tts-be is a backend for a timetable selector that aims to help students better choose their class schedules. Versions prior to 2.1.0 have a Broken Access Control vulnerability across several API endpoints (such as /api/student/{id}/photo and /api/courseunit/{id}/exchange/metadata). By chaining these unauthenticated endpoints, a remote attacker can use the backend as an open proxy to bypass authorization checks, allowing for the enumeration and extraction of sensitive Personally Identifiable Information (PII) from upstream university systems. The exposed data includes full names, student IDs, class schedules, and photos. This issue was fixed in version 2.1.0.