Require verified domain ownership for Microsoft OIDC

Stop trusting the email on its own: set trustProviderEmail to false so
email_verified is required, and additionally require the "xms_edov"
claim, which Azure sets only after verifying the issuing tenant owns
the email's domain. A token that lacks it is rejected before any
identity is matched.

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

pkg/iam/oidc/service_test.go

@@ -0,0 +1,83 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package oidc
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/coredata"
+)
+
+func newTestService(t *testing.T) *Service {
+	t.Helper()
+
+	return NewService(
+		nil,
+		"https://app.probo.test",
+		ProviderConfig{ClientID: "google-client", ClientSecret: "s", Enabled: true},
+		ProviderConfig{ClientID: "microsoft-client", ClientSecret: "s", Enabled: true},
+		log.NewLogger(),
+	)
+}
+
+// TestMicrosoftRequiresDomainOwnerVerified pins the nOAuth mitigation: the
+// Microsoft provider must not trust the email claim on email_verified alone and
+// must require the xms_edov domain-ownership claim.
+func TestMicrosoftRequiresDomainOwnerVerified(t *testing.T) {
+	t.Parallel()
+
+	s := newTestService(t)
+
+	microsoft := s.providers[coredata.OIDCProviderMicrosoft]
+	require.NotNil(t, microsoft)
+	assert.False(t, microsoft.trustProviderEmail, "Microsoft email must not be trusted unconditionally")
+	assert.True(t, microsoft.requireEmailDomainOwnerVerified, "Microsoft must require xms_edov")
+
+	google := s.providers[coredata.OIDCProviderGoogle]
+	require.NotNil(t, google)
+	assert.False(t, google.requireEmailDomainOwnerVerified, "Google verifies its domains and does not use xms_edov")
+}
+
+func TestIsEmailDomainOwnerVerified(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		value any
+		want  bool
+	}{
+		{"bool true", true, true},
+		{"bool false", false, false},
+		{"string true", "true", true},
+		{"string True", "True", true},
+		{"string false", "false", false},
+		{"absent", nil, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(
+			tt.name,
+			func(t *testing.T) {
+				t.Parallel()
+
+				claims := &idTokenClaims{EmailDomainOwnerVerified: tt.value}
+				assert.Equal(t, tt.want, claims.isEmailDomainOwnerVerified())
+			},
+		)
+	}
+}