tl;dr:
1. Notice we control value of one header
2. Notice we can inject utf CRLF sequence
3. Inject `Connection: keep-alive` header
4. Inject another request for the `/flag`

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-14-dragon-finals/cat_flag

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-11-14-dragon-finals/cat_flag).