ainevsia@ubuntu:~/cc$ whoami
ainevsia
ainevsia@ubuntu:~/cc$ ls -la | grep flag
---s--x--- 1 admin ainevsia 8768 Sep 5 06:49 catflag
-r-------- 1 admin root 16 Sep 5 06:47 flag
ainevsia@ubuntu:~/cc$ ./catflag
this{is}a{flag}
As a user with the gid, once i run catflag, i will get the user's privilege.