Reject empty SAML NameIDs on login

Empty NameID values were stored as '' and occupied the
unique saml_subject index, causing duplicate-key failures
on later logins. Reject blank NameIDs during assertion
validation, return a clear error when a NameID is already
linked to another account, and stop returning internal
errors from the SAML consume endpoint.

Signed-off-by: Sacha Al Himdani <sacha@probo.com>

pkg/iam/saml/errors.go

@@ -104,3 +104,25 @@ func NewUserInactiveError(profileID gid.GID) error {
 func (e ErrUserInactive) Error() string {
 	return fmt.Sprintf("user %q is inactive", e.ProfileID)
 }
+
+type ErrSAMLSubjectAlreadyInUse struct {
+	AssertionID string
+}
+
+func NewSAMLSubjectAlreadyInUseError(assertionID string) error {
+	return &ErrSAMLSubjectAlreadyInUse{AssertionID: assertionID}
+}
+
+func (e ErrSAMLSubjectAlreadyInUse) Error() string {
+	return fmt.Sprintf("SAML NameID is already linked to another account (assertion %q)", e.AssertionID)
+}
+
+type ErrSAMLSubjectRequired struct{}
+
+func NewSAMLSubjectRequiredError() error {
+	return &ErrSAMLSubjectRequired{}
+}
+
+func (e ErrSAMLSubjectRequired) Error() string {
+	return "NameID value is required"
+}

pkg/iam/saml/service.go

@@ -257,14 +257,16 @@ func (s *Service) HandleAssertion(
 				return NewEmailDomainMismatchError(email, config.EmailDomain)
 			}
 
+			samlSubject := strings.TrimSpace(assertion.Subject.NameID.Value)
+
 			err = identity.LoadByEmail(ctx, tx, email)
-			if err == coredata.ErrResourceNotFound && !config.AutoSignupEnabled {
+			if errors.Is(err, coredata.ErrResourceNotFound) && !config.AutoSignupEnabled {
 				return NewSAMLAutoSignupDisabledError(config.ID)
-			} else if err == coredata.ErrResourceNotFound && config.AutoSignupEnabled {
+			} else if errors.Is(err, coredata.ErrResourceNotFound) && config.AutoSignupEnabled {
 				*identity = coredata.Identity{
 					ID:                   gid.New(gid.NilTenant, coredata.IdentityEntityType),
 					EmailAddress:         email,
-					SAMLSubject:          &assertion.Subject.NameID.Value,
+					SAMLSubject:          &samlSubject,
 					FullName:             fullname,
 					HashedPassword:       nil,
 					EmailAddressVerified: true,
@@ -272,8 +274,11 @@ func (s *Service) HandleAssertion(
 					UpdatedAt:            now,
 				}
 
-				err := identity.Insert(ctx, tx)
-				if err != nil {
+				if err := identity.Insert(ctx, tx); err != nil {
+					if errors.Is(err, coredata.ErrSAMLSubjectAlreadyExists) {
+						return NewSAMLSubjectAlreadyInUseError(assertion.ID)
+					}
+
 					return fmt.Errorf("cannot insert identity: %w", err)
 				}
 			} else if err != nil {
@@ -282,16 +287,18 @@ func (s *Service) HandleAssertion(
 				identity.EmailAddress = email
 				identity.FullName = fullname
 
-				// Identity can exist (e.g. provisioned via SCIM) but not have a SAML subject
-				if identity.SAMLSubject == nil {
-					identity.SAMLSubject = &assertion.Subject.NameID.Value
+				if !hasSAMLSubject(identity) {
+					identity.SAMLSubject = &samlSubject
 				}
 
 				identity.EmailAddressVerified = true
 				identity.UpdatedAt = now
 
-				err = identity.Update(ctx, tx)
-				if err != nil {
+				if err = identity.Update(ctx, tx); err != nil {
+					if errors.Is(err, coredata.ErrSAMLSubjectAlreadyExists) {
+						return NewSAMLSubjectAlreadyInUseError(assertion.ID)
+					}
+
 					return fmt.Errorf("cannot update identity: %w", err)
 				}
 			}
@@ -415,6 +422,10 @@ func (s *Service) validateAssertion(assertion *saml.Assertion, config *coredata.
 		return fmt.Errorf("subject or NameID missing")
 	}
 
+	if strings.TrimSpace(assertion.Subject.NameID.Value) == "" {
+		return NewSAMLSubjectRequiredError()
+	}
+
 	if assertion.Issuer.Value != config.IdPEntityID {
 		return fmt.Errorf("assertion issuer %q does not match expected issuer %q",
 			assertion.Issuer.Value, config.IdPEntityID)
@@ -479,3 +490,7 @@ func (s *Service) baseServiceProvider() *saml.ServiceProvider {
 		AllowIDPInitiated: true,
 	}
 }
+
+func hasSAMLSubject(identity *coredata.Identity) bool {
+	return identity.SAMLSubject != nil && strings.TrimSpace(*identity.SAMLSubject) != ""
+}

pkg/iam/saml/service_test.go

@@ -0,0 +1,117 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package saml
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/crewjam/saml"
+	"go.probo.inc/probo/pkg/coredata"
+)
+
+func TestValidateAssertionRejectsEmptySAMLSubject(t *testing.T) {
+	t.Parallel()
+
+	s := &Service{}
+	config := &coredata.SAMLConfiguration{IdPEntityID: "https://idp.example"}
+	now := time.Now()
+
+	tests := []struct {
+		name  string
+		value string
+	}{
+		{name: "empty", value: ""},
+		{name: "whitespace", value: "   "},
+	}
+
+	for _, tt := range tests {
+		t.Run(
+			tt.name,
+			func(t *testing.T) {
+				t.Parallel()
+
+				err := s.validateAssertion(
+					&saml.Assertion{
+						ID: "assertion-1",
+						Issuer: saml.Issuer{
+							Value: config.IdPEntityID,
+						},
+						Subject: &saml.Subject{
+							NameID: &saml.NameID{Value: tt.value},
+						},
+						Conditions: &saml.Conditions{
+							NotOnOrAfter: now.Add(time.Hour),
+						},
+					},
+					config,
+					now,
+				)
+				if err == nil {
+					t.Fatal("expected error for empty SAML subject")
+				}
+
+				if _, ok := errors.AsType[*ErrSAMLSubjectRequired](err); !ok {
+					t.Fatalf("expected *ErrSAMLSubjectRequired, got %T: %v", err, err)
+				}
+
+				if got := err.Error(); got != "NameID value is required" {
+					t.Fatalf("unexpected error: %v", err)
+				}
+			},
+		)
+	}
+}
+
+func TestHasSAMLSubject(t *testing.T) {
+	t.Parallel()
+
+	empty := ""
+	whitespace := "  "
+	value := "user@example.com"
+
+	tests := []struct {
+		name     string
+		subject  *string
+		expected bool
+	}{
+		{name: "nil", subject: nil, expected: false},
+		{name: "empty", subject: &empty, expected: false},
+		{name: "whitespace", subject: &whitespace, expected: false},
+		{name: "present", subject: &value, expected: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(
+			tt.name,
+			func(t *testing.T) {
+				t.Parallel()
+
+				identity := &coredata.Identity{SAMLSubject: tt.subject}
+
+				if got := hasSAMLSubject(identity); got != tt.expected {
+					t.Fatalf("hasSAMLSubject() = %v, want %v", got, tt.expected)
+				}
+			},
+		)
+	}
+}