AzureAD · Avery-Dunn · Jul 18, 2025
@@ -67,6 +67,7 @@ def __init__(
             performed.
         """
         self._http_client = http_client
+        self._oidc_authority_url = oidc_authority_url
         if oidc_authority_url:
             logger.debug("Initializing with OIDC authority: %s", oidc_authority_url)
             tenant_discovery_endpoint = self._initialize_oidc_authority(
@@ -95,11 +96,19 @@ def __init__(
             raise ValueError(error_message)
         logger.debug(
             'openid_config("%s") = %s', tenant_discovery_endpoint, openid_config)
+        self._issuer = openid_config.get('issuer')
         self.authorization_endpoint = openid_config['authorization_endpoint']
         self.token_endpoint = openid_config['token_endpoint']
         self.device_authorization_endpoint = openid_config.get('device_authorization_endpoint')
         _, _, self.tenant = canonicalize(self.token_endpoint)  # Usually a GUID
 
+        # Validate the issuer if using OIDC authority
+        if self._oidc_authority_url and not self.has_valid_issuer():
+            raise ValueError((
+                "The issuer '{iss}' does not match the authority '{auth}' or a known pattern. "
+                "When using the 'oidc_authority' parameter in ClientApplication, the authority "
+                "will be validated against the issuer from {auth}/.well-known/openid-configuration ."
+            ).format(iss=self._issuer, auth=oidc_authority_url))
     def _initialize_oidc_authority(self, oidc_authority_url):
         authority, self.instance, tenant = canonicalize(oidc_authority_url)
         self.is_adfs = tenant.lower() == 'adfs'  # As a convention
@@ -174,6 +183,53 @@ def user_realm_discovery(self, username, correlation_id=None, response=None):
             self.__class__._domains_without_user_realm_discovery.add(self.instance)
         return {}  # This can guide the caller to fall back normal ROPC flow
 
+    def has_valid_issuer(self) -> bool:
+        """
+            Returns True if the issuer from OIDC discovery is valid for this authority.
+
+            An issuer is valid if one of the following is true:
+            - It exactly matches the authority URL
+            - It has a known Microsoft host (e.g., login.microsoftonline.com)
+            - It has the same scheme and host as the authority (path can be different)
+            - For CIAM, the issuer follows the pattern of {tenant}.ciamlogin.com (tenant comes from the authority)
+            """
+        if self._oidc_authority_url == self._issuer:
+            # The issuer matches the authority URL exactly
+            return True
+
+        issuer = urlparse(self._issuer) if self._issuer else None
+        if not issuer:
+            return False
+
+        # Check if issuer has a known Microsoft host
+        if issuer.hostname in WELL_KNOWN_AUTHORITY_HOSTS:
+            return True
+
+        # Check if issuer has the same scheme and host as the authority
+        authority = urlparse(self._oidc_authority_url)
+        if authority.scheme == issuer.scheme and authority.netloc == issuer.netloc:
+            return True
+
+        # Check CIAM scenario: issuer follows the pattern {tenant}.ciamlogin.com
+        # Extract tenant from authority URL - could be in the host or path
+        tenant = None
+        if authority.hostname.endswith(_CIAM_DOMAIN_SUFFIX):
+            # Normal CIAM host: {tenant}.ciamlogin.com
+            tenant = authority.hostname.rsplit(_CIAM_DOMAIN_SUFFIX, 1)[0]
+        else:
+            # Custom CIAM host: extract tenant from path
+            parts = authority.path.split('/')
+            if len(parts) >= 2 and parts[1]:
+                tenant = parts[1]  # First path segment after the initial '/'
+
+        if tenant and issuer.hostname.endswith(_CIAM_DOMAIN_SUFFIX):
+            # Check if issuer follows the pattern {tenant}.ciamlogin.com
+            expected_issuer_host = f"{tenant}{_CIAM_DOMAIN_SUFFIX}"
+            if issuer.hostname == expected_issuer_host:
+                return True
+
+        # None of the conditions matched
+        return False
 
 def canonicalize(authority_or_auth_endpoint):
     # Returns (url_parsed_result, hostname_in_lowercase, tenant)
@@ -222,4 +278,3 @@ def tenant_discovery(tenant_discovery_endpoint, http_client, **kwargs):
     resp.raise_for_status()
     raise RuntimeError(  # A fallback here, in case resp.raise_for_status() is no-op
         "Unable to complete OIDC Discovery: %d, %s" % (resp.status_code, resp.text))
-
@@ -796,6 +796,7 @@ def test_should_fallback_when_pymsalruntime_failed_to_initialize_broker(self):
 @patch("msal.authority.tenant_discovery", new=Mock(return_value={
     "authorization_endpoint": "https://contoso.com/placeholder",
     "token_endpoint": "https://contoso.com/placeholder",
+    "issuer": "https://contoso.com/placeholder",
     }))
 @patch("msal.application._init_broker", new=Mock())  # Pretend pymsalruntime installed and working
 class TestBrokerFallbackWithDifferentAuthorities(unittest.TestCase):

@@ -6,6 +6,7 @@
 
 import msal
 from msal.authority import *
+from msal.authority import _CIAM_DOMAIN_SUFFIX  # Explicitly import the private constant
 from tests import unittest
 from tests.http_client import MinimalHttpClient
 
@@ -100,12 +101,12 @@ def test_authority_with_path_should_be_used_as_is(self, oidc_discovery):
 
 
 @patch("msal.authority._instance_discovery")
-@patch("msal.authority.tenant_discovery", return_value={
-    "authorization_endpoint": "https://contoso.com/authorize",
-    "token_endpoint": "https://contoso.com/token",
-    })
+@patch("msal.authority.tenant_discovery")  # Moved return_value out of the decorator
 class OidcAuthorityTestCase(unittest.TestCase):
     authority = "https://contoso.com/tenant"
+    authorization_endpoint = "https://contoso.com/authorize"
+    token_endpoint = "https://contoso.com/token"
+    issuer = "https://contoso.com/tenant"  # Added as class variable for inheritance
 
     def setUp(self):
         # setUp() gives subclass a dynamic setup based on their authority
@@ -115,25 +116,37 @@ def setUp(self):
             # Here the test is to confirm the OIDC endpoint contains no "/v2.0"
             self.authority + "/.well-known/openid-configuration")
 
+    def setup_tenant_discovery(self, tenant_discovery):
+        """Configure the tenant_discovery mock with class-specific values"""
+        tenant_discovery.return_value = {
+            "authorization_endpoint": self.authorization_endpoint,
+            "token_endpoint": self.token_endpoint,
+            "issuer": self.issuer,
+        }
+
     def test_authority_obj_should_do_oidc_discovery_and_skip_instance_discovery(
             self, oidc_discovery, instance_discovery):
+        self.setup_tenant_discovery(oidc_discovery)
+
         c = MinimalHttpClient()
         a = Authority(None, c, oidc_authority_url=self.authority)
         instance_discovery.assert_not_called()
         oidc_discovery.assert_called_once_with(self.oidc_discovery_endpoint, c)
-        self.assertEqual(a.authorization_endpoint, 'https://contoso.com/authorize')
-        self.assertEqual(a.token_endpoint, 'https://contoso.com/token')
+        self.assertEqual(a.authorization_endpoint, self.authorization_endpoint)
+        self.assertEqual(a.token_endpoint, self.token_endpoint)
 
     def test_application_obj_should_do_oidc_discovery_and_skip_instance_discovery(
             self, oidc_discovery, instance_discovery):
+        self.setup_tenant_discovery(oidc_discovery)
+
         app = msal.ClientApplication(
             "id", authority=None, oidc_authority=self.authority)
         instance_discovery.assert_not_called()
         oidc_discovery.assert_called_once_with(
             self.oidc_discovery_endpoint, app.http_client)
         self.assertEqual(
-            app.authority.authorization_endpoint, 'https://contoso.com/authorize')
-        self.assertEqual(app.authority.token_endpoint, 'https://contoso.com/token')
+            app.authority.authorization_endpoint, self.authorization_endpoint)
+        self.assertEqual(app.authority.token_endpoint, self.token_endpoint)
 
 
 class DstsAuthorityTestCase(OidcAuthorityTestCase):
@@ -144,14 +157,14 @@ class DstsAuthorityTestCase(OidcAuthorityTestCase):
         "https://some.url.dsts.core.azure-test.net/dstsv2/common/oauth2/authorize")
     token_endpoint = (
         "https://some.url.dsts.core.azure-test.net/dstsv2/common/oauth2/token")
+    issuer = "https://test-instance1-dsts.dsts.core.azure-test.net/dstsv2/common"
 
     @patch("msal.authority._instance_discovery")
-    @patch("msal.authority.tenant_discovery", return_value={
-        "authorization_endpoint": authorization_endpoint,
-        "token_endpoint": token_endpoint,
-    })  # We need to create new patches (i.e. mocks) for non-inherited test cases
+    @patch("msal.authority.tenant_discovery")  # Remove the hard-coded return_value
     def test_application_obj_should_accept_dsts_url_as_an_authority(
             self, oidc_discovery, instance_discovery):
+        self.setup_tenant_discovery(oidc_discovery)
+
         app = msal.ClientApplication("id", authority=self.authority)
         instance_discovery.assert_not_called()
         oidc_discovery.assert_called_once_with(
@@ -274,3 +287,93 @@ def _test_turning_off_instance_discovery_should_skip_authority_validation_and_in
         app.get_accounts()  # This could make an instance metadata call for authority aliases
         instance_metadata.assert_not_called()
 
+
+class TestAuthorityIssuerValidation(unittest.TestCase):
+    """Test cases for authority.has_valid_issuer method """
+
+    def setUp(self):
+        self.http_client = MinimalHttpClient()
+
+    def _create_authority_with_issuer(self, oidc_authority_url, issuer, tenant_discovery_mock):
+        tenant_discovery_mock.return_value = {
+            "authorization_endpoint": "https://example.com/oauth2/authorize",
+            "token_endpoint": "https://example.com/oauth2/token",
+            "issuer": issuer,
+        }
+        authority = Authority(
+            None, 
+            self.http_client, 
+            oidc_authority_url=oidc_authority_url
+        )
+        return authority
+
+    @patch("msal.authority.tenant_discovery")
+    def test_exact_match_issuer(self, tenant_discovery_mock):
+        """Test when issuer exactly matches the OIDC authority URL"""
+        authority_url = "https://example.com/tenant"
+        authority = self._create_authority_with_issuer(authority_url, authority_url, tenant_discovery_mock)
+        self.assertTrue(authority.has_valid_issuer(), "Issuer should be valid when it exactly matches the authority URL")
+
+    @patch("msal.authority.tenant_discovery")
+    def test_no_issuer(self, tenant_discovery_mock):
+        """Test when no issuer is returned from OIDC discovery"""
+        authority_url = "https://example.com/tenant"
+        tenant_discovery_mock.return_value = {
+            "authorization_endpoint": "https://example.com/oauth2/authorize",
+            "token_endpoint": "https://example.com/oauth2/token",
+            # No issuer key
+        }
+        # Since initialization now checks for valid issuer, we expect it to raise ValueError
+        with self.assertRaises(ValueError) as context:
+            Authority(None, self.http_client, oidc_authority_url=authority_url)
+        self.assertIn("issuer", str(context.exception).lower())
+
+    @patch("msal.authority.tenant_discovery")
+    def test_microsoft_host_issuer(self, tenant_discovery_mock):
+        """Test when issuer has a known Microsoft host"""
+        authority_url = "https://custom-domain.com/tenant"
+        issuer = f"https://{WORLD_WIDE}/tenant"
+        authority = self._create_authority_with_issuer(authority_url, issuer, tenant_discovery_mock)
+        self.assertTrue(authority.has_valid_issuer(), "Issuer should be valid when it has a known Microsoft host")
+
+    @patch("msal.authority.tenant_discovery")
+    def test_same_scheme_and_host_different_path(self, tenant_discovery_mock):
+        """Test when issuer has same scheme and host but different path"""
+        authority_url = "https://example.com/tenant"
+        issuer = "https://example.com/different/path"
+        authority = self._create_authority_with_issuer(authority_url, issuer, tenant_discovery_mock)
+        self.assertTrue(authority.has_valid_issuer(), "Issuer should be valid when it has the same scheme and host")
+
+    @patch("msal.authority.tenant_discovery")
+    def test_ciam_authority_with_matching_tenant(self, tenant_discovery_mock):
+        """Test CIAM authority with matching tenant in path"""
+        authority_url = "https://custom-domain.com/tenant_name"
+        issuer = f"https://tenant_name{_CIAM_DOMAIN_SUFFIX}"
+        authority = self._create_authority_with_issuer(authority_url, issuer, tenant_discovery_mock)
+        self.assertTrue(authority.has_valid_issuer(), "Issuer should be valid for CIAM pattern with matching tenant")
+
+    @patch("msal.authority.tenant_discovery")
+    def test_ciam_authority_with_host_tenant(self, tenant_discovery_mock):
+        """Test CIAM authority with tenant in hostname"""
+        tenant_name = "tenant_name"
+        authority_url = f"https://{tenant_name}{_CIAM_DOMAIN_SUFFIX}/custom/path"
+        issuer = f"https://{tenant_name}{_CIAM_DOMAIN_SUFFIX}"
+        authority = self._create_authority_with_issuer(authority_url, issuer, tenant_discovery_mock)
+        self.assertTrue(authority.has_valid_issuer(), "Issuer should be valid for CIAM pattern with tenant in hostname")
+
+    @patch("msal.authority.tenant_discovery")
+    def test_invalid_issuer(self, tenant_discovery_mock):
+        """Test when issuer is completely different from authority"""
+        authority_url = "https://example.com/tenant"
+        issuer = "https://malicious-site.com/tenant"
+        tenant_discovery_mock.return_value = {
+            "authorization_endpoint": "https://example.com/oauth2/authorize",
+            "token_endpoint": "https://example.com/oauth2/token",
+            "issuer": issuer,
+        }
+        # Since initialization now checks for valid issuer, we expect it to raise ValueError
+        with self.assertRaises(ValueError) as context:
+            Authority(None, self.http_client, oidc_authority_url=authority_url)
+        self.assertIn("issuer", str(context.exception).lower())
+        self.assertIn(issuer, str(context.exception))
+        self.assertIn(authority_url, str(context.exception))