This problem has a buffer overflow vulnerability! Can you get a shell, then use that shell to read flag.txt? You can solve this problem interactively here[1], and the source can be found here[2].

hint:Can you overflow into the 'secret' variable?

[1]https://picoctf.com/problem-static/binary/Overflow1/overflow1.html#1
[2]https://picoctf.com/problem-static/binary/Overflow1/overflow1.c