Writeup

Initially I tried to overflow as much as possible and seeing which string the check finally became from my input. Trying things like abcdefg all the way to Z t o find out at which character does it start overflowing to the check.

Then I knew that I had to input my stdin as a piped input, since it's hard to just type ASCII strings into my terminal. So I used the command

All that's finally left is to keep the shell open before it is closed by the program. We keep it open by running the command for cat afterwards with a - which interpreted as stdin

# or
(python -c 'print("a"*40 + "\xef\xbe\xad\xde")'; cat -) | ./ch13

and we get the password!