diff --git a/cloud/auth.py b/cloud/auth.py
index 0b90cdd..48c50a6 100644
--- a/cloud/auth.py
+++ b/cloud/auth.py
@@ -334,7 +334,13 @@ class AuthStore:
         role = self._member_role(project_id, user_id)
         if role is None:
             return False
-        return _ROLE_LEVEL[role] >= _ROLE_LEVEL[min_role]
+        # 未知角色（如直接写库留下的 'member'）一律按无权限处理，
+        # 绝不能 KeyError 冒泡成 500：权限检查的失败语义只能是
+        # False / PermissionError，绝不是服务端异常。
+        level = _ROLE_LEVEL.get(role)
+        if level is None:
+            return False
+        return level >= _ROLE_LEVEL[min_role]
 
     def require_permission(self, user_id, project_id, action):
         """不满足权限时 raise PermissionError。"""
diff --git a/cloud/tests/test_auth.py b/cloud/tests/test_auth.py
index 4be67b8..1557677 100644
--- a/cloud/tests/test_auth.py
+++ b/cloud/tests/test_auth.py
@@ -93,6 +93,31 @@ class TestAuth(unittest.TestCase):
         with self.assertRaises(PermissionError):
             self.store.require_permission(outsider_id, project_id, "read")
 
+    def test_unknown_role_denied_not_500(self):
+        # 回归：project_members 里直接写库留下的未知角色（如
+        # 'member'，set_member 本身会拦，但旧行/直写库拦不住）
+        # 在生产上曾让 check_permission 以 KeyError 冒泡成 500。
+        # 权限检查的失败语义只能是 False / PermissionError。
+        project_id, _o, viewer_id, _c, _m, _outsider = (
+            self._project_with_roles()
+        )
+        db_path = os.path.join(self.tmp.name, "auth.db")
+        conn = sqlite3.connect(db_path)
+        try:
+            conn.execute(
+                "UPDATE project_members SET role = ?"
+                " WHERE project_id = ? AND user_id = ?",
+                ("member", project_id, viewer_id))
+            conn.commit()
+        finally:
+            conn.close()
+        self.assertFalse(
+            self.store.check_permission(viewer_id, project_id, "read"))
+        self.assertFalse(
+            self.store.check_permission(viewer_id, project_id, "create_task"))
+        with self.assertRaises(PermissionError):
+            self.store.require_permission(viewer_id, project_id, "read")
+
     def test_permission_grants(self):
         project_id, owner_id, viewer_id, contributor_id, maintainer_id, _ = (
             self._project_with_roles()