Kimai Favorite Timesheet Add and Remove Endpoints Allows Cross-User Bookmark Manipulation

Summary

### Summary Kimai 2.56.0 contains an authenticated improper authorization / IDOR vulnerability in the favorite timesheet add and remove endpoints. A low-privileged user who knows another user's `timesheet.id` can add that record to, or remove it from, the victim's `favorite/recent` bookmark list. This allows cross-user manipulation of per-user favorite state without administrative privileges. ### Details The issue affects the following routes: - `GET /en/favorite/timesheet/add/{id}` - `GET /en/favorite/timesheet/remove/{id}` Both endpoints accept a user-controlled timesheet identifier and only require the caller to hold the generic `start_own_timesheet` permission. They do not verify that the referenced `Timesheet` object belongs to the currently authenticated user. - In `src/Controller/FavoriteController.php`, the controller methods accept a `Timesheet` object directly and forward it to the favorite service. - The root cause becomes more obvious in `src/Timesheet/FavoriteRecordService.php`. The bookmark owner is derived from `$timesheet->getUser()` instead of the current session user. - Because of this design, any authenticated user who can reference another user's timesheet ID can modify the victim's `favorite/recent` bookmark data. *A PoC was provided, but removed for security reasons.* ### Impact This vulnerability allows any authenticated low-privileged user to manipulate another user's favorite bookmark state across accounts. An attacker can inject arbitrary victim-owned timesheet entries into the victim's quick-entry workflow, remove existing favorites, and repeatedly disturb the victim's normal timesheet usage without needing administrative privileges. The issue does not directly disclose sensitive data, but it is a real cross-user business-state tampering vulnerability with clear integrity impact. Because the add and remove endpoints can be combined, an attacker can reliably insert, remove, and reorder entries in another user's `favorite/recent` list.