The challenge allows the use of cowsay on a website.

You can create a custom one, if you select ‘custom’ from the input list, meaning that you want to write a custom file that will be executed from cowsay with the ‘-f’ parameter. So the only things that I can work with is the body had to work with the body, but every command inside was not executed, just printed.

Then I started to read the source code

I searched for the execution of a cowfile, since it was our case, and a line got my attention:

177 - do $full;

where $full is the path of our cowfile.

It execute the file as if it was Perl! Then I started to read a cowfile, and the solution was obvious.

It reads the file till ‘EOC’, and after that we can inject our code that will be executed. So we can just give as input

system("cat flag");

and we will get the flag!