diff --git a/msal/authority.py b/msal/authority.py
index 2e3a818..56b3d4c 100644
--- a/msal/authority.py
+++ b/msal/authority.py
@@ -1,4 +1,7 @@
-import re
+try:
+    from urllib.parse import urlparse
+except ImportError:  # Fall back to Python 2
+    from urlparse import urlparse
 import logging
 
 import requests
@@ -15,7 +18,12 @@ WELL_KNOWN_AUTHORITY_HOSTS = set([
     'login.microsoftonline.us',
     'login.microsoftonline.de',
     ])
-
+WELL_KNOWN_B2C_HOSTS = [
+    "b2clogin.com",
+    "b2clogin.cn",
+    "b2clogin.us",
+    "b2clogin.de",
+    ]
 
 class Authority(object):
     """This class represents an (already-validated) authority.
@@ -23,6 +31,8 @@ class Authority(object):
     Once constructed, it contains members named "*_endpoint" for this instance.
     TODO: It will also cache the previously-validated authority instances.
     """
+    _domains_without_user_realm_discovery = set([])
+
     def __init__(self, authority_url, validate_authority=True,
             verify=True, proxies=None, timeout=None,
             ):
@@ -37,18 +47,30 @@ class Authority(object):
         self.verify = verify
         self.proxies = proxies
         self.timeout = timeout
-        canonicalized, self.instance, tenant = canonicalize(authority_url)
-        tenant_discovery_endpoint = (
-            'https://{}/{}{}/.well-known/openid-configuration'.format(
-                self.instance,
-                tenant,
-                "" if tenant == "adfs" else "/v2.0" # the AAD v2 endpoint
-                ))
-        if (tenant != "adfs" and validate_authority
+        authority, self.instance, tenant = canonicalize(authority_url)
+        is_b2c = any(self.instance.endswith("." + d) for d in WELL_KNOWN_B2C_HOSTS)
+        if (tenant != "adfs" and (not is_b2c) and validate_authority
                 and self.instance not in WELL_KNOWN_AUTHORITY_HOSTS):
-            tenant_discovery_endpoint = instance_discovery(
-                canonicalized + "/oauth2/v2.0/authorize",
+            payload = instance_discovery(
+                "https://{}{}/oauth2/v2.0/authorize".format(
+                    self.instance, authority.path),
                 verify=verify, proxies=proxies, timeout=timeout)
+            if payload.get("error") == "invalid_instance":
+                raise ValueError(
+                    "invalid_instance: "
+                    "The authority you provided, %s, is not whitelisted. "
+                    "If it is indeed your legit customized domain name, "
+                    "you can turn off this check by passing in "
+                    "validate_authority=False"
+                    % authority_url)
+            tenant_discovery_endpoint = payload['tenant_discovery_endpoint']
+        else:
+            tenant_discovery_endpoint = (
+                'https://{}{}{}/.well-known/openid-configuration'.format(
+                    self.instance,
+                    authority.path,  # In B2C scenario, it is "/tenant/policy"
+                    "" if tenant == "adfs" else "/v2.0" # the AAD v2 endpoint
+                    ))
         openid_config = tenant_discovery(
             tenant_discovery_endpoint,
             verify=verify, proxies=proxies, timeout=timeout)
@@ -58,42 +80,44 @@ class Authority(object):
         _, _, self.tenant = canonicalize(self.token_endpoint)  # Usually a GUID
         self.is_adfs = self.tenant.lower() == 'adfs'
 
-    def user_realm_discovery(self, username):
-        resp = requests.get(
-            "https://{netloc}/common/userrealm/{username}?api-version=1.0".format(
-                netloc=self.instance, username=username),
-            headers={'Accept':'application/json'},
-            verify=self.verify, proxies=self.proxies, timeout=self.timeout)
-        resp.raise_for_status()
-        return resp.json()
-        # It will typically contain "ver", "account_type",
+    def user_realm_discovery(self, username, response=None):
+        # It will typically return a dict containing "ver", "account_type",
         # "federation_protocol", "cloud_audience_urn",
         # "federation_metadata_url", "federation_active_auth_url", etc.
+        if self.instance not in self.__class__._domains_without_user_realm_discovery:
+            resp = response or requests.get(
+                "https://{netloc}/common/userrealm/{username}?api-version=1.0".format(
+                    netloc=self.instance, username=username),
+                headers={'Accept':'application/json'},
+                verify=self.verify, proxies=self.proxies, timeout=self.timeout)
+            if resp.status_code != 404:
+                resp.raise_for_status()
+                return resp.json()
+            self.__class__._domains_without_user_realm_discovery.add(self.instance)
+        return {}  # This can guide the caller to fall back normal ROPC flow
+
 
-def canonicalize(url):
-    # Returns (canonicalized_url, netloc, tenant). Raises ValueError on errors.
-    match_object = re.match(r'https://([^/]+)/([^/?#]+)', url.lower())
-    if not match_object:
+def canonicalize(authority_url):
+    authority = urlparse(authority_url)
+    parts = authority.path.split("/")
+    if authority.scheme != "https" or len(parts) < 2 or not parts[1]:
         raise ValueError(
             "Your given address (%s) should consist of "
             "an https url with a minimum of one segment in a path: e.g. "
-            "https://login.microsoftonline.com/<tenant_name>" % url)
-    return match_object.group(0), match_object.group(1), match_object.group(2)
+            "https://login.microsoftonline.com/<tenant> "
+            "or https://<tenant_name>.b2clogin.com/<tenant_name>.onmicrosoft.com/policy"
+            % authority_url)
+    return authority, authority.netloc, parts[1]
 
-def instance_discovery(url, response=None, **kwargs):
-    # Returns tenant discovery endpoint
-    resp = requests.get(  # Note: This URL seemingly returns V1 endpoint only
+def instance_discovery(url, **kwargs):
+    return requests.get(  # Note: This URL seemingly returns V1 endpoint only
         'https://{}/common/discovery/instance'.format(
             WORLD_WIDE  # Historically using WORLD_WIDE. Could use self.instance too
                 # See https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/4.0.0/src/Microsoft.Identity.Client/Instance/AadInstanceDiscovery.cs#L101-L103
                 # and https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/4.0.0/src/Microsoft.Identity.Client/Instance/AadAuthority.cs#L19-L33
             ),
         params={'authorization_endpoint': url, 'api-version': '1.0'},
-        **kwargs)
-    payload = response or resp.json()
-    if 'tenant_discovery_endpoint' not in payload:
-        raise MsalServiceError(status_code=resp.status_code, **payload)
-    return payload['tenant_discovery_endpoint']
+        **kwargs).json()
 
 def tenant_discovery(tenant_discovery_endpoint, **kwargs):
     # Returns Openid Configuration

Support Azure AD B2C
Support [Azure AD B2C](https://azure.microsoft.com/en-us/services/active-directory-b2c/)
diff --git a/tests/test_authority.py b/tests/test_authority.py
index d7fc5ca..340b493 100644
--- a/tests/test_authority.py
+++ b/tests/test_authority.py
@@ -1,8 +1,11 @@
+import os
+
 from msal.authority import *
 from msal.exceptions import MsalServiceError
 from tests import unittest
 
 
+@unittest.skipIf(os.getenv("TRAVIS_TAG"), "Skip network io during tagged release")
 class TestAuthority(unittest.TestCase):
 
     def test_wellknown_host_and_tenant(self):
@@ -26,7 +29,7 @@ class TestAuthority(unittest.TestCase):
         self.assertNotIn('v2.0', a.token_endpoint)
 
     def test_unknown_host_wont_pass_instance_discovery(self):
-        with self.assertRaisesRegexp(MsalServiceError, "invalid_instance"):
+        with self.assertRaisesRegexp(ValueError, "invalid_instance"):
             Authority('https://unknown.host/tenant_doesnt_matter_in_this_case')
 
     def test_invalid_host_skipping_validation_meets_connection_error_down_the_road(self):
@@ -37,19 +40,19 @@ class TestAuthority(unittest.TestCase):
 class TestAuthorityInternalHelperCanonicalize(unittest.TestCase):
 
     def test_canonicalize_tenant_followed_by_extra_paths(self):
-        self.assertEqual(
-            canonicalize("https://example.com/tenant/subpath?foo=bar#fragment"),
-            ("https://example.com/tenant", "example.com", "tenant"))
+        _, i, t = canonicalize("https://example.com/tenant/subpath?foo=bar#fragment")
+        self.assertEqual("example.com", i)
+        self.assertEqual("tenant", t)
 
     def test_canonicalize_tenant_followed_by_extra_query(self):
-        self.assertEqual(
-            canonicalize("https://example.com/tenant?foo=bar#fragment"),
-            ("https://example.com/tenant", "example.com", "tenant"))
+        _, i, t = canonicalize("https://example.com/tenant?foo=bar#fragment")
+        self.assertEqual("example.com", i)
+        self.assertEqual("tenant", t)
 
     def test_canonicalize_tenant_followed_by_extra_fragment(self):
-        self.assertEqual(
-            canonicalize("https://example.com/tenant#fragment"),
-            ("https://example.com/tenant", "example.com", "tenant"))
+        _, i, t = canonicalize("https://example.com/tenant#fragment")
+        self.assertEqual("example.com", i)
+        self.assertEqual("tenant", t)
 
     def test_canonicalize_rejects_non_https(self):
         with self.assertRaises(ValueError):
@@ -64,20 +67,22 @@ class TestAuthorityInternalHelperCanonicalize(unittest.TestCase):
             canonicalize("https://no.tenant.example.com/")
 
 
-class TestAuthorityInternalHelperInstanceDiscovery(unittest.TestCase):
-
-    def test_instance_discovery_happy_case(self):
-        self.assertEqual(
-            instance_discovery("https://login.windows.net/tenant"),
-            "https://login.windows.net/tenant/.well-known/openid-configuration")
-
-    def test_instance_discovery_with_unknown_instance(self):
-        with self.assertRaisesRegexp(MsalServiceError, "invalid_instance"):
-            instance_discovery('https://unknown.host/tenant_doesnt_matter_here')
-
-    def test_instance_discovery_with_mocked_response(self):
-        mock_response = {'tenant_discovery_endpoint': 'http://a.com/t/openid'}
-        endpoint = instance_discovery(
-            "https://login.microsoftonline.in/tenant.com", response=mock_response)
-        self.assertEqual(endpoint, mock_response['tenant_discovery_endpoint'])
+@unittest.skipIf(os.getenv("TRAVIS_TAG"), "Skip network io during tagged release")
+class TestAuthorityInternalHelperUserRealmDiscovery(unittest.TestCase):
+    def test_memorize(self):
+        # We use a real authority so the constructor can finish tenant discovery
+        authority = "https://login.microsoftonline.com/common"
+        self.assertNotIn(authority, Authority._domains_without_user_realm_discovery)
+        a = Authority(authority, validate_authority=False)
+
+        # We now pretend this authority supports no User Realm Discovery
+        class MockResponse(object):
+            status_code = 404
+        a.user_realm_discovery("john.doe@example.com", response=MockResponse())
+        self.assertIn(
+            "login.microsoftonline.com",
+            Authority._domains_without_user_realm_discovery,
+            "user_realm_discovery() should memorize domains not supporting URD")
+        a.user_realm_discovery("john.doe@example.com",
+            response="This would cause exception if memorization did not work")
 
diff --git a/tests/test_e2e.py b/tests/test_e2e.py
index 2bf8050..770e462 100644
--- a/tests/test_e2e.py
+++ b/tests/test_e2e.py
@@ -13,6 +13,21 @@ logger = logging.getLogger(__name__)
 logging.basicConfig(level=logging.INFO)
 
 
+def _get_app_and_auth_code(
+        client_id,
+        client_secret=None,
+        authority="https://login.microsoftonline.com/common",
+        port=44331,
+        scopes=["https://graph.microsoft.com/.default"],  # Microsoft Graph
+        ):
+    from msal.oauth2cli.authcode import obtain_auth_code
+    app = msal.ClientApplication(client_id, client_secret, authority=authority)
+    redirect_uri = "http://localhost:%d" % port
+    ac = obtain_auth_code(port, auth_uri=app.get_authorization_request_url(
+        scopes, redirect_uri=redirect_uri))
+    assert ac is not None
+    return (app, ac, redirect_uri)
+
 @unittest.skipIf(os.getenv("TRAVIS_TAG"), "Skip e2e tests during tagged release")
 class E2eTestCase(unittest.TestCase):
 
@@ -49,9 +64,15 @@ class E2eTestCase(unittest.TestCase):
         result_from_cache = self.app.acquire_token_silent(scope, account=account)
         self.assertIsNotNone(result_from_cache,
                 "We should get a result from acquire_token_silent(...) call")
-        self.assertNotEqual(
-            result_from_wire['access_token'], result_from_cache['access_token'],
-            "We should get a fresh AT (via RT)")
+        self.assertIsNotNone(
+            # We used to assert it this way:
+            #   result_from_wire['access_token'] != result_from_cache['access_token']
+            # but ROPC in B2C tends to return the same AT we obtained seconds ago.
+            # Now looking back, "refresh_token grant would return a brand new AT"
+            # was just an empirical observation but never a committment in specs,
+            # so we adjust our way to assert here.
+            (result_from_cache or {}).get("access_token"),
+            "We should get an AT from acquire_token_silent(...) call")
 
     def assertCacheWorksForApp(self, result_from_wire, scope):
         # Going to test acquire_token_silent(...) to locate an AT from cache
@@ -70,7 +91,10 @@ class E2eTestCase(unittest.TestCase):
             username, password, scopes=scope)
         self.assertLoosely(result)
         # self.assertEqual(None, result.get("error"), str(result))
-        self.assertCacheWorksForUser(result, scope, username=username)
+        self.assertCacheWorksForUser(
+            result, scope,
+            username=username if ".b2clogin.com" not in authority else None,
+            )
 
 
 THIS_FOLDER = os.path.dirname(__file__)
@@ -95,23 +119,17 @@ class FileBasedTestCase(E2eTestCase):
         self._test_username_password(**self.config)
 
     def _get_app_and_auth_code(self):
-        from msal.oauth2cli.authcode import obtain_auth_code
-        app = msal.ClientApplication(
+        return _get_app_and_auth_code(
             self.config["client_id"],
-            client_credential=self.config.get("client_secret"),
-            authority=self.config.get("authority"))
-        port = self.config.get("listen_port", 44331)
-        redirect_uri = "http://localhost:%s" % port
-        auth_request_uri = app.get_authorization_request_url(
-            self.config["scope"], redirect_uri=redirect_uri)
-        ac = obtain_auth_code(port, auth_uri=auth_request_uri)
-        self.assertNotEqual(ac, None)
-        return (app, ac, redirect_uri)
+            client_secret=self.config.get("client_secret"),
+            authority=self.config.get("authority"),
+            port=self.config.get("listen_port", 44331),
+            scopes=self.config["scope"],
+            )
 
     def test_auth_code(self):
         self.skipUnlessWithConfig(["client_id", "scope"])
         (self.app, ac, redirect_uri) = self._get_app_and_auth_code()
-
         result = self.app.acquire_token_by_authorization_code(
             ac, self.config["scope"], redirect_uri=redirect_uri)
         logger.debug("%s.cache = %s",
@@ -314,7 +332,7 @@ class LabBasedTestCase(E2eTestCase):
         lab_name = lab_name.lower()
         if lab_name not in cls._secrets:
             logger.info("Querying lab user password for %s", lab_name)
-            # Note: Short link won't work "https://aka.ms/GetLabUserSecret?Secret=%s"
+            # Short link only works in browser "https://aka.ms/GetLabUserSecret?Secret=%s"
             # So we use the official link written in here
             # https://microsoft.sharepoint-df.com/teams/MSIDLABSExtended/SitePages/Programmatically-accessing-LAB-API%27s.aspx
             url = ("https://request.msidlab.com/api/GetLabUserSecret?code=KpY5uCcoKo0aW8VOL/CUO3wnu9UF2XbSnLFGk56BDnmQiwD80MQ7HA==&Secret=%s"
@@ -417,3 +435,49 @@ class LabBasedTestCase(E2eTestCase):
             result = cca.acquire_token_silent(downstream_scopes, account)
             self.assertEqual(cca_result["access_token"], result["access_token"])
 
+    def _build_b2c_authority(self, policy):
+        base = "https://msidlabb2c.b2clogin.com/msidlabb2c.onmicrosoft.com"
+        return base + "/" + policy  # We do not support base + "?p=" + policy
+
+    @unittest.skipIf(os.getenv("TRAVIS"), "Browser automation is not yet implemented")
+    def test_b2c_acquire_token_by_auth_code(self):
+        """
+        When prompted, you can manually login using this account:
+
+            username="b2clocal@msidlabb2c.onmicrosoft.com"
+                # This won't work https://msidlab.com/api/user?usertype=b2c
+            password="***"  # From https://aka.ms/GetLabUserSecret?Secret=msidlabb2c
+        """
+        scopes = ["https://msidlabb2c.onmicrosoft.com/msaapp/user_impersonation"]
+        (self.app, ac, redirect_uri) = _get_app_and_auth_code(
+            "b876a048-55a5-4fc5-9403-f5d90cb1c852",
+            client_secret=self.get_lab_user_secret("MSIDLABB2C-MSAapp-AppSecret"),
+            authority=self._build_b2c_authority("B2C_1_SignInPolicy"),
+            port=3843,  # Lab defines 4 of them: [3843, 4584, 4843, 60000]
+            scopes=scopes,
+            )
+        result = self.app.acquire_token_by_authorization_code(
+            ac, scopes, redirect_uri=redirect_uri)
+        logger.debug(
+            "%s: cache = %s, id_token_claims = %s",
+            self.id(),
+            json.dumps(self.app.token_cache._cache, indent=4),
+            json.dumps(result.get("id_token_claims"), indent=4),
+            )
+        self.assertIn(
+            "access_token", result,
+            "{error}: {error_description}".format(
+                # Note: No interpolation here, cause error won't always present
+                error=result.get("error"),
+                error_description=result.get("error_description")))
+        self.assertCacheWorksForUser(result, scopes, username=None)
+
+    def test_b2c_acquire_token_by_ropc(self):
+        self._test_username_password(
+            authority=self._build_b2c_authority("B2C_1_ROPC_Auth"),
+            client_id="e3b9ad76-9763-4827-b088-80c7a7888f79",
+            username="b2clocal@msidlabb2c.onmicrosoft.com",
+            password=self.get_lab_user_secret("msidlabb2c"),
+            scope=["https://msidlabb2c.onmicrosoft.com/msidlabb2capi/read"],
+            )
+

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest mock responses requests-mock
apt-get update apt-get install -y gcc
3.9
requirements/base.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
certifi==2025.1.31
cffi==1.17.1
charset-normalizer==3.4.1
cryptography==44.0.2
exceptiongroup==1.2.2
idna==3.10
iniconfig==2.1.0
mock==5.2.0
-e git+https://github.com/AzureAD/microsoft-authentication-library-for-python.git@45507939078915f70dfe35b5ad58bf0f2fba41ef#egg=msal
packaging==24.2
pluggy==1.5.0
pycparser==2.22
PyJWT==1.7.1
pytest==8.3.5
PyYAML==6.0.2
requests==2.32.3
requests-mock==1.12.1
responses==0.25.7
tomli==2.2.1
urllib3==2.3.0

name: microsoft-authentication-library-for-python
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - certifi==2025.1.31
      - cffi==1.17.1
      - charset-normalizer==3.4.1
      - cryptography==44.0.2
      - exceptiongroup==1.2.2
      - idna==3.10
      - iniconfig==2.1.0
      - mock==5.2.0
      - packaging==24.2
      - pluggy==1.5.0
      - pycparser==2.22
      - pyjwt==1.7.1
      - pytest==8.3.5
      - pyyaml==6.0.2
      - requests==2.32.3
      - requests-mock==1.12.1
      - responses==0.25.7
      - tomli==2.2.1
      - urllib3==2.3.0
prefix: /opt/conda/envs/microsoft-authentication-library-for-python

tests/test_authority.py::TestAuthority::test_unknown_host_wont_pass_instance_discovery tests/test_authority.py::TestAuthorityInternalHelperUserRealmDiscovery::test_memorize
tests/test_authority.py::TestAuthority::test_wellknown_host_and_tenant
tests/test_authority.py::TestAuthority::test_invalid_host_skipping_validation_meets_connection_error_down_the_road tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_rejects_non_https tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_rejects_tenantless tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_rejects_tenantless_host_with_trailing_slash tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_tenant_followed_by_extra_fragment tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_tenant_followed_by_extra_paths tests/test_authority.py::TestAuthorityInternalHelperCanonicalize::test_canonicalize_tenant_followed_by_extra_query
msal/authority.py
msal/authority.py