2 files changed, 21 insertions, 12 deletions
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 161b3c933c..98d3d2d97f 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -627,23 +627,28 @@ class AuthHandler(BaseHandler):
 
     async def add_oob_auth(
         self, stagetype: str, authdict: Dict[str, Any], clientip: str
-    ) -> bool:
+    ) -> None:
         """
         Adds the result of out-of-band authentication into an existing auth
         session. Currently used for adding the result of fallback auth.
+
+        Raises:
+            LoginError if the stagetype is unknown or the session is missing.
+            LoginError is raised by check_auth if authentication fails.
         """
         if stagetype not in self.checkers:
-            raise LoginError(400, "", Codes.MISSING_PARAM)
+            raise LoginError(
+                400, f"Unknown UIA stage type: {stagetype}", Codes.INVALID_PARAM
+            )
         if "session" not in authdict:
-            raise LoginError(400, "", Codes.MISSING_PARAM)
+            raise LoginError(400, "Missing session ID", Codes.MISSING_PARAM)
 
+        # If authentication fails a LoginError is raised. Otherwise, store
+        # the successful result.
         result = await self.checkers[stagetype].check_auth(authdict, clientip)
-        if result:
-            await self.store.mark_ui_auth_stage_complete(
-                authdict["session"], stagetype, result
-            )
-            return True
-        return False
+        await self.store.mark_ui_auth_stage_complete(
+            authdict["session"], stagetype, result
+        )
 
     def get_session_id(self, clientdict: Dict[str, Any]) -> Optional[str]:
         """
diff --git a/synapse/handlers/ui_auth/checkers.py b/synapse/handlers/ui_auth/checkers.py
index 5414ce77d8..270541cc76 100644
--- a/synapse/handlers/ui_auth/checkers.py
+++ b/synapse/handlers/ui_auth/checkers.py
@@ -49,7 +49,7 @@ class UserInteractiveAuthChecker:
             clientip: The IP address of the client.
 
         Raises:
-            SynapseError if authentication failed
+            LoginError if authentication failed.
 
         Returns:
             The result of authentication (to pass back to the client?)
@@ -131,7 +131,9 @@ class RecaptchaAuthChecker(UserInteractiveAuthChecker):
             )
             if resp_body["success"]:
                 return True
-        raise LoginError(401, "", errcode=Codes.UNAUTHORIZED)
+        raise LoginError(
+            401, "Captcha authentication failed", errcode=Codes.UNAUTHORIZED
+        )
 
 
 class _BaseThreepidAuthChecker:
@@ -191,7 +193,9 @@ class _BaseThreepidAuthChecker:
             raise AssertionError("Unrecognized threepid medium: %s" % (medium,))
 
         if not threepid:
-            raise LoginError(401, "", errcode=Codes.UNAUTHORIZED)
+            raise LoginError(
+                401, "Unable to get validated threepid", errcode=Codes.UNAUTHORIZED
+            )
 
         if threepid["medium"] != medium:
             raise LoginError(