+}

management/internals/modules/reverseproxy/service/manager/domain_validation_test.go

@@ -0,0 +1,127 @@
+package manager
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/otel/metric/noop"
+
+	domainmanager "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/domain/manager"
+	proxymanager "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/proxy/manager"
+	rpservice "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/service"
+	"github.com/netbirdio/netbird/management/server/activity"
+	"github.com/netbirdio/netbird/management/server/mock_server"
+	"github.com/netbirdio/netbird/management/server/permissions"
+	"github.com/netbirdio/netbird/management/server/store"
+	"github.com/netbirdio/netbird/shared/management/status"
+)
+
+const validationTestCluster = "eu.proxy.test"
+
+// withRealDomainManager swaps the stub cluster deriver for the real domain
+// manager backed by the same store, so service creation is gated by the actual
+// domain rows rather than by a test double that always agrees.
+func withRealDomainManager(t *testing.T, mgr *Manager, testStore store.Store) {
+	t.Helper()
+
+	ctx := context.Background()
+	proxyMgr, err := proxymanager.NewManager(testStore, noop.NewMeterProvider().Meter(""))
+	require.NoError(t, err)
+
+	_, err = proxyMgr.Connect(ctx, "proxy-1", "session-1", validationTestCluster, "127.0.0.1", nil, nil)
+	require.NoError(t, err)
+
+	accountMgr := &mock_server.MockAccountManager{
+		StoreEventFunc: func(context.Context, string, string, string, activity.ActivityDescriber, map[string]any) {},
+	}
+	mgr.clusterDeriver = domainmanager.NewManager(testStore, proxyMgr, permissions.NewManager(testStore), accountMgr)
+}
+
+func newTestService(domain string) *rpservice.Service {
+	return &rpservice.Service{
+		Name:    "test-service",
+		Domain:  domain,
+		Enabled: true,
+		Mode:    rpservice.ModeHTTP,
+		Targets: []*rpservice.Target{{
+			Host:       "10.0.0.1",
+			Port:       8080,
+			Protocol:   "http",
+			TargetId:   testPeerID,
+			TargetType: "peer",
+			Enabled:    true,
+		}},
+	}
+}
+
+// A service must not bind to a domain the account has not validated, and
+// nothing may be persisted for the attempt.
+func TestCreateService_RefusesUnvalidatedDomain(t *testing.T) {
+	ctx := context.Background()
+	mgr, testStore := setupIntegrationTest(t)
+	withRealDomainManager(t, mgr, testStore)
+
+	_, err := testStore.CreateCustomDomain(ctx, testAccountID, "unproven.example.com", validationTestCluster, false)
+	require.NoError(t, err)
+
+	_, err = mgr.CreateService(ctx, testAccountID, testUserID, newTestService("unproven.example.com"))
+	require.Error(t, err, "an unvalidated domain must not bind a service")
+	assert.Contains(t, err.Error(), "not validated", "the API error should name the actual problem")
+
+	sErr, ok := status.FromError(err)
+	require.True(t, ok, "error should be a typed status error")
+	assert.Equal(t, status.PreconditionFailed, sErr.Type())
+
+	services, err := testStore.GetAccountServices(ctx, store.LockingStrengthNone, testAccountID)
+	require.NoError(t, err)
+	assert.Empty(t, services, "no service row should be written for a refused domain")
+}
+
+// The negative control: a validated domain still binds a service and derives
+// its cluster exactly as before.
+func TestCreateService_ValidatedDomainBindsService(t *testing.T) {
+	ctx := context.Background()
+	mgr, testStore := setupIntegrationTest(t)
+	withRealDomainManager(t, mgr, testStore)
+
+	_, err := testStore.CreateCustomDomain(ctx, testAccountID, "proven.example.com", validationTestCluster, true)
+	require.NoError(t, err)
+
+	created, err := mgr.CreateService(ctx, testAccountID, testUserID, newTestService("app.proven.example.com"))
+	require.NoError(t, err)
+	assert.Equal(t, validationTestCluster, created.ProxyCluster, "service should bind to the domain's target cluster")
+
+	services, err := testStore.GetAccountServices(ctx, store.LockingStrengthNone, testAccountID)
+	require.NoError(t, err)
+	require.Len(t, services, 1, "the service should be persisted")
+	assert.Equal(t, "app.proven.example.com", services[0].Domain)
+}
+
+// An update must not be a way around the creation gate: moving a live service
+// onto an unvalidated domain has to fail rather than silently keep the old
+// cluster and start serving the new hostname.
+func TestUpdateService_RefusesMoveToUnvalidatedDomain(t *testing.T) {
+	ctx := context.Background()
+	mgr, testStore := setupIntegrationTest(t)
+	withRealDomainManager(t, mgr, testStore)
+
+	_, err := testStore.CreateCustomDomain(ctx, testAccountID, "proven.example.com", validationTestCluster, true)
+	require.NoError(t, err)
+	_, err = testStore.CreateCustomDomain(ctx, testAccountID, "unproven.example.com", validationTestCluster, false)
+	require.NoError(t, err)
+
+	created, err := mgr.CreateService(ctx, testAccountID, testUserID, newTestService("app.proven.example.com"))
+	require.NoError(t, err)
+
+	moved := *created
+	moved.Domain = "app.unproven.example.com"
+	_, err = mgr.UpdateService(ctx, testAccountID, testUserID, &moved)
+	require.Error(t, err, "moving to an unvalidated domain must fail")
+	assert.Contains(t, err.Error(), "not validated")
+
+	stored, err := testStore.GetServiceByID(ctx, store.LockingStrengthNone, testAccountID, created.ID)
+	require.NoError(t, err)
+	assert.Equal(t, "app.proven.example.com", stored.Domain, "the service must keep its original domain")
+}

management/internals/modules/reverseproxy/service/manager/manager.go

@@ -606,16 +606,19 @@ func (m *Manager) resolveEffectiveCluster(ctx context.Context, accountID string,
 		return existing.ProxyCluster, nil
 	}
 
-	if m.clusterDeriver != nil {
-		derived, err := m.clusterDeriver.DeriveClusterFromDomain(ctx, accountID, svc.Domain)
-		if err != nil {
-			log.WithError(err).Warnf("could not derive cluster from domain %s", svc.Domain)
-		} else {
-			return derived, nil
-		}
+	if m.clusterDeriver == nil {
+		return existing.ProxyCluster, nil
 	}
 
-	return existing.ProxyCluster, nil
+	// Falling back to the old cluster here would let an update move a service
+	// onto a domain the account has not validated, bypassing the check that
+	// creation makes.
+	derived, err := m.clusterDeriver.DeriveClusterFromDomain(ctx, accountID, svc.Domain)
+	if err != nil {
+		return "", status.Errorf(status.PreconditionFailed, "could not derive cluster from domain %s: %v", svc.Domain, err)
+	}
+
+	return derived, nil
 }
 
 func (m *Manager) executeServiceUpdate(ctx context.Context, transaction store.Store, accountID string, service *service.Service, updateInfo *serviceUpdateInfo, customPorts *bool, effectiveCluster string) error {

management/server/store/sql_store.go

@@ -5658,6 +5658,23 @@ func (s *SqlStore) ListCustomDomains(ctx context.Context, accountID string) ([]*
 	return domains, nil
 }
 
+// GetCustomDomainByName returns the custom domain row holding the given name,
+// regardless of which account owns it.
+func (s *SqlStore) GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error) {
+	customDomain := &domain.Domain{}
+	result := s.db.Take(customDomain, "domain = ?", domainName)
+	if result.Error != nil {
+		if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+			return nil, status.Errorf(status.NotFound, "custom domain %s not found", domainName)
+		}
+
+		log.WithContext(ctx).Errorf("failed to get custom domain by name from store: %v", result.Error)
+		return nil, status.Errorf(status.Internal, "failed to get custom domain from store")
+	}
+
+	return customDomain, nil
+}
+
 func (s *SqlStore) CreateCustomDomain(ctx context.Context, accountID string, domainName string, targetCluster string, validated bool) (*domain.Domain, error) {
 	newDomain := &domain.Domain{
 		ID:            xid.New().String(), // Generate our own ID because gorm doesn't always configure the database to handle this for us.

management/server/store/store.go

@@ -294,6 +294,7 @@ type Store interface {
 	GetCustomDomain(ctx context.Context, accountID string, domainID string) (*domain.Domain, error)
 	ListFreeDomains(ctx context.Context, accountID string) ([]string, error)
 	ListCustomDomains(ctx context.Context, accountID string) ([]*domain.Domain, error)
+	GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error)
 	CreateCustomDomain(ctx context.Context, accountID string, domainName string, targetCluster string, validated bool) (*domain.Domain, error)
 	UpdateCustomDomain(ctx context.Context, accountID string, d *domain.Domain) (*domain.Domain, error)
 	DeleteCustomDomain(ctx context.Context, accountID string, domainID string) error

management/server/store/store_mock.go

@@ -1892,6 +1892,21 @@ func (mr *MockStoreMockRecorder) GetCustomDomain(ctx, accountID, domainID interf
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetCustomDomain", reflect.TypeOf((*MockStore)(nil).GetCustomDomain), ctx, accountID, domainID)
 }
 
+// GetCustomDomainByName mocks base method.
+func (m *MockStore) GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetCustomDomainByName", ctx, domainName)
+	ret0, _ := ret[0].(*domain.Domain)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetCustomDomainByName indicates an expected call of GetCustomDomainByName.
+func (mr *MockStoreMockRecorder) GetCustomDomainByName(ctx, domainName interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetCustomDomainByName", reflect.TypeOf((*MockStore)(nil).GetCustomDomainByName), ctx, domainName)
+}
+
 // GetCustomDomainsCounts mocks base method.
 func (m *MockStore) GetCustomDomainsCounts(ctx context.Context) (int64, int64, error) {
 	m.ctrl.T.Helper()