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.go

@@ -64,6 +64,10 @@ type (
 		// the email_verified claim does not need to be present in
 		// the ID token.
 		trustProviderEmail bool
+
+		// requireEmailDomainOwnerVerified requires the "xms_edov"
+		// claim (nOAuth mitigation).
+		requireEmailDomainOwnerVerified bool
 	}
 
 	UserInfo struct {
@@ -112,6 +116,9 @@ type (
 		EmailVerified any     `json:"email_verified"`
 		Name          string  `json:"name"`
 		HostedDomain  string  `json:"hd"`
+
+		// EmailDomainOwnerVerified is Microsoft's "xms_edov" domain-ownership claim.
+		EmailDomainOwnerVerified any `json:"xms_edov"`
 	}
 )
 
@@ -141,6 +148,17 @@ func (c *idTokenClaims) isEmailVerified() bool {
 	return false
 }
 
+func (c *idTokenClaims) isEmailDomainOwnerVerified() bool {
+	switch v := c.EmailDomainOwnerVerified.(type) {
+	case bool:
+		return v
+	case string:
+		return strings.EqualFold(v, "true")
+	}
+
+	return false
+}
+
 var (
 	googleEndpoint = oauth2.Endpoint{
 		AuthURL:  "https://accounts.google.com/o/oauth2/v2/auth",
@@ -208,8 +226,9 @@ func NewService(
 				RedirectURL:  baseURL + "/api/connect/v1/oidc/microsoft/callback",
 				Scopes:       []string{"openid", "email", "profile"},
 			},
-			jwksURL:            microsoftJWKSURL,
-			trustProviderEmail: true,
+			jwksURL:                         microsoftJWKSURL,
+			trustProviderEmail:              false,
+			requireEmailDomainOwnerVerified: true,
 			issuerValidator: func(iss string) bool {
 				return strings.HasPrefix(iss, "https://login.microsoftonline.com/") &&
 					strings.HasSuffix(iss, "/v2.0")
@@ -404,6 +423,10 @@ func (s *Service) HandleCallback(
 		return nil, "", nil, NewEmailNotVerifiedError()
 	}
 
+	if info.requireEmailDomainOwnerVerified && !claims.isEmailDomainOwnerVerified() {
+		return nil, "", nil, NewEmailNotVerifiedError()
+	}
+
 	if !info.enterpriseChecker(claims) {
 		return nil, "", nil, NewPersonalAccountNotAllowedError()
 	}

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())
+			},
+		)
+	}
+}