diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5ccadc42b..6b7abb366 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,18 @@
 
 [1]: https://pypi.org/project/google-auth/#history
 
+### [1.33.1](https://www.github.com/googleapis/google-auth-library-python/compare/v1.33.0...v1.33.1) (2021-07-20)
+
+
+### Bug Fixes
+
+* fallback to source creds expiration in downscoped tokens ([#805](https://www.github.com/googleapis/google-auth-library-python/issues/805)) ([dfad661](https://www.github.com/googleapis/google-auth-library-python/commit/dfad66128c6ee7513e5565d39bc7b002055dd0d5))
+
+
+### Reverts
+
+* revert "feat: service account is able to use a private token endpoint ([#784](https://www.github.com/googleapis/google-auth-library-python/issues/784))" ([#808](https://www.github.com/googleapis/google-auth-library-python/issues/808)) ([d94e65c](https://www.github.com/googleapis/google-auth-library-python/commit/d94e65c0e441183403608d762b92b30b77e21eeb))
+
 ## [1.33.0](https://www.github.com/googleapis/google-auth-library-python/compare/v1.32.1...v1.33.0) (2021-07-14)
 
 
diff --git a/google/auth/downscoped.py b/google/auth/downscoped.py
index 800f2894c..96a4e6547 100644
--- a/google/auth/downscoped.py
+++ b/google/auth/downscoped.py
@@ -479,8 +479,16 @@ def refresh(self, request):
             additional_options=self._credential_access_boundary.to_json(),
         )
         self.token = response_data.get("access_token")
-        lifetime = datetime.timedelta(seconds=response_data.get("expires_in"))
-        self.expiry = now + lifetime
+        # For downscoping CAB flow, the STS endpoint may not return the expiration
+        # field for some flows. The generated downscoped token should always have
+        # the same expiration time as the source credentials. When no expires_in
+        # field is returned in the response, we can just get the expiration time
+        # from the source credentials.
+        if response_data.get("expires_in"):
+            lifetime = datetime.timedelta(seconds=response_data.get("expires_in"))
+            self.expiry = now + lifetime
+        else:
+            self.expiry = self._source_credentials.expiry
 
     @_helpers.copy_docstring(credentials.CredentialsWithQuotaProject)
     def with_quota_project(self, quota_project_id):
diff --git a/google/auth/version.py b/google/auth/version.py
index e74f1e70f..6327f8588 100644
--- a/google/auth/version.py
+++ b/google/auth/version.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "1.33.0"
+__version__ = "1.33.1"
diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index 8f18f26ea..dd3658994 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -80,7 +80,6 @@
 from google.oauth2 import _client
 
 _DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
-_GOOGLE_OAUTH2_TOKEN_ENDPOINT = "https://oauth2.googleapis.com/token"
 
 
 class Credentials(
@@ -383,7 +382,7 @@ def _make_authorization_grant_assertion(self):
             # The issuer must be the service account email.
             "iss": self._service_account_email,
             # The audience must be the auth token endpoint's URI
-            "aud": _GOOGLE_OAUTH2_TOKEN_ENDPOINT,
+            "aud": self._token_uri,
             "scope": _helpers.scopes_to_string(self._scopes or ()),
         }
 
@@ -644,7 +643,7 @@ def _make_authorization_grant_assertion(self):
             # The issuer must be the service account email.
             "iss": self.service_account_email,
             # The audience must be the auth token endpoint's URI
-            "aud": _GOOGLE_OAUTH2_TOKEN_ENDPOINT,
+            "aud": self._token_uri,
             # The target audience specifies which service the ID token is
             # intended for.
             "target_audience": self._target_audience,
diff --git a/system_tests/secrets.tar.enc b/system_tests/secrets.tar.enc
index e416918d4..46b521c76 100644
Binary files a/system_tests/secrets.tar.enc and b/system_tests/secrets.tar.enc differ
diff --git a/tests/oauth2/test_service_account.py b/tests/oauth2/test_service_account.py
index 370438f48..5852d3714 100644
--- a/tests/oauth2/test_service_account.py
+++ b/tests/oauth2/test_service_account.py
@@ -167,7 +167,7 @@ def test__make_authorization_grant_assertion(self):
         token = credentials._make_authorization_grant_assertion()
         payload = jwt.decode(token, PUBLIC_CERT_BYTES)
         assert payload["iss"] == self.SERVICE_ACCOUNT_EMAIL
-        assert payload["aud"] == service_account._GOOGLE_OAUTH2_TOKEN_ENDPOINT
+        assert payload["aud"] == self.TOKEN_URI
 
     def test__make_authorization_grant_assertion_scoped(self):
         credentials = self.make_credentials()
@@ -440,7 +440,7 @@ def test__make_authorization_grant_assertion(self):
         token = credentials._make_authorization_grant_assertion()
         payload = jwt.decode(token, PUBLIC_CERT_BYTES)
         assert payload["iss"] == self.SERVICE_ACCOUNT_EMAIL
-        assert payload["aud"] == service_account._GOOGLE_OAUTH2_TOKEN_ENDPOINT
+        assert payload["aud"] == self.TOKEN_URI
         assert payload["target_audience"] == self.TARGET_AUDIENCE
 
     @mock.patch("google.oauth2._client.id_token_jwt_grant", autospec=True)
diff --git a/tests/test_downscoped.py b/tests/test_downscoped.py
index ac60e5b00..795ec2942 100644
--- a/tests/test_downscoped.py
+++ b/tests/test_downscoped.py
@@ -80,10 +80,11 @@
 
 
 class SourceCredentials(credentials.Credentials):
-    def __init__(self, raise_error=False):
+    def __init__(self, raise_error=False, expires_in=3600):
         super(SourceCredentials, self).__init__()
         self._counter = 0
         self._raise_error = raise_error
+        self._expires_in = expires_in
 
     def refresh(self, request):
         if self._raise_error:
@@ -93,7 +94,7 @@ def refresh(self, request):
         now = _helpers.utcnow()
         self._counter += 1
         self.token = "ACCESS_TOKEN_{}".format(self._counter)
-        self.expiry = now + datetime.timedelta(seconds=3600)
+        self.expiry = now + datetime.timedelta(seconds=self._expires_in)
 
 
 def make_availability_condition(expression, title=None, description=None):
@@ -539,6 +540,47 @@ def test_refresh(self, unused_utcnow):
             # Confirm source credentials called with the same request instance.
             wrapped_souce_cred_refresh.assert_called_with(request)
 
+    @mock.patch("google.auth._helpers.utcnow", return_value=datetime.datetime.min)
+    def test_refresh_without_response_expires_in(self, unused_utcnow):
+        response = SUCCESS_RESPONSE.copy()
+        # Simulate the response is missing the expires_in field.
+        # The downscoped token expiration should match the source credentials
+        # expiration.
+        del response["expires_in"]
+        expected_expires_in = 1800
+        # Simulate the source credentials generates a token with 1800 second
+        # expiration time. The generated downscoped token should have the same
+        # expiration time.
+        source_credentials = SourceCredentials(expires_in=expected_expires_in)
+        expected_expiry = datetime.datetime.min + datetime.timedelta(
+            seconds=expected_expires_in
+        )
+        headers = {"Content-Type": "application/x-www-form-urlencoded"}
+        request_data = {
+            "grant_type": GRANT_TYPE,
+            "subject_token": "ACCESS_TOKEN_1",
+            "subject_token_type": SUBJECT_TOKEN_TYPE,
+            "requested_token_type": REQUESTED_TOKEN_TYPE,
+            "options": urllib.parse.quote(json.dumps(CREDENTIAL_ACCESS_BOUNDARY_JSON)),
+        }
+        request = self.make_mock_request(status=http_client.OK, data=response)
+        credentials = self.make_credentials(source_credentials=source_credentials)
+
+        # Spy on calls to source credentials refresh to confirm the expected request
+        # instance is used.
+        with mock.patch.object(
+            source_credentials, "refresh", wraps=source_credentials.refresh
+        ) as wrapped_souce_cred_refresh:
+            credentials.refresh(request)
+
+            self.assert_request_kwargs(request.call_args[1], headers, request_data)
+            assert credentials.valid
+            assert credentials.expiry == expected_expiry
+            assert not credentials.expired
+            assert credentials.token == response["access_token"]
+            # Confirm source credentials called with the same request instance.
+            wrapped_souce_cred_refresh.assert_called_with(request)
+
     def test_refresh_token_exchange_error(self):
         request = self.make_mock_request(
             status=http_client.BAD_REQUEST, data=ERROR_RESPONSE
diff --git a/tests_async/oauth2/test_service_account_async.py b/tests_async/oauth2/test_service_account_async.py
index 3dce13d82..40794536c 100644
--- a/tests_async/oauth2/test_service_account_async.py
+++ b/tests_async/oauth2/test_service_account_async.py
@@ -152,10 +152,7 @@ def test__make_authorization_grant_assertion(self):
         token = credentials._make_authorization_grant_assertion()
         payload = jwt.decode(token, test_service_account.PUBLIC_CERT_BYTES)
         assert payload["iss"] == self.SERVICE_ACCOUNT_EMAIL
-        assert (
-            payload["aud"]
-            == service_account.service_account._GOOGLE_OAUTH2_TOKEN_ENDPOINT
-        )
+        assert payload["aud"] == self.TOKEN_URI
 
     def test__make_authorization_grant_assertion_scoped(self):
         credentials = self.make_credentials()
@@ -314,10 +311,7 @@ def test__make_authorization_grant_assertion(self):
         token = credentials._make_authorization_grant_assertion()
         payload = jwt.decode(token, test_service_account.PUBLIC_CERT_BYTES)
         assert payload["iss"] == self.SERVICE_ACCOUNT_EMAIL
-        assert (
-            payload["aud"]
-            == service_account.service_account._GOOGLE_OAUTH2_TOKEN_ENDPOINT
-        )
+        assert payload["aud"] == self.TOKEN_URI
         assert payload["target_audience"] == self.TARGET_AUDIENCE
 
     @mock.patch("google.oauth2._client_async.id_token_jwt_grant", autospec=True)