tl;dr
1. SQLi via "Host" header
2. `procedure analyse()` to get name of the table and columns
3. `union select...` + `to_base64` to grab the flag

Whole writeup: https://github.com/p4-team/ctf/tree/master/2017-11-09-defcamp-final/state_agency

Original writeup (https://github.com/p4-team/ctf/tree/master/2017-11-09-defcamp-final/state_agency).