Rating:

tl;dr:
1. Extract list of endpoints and how to use them
2. Register account
3. Find secret logs endpoint and get admin auth token
4. Break admin rng number from the token
5. Recover list of admin links by trying to add new one with existing name
6. Share flag link with your user

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/sharel

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/sharel).