The binary has a FSB vulnerability.
We can overwrite the GOT address for `puts` and call the `flag` function.

[writeup](https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Binary-120pts-Purchases)

Original writeup (https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Binary-120pts-Purchases).