diff --git a/google/auth/downscoped.py b/google/auth/downscoped.py
index 800f289..96a4e65 100644
--- a/google/auth/downscoped.py
+++ b/google/auth/downscoped.py
@@ -479,8 +479,16 @@
             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/tests/test_downscoped.py b/tests/test_downscoped.py
index ac60e5b..795ec29 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 @@
         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 @@
             # 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