Temporary storage of plaintext passwords during password changes in matrix synapse

@@ -0,0 +1 @@
+Avoid temporary storage of sensitive information.

@@ -186,15 +186,15 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                 params, session_id = await self.auth_handler.validate_user_via_ui_auth(
                     requester,
                     request,
-                    body.dict(exclude_unset=True),
+                    body.dict(exclude_unset=True, exclude={"new_password"}),
                     "modify your account password",
                 )
                 user_id = requester.user.to_string()
             else:
                 result, params, session_id = await self.auth_handler.check_ui_auth(
                     [[LoginType.EMAIL_IDENTITY]],
                     request,
-                    body.dict(exclude_unset=True),
+                    body.dict(exclude_unset=True, exclude={"new_password"}),
                     "modify your account password",
                 )
 

@@ -31,6 +31,7 @@
 from synapse.rest.client import account, login, register, room
 from synapse.rest.synapse.client.password_reset import PasswordResetSubmitTokenResource
 from synapse.server import HomeServer
+from synapse.storage._base import db_to_json
 from synapse.types import JsonDict, UserID
 from synapse.util import Clock
 
@@ -134,6 +135,18 @@ def test_basic_password_reset(self) -> None:
         # Assert we can't log in with the old password
         self.attempt_wrong_password_login("kermit", old_password)
 
+        # Check that the UI Auth information doesn't store the password in the database.
+        #
+        # Note that we don't have the UI Auth session ID, so just pull out the single
+        # row.
+        ui_auth_data = self.get_success(
+            self.store.db_pool.simple_select_one(
+                "ui_auth_sessions", keyvalues={}, retcols=("clientdict",)
+            )
+        )
+        client_dict = db_to_json(ui_auth_data["clientdict"])
+        self.assertNotIn("new_password", client_dict)
+
     @override_config({"rc_3pid_validation": {"burst_count": 3}})
     def test_ratelimit_by_email(self) -> None:
         """Test that we ratelimit /requestToken for the same email."""