Formatting

The challenge is a python file. If we run the service using the netcat utility and give some random input, it returns the same string. After reading the source code, I found that format string is used to return the string. There is vulnerability in str format in python. You can read more about it here. We can exploit to format string to get the global variable. Use the following payload.

hello_{a.__init__.__globals__}

and we get the flag

flag >> ictf{c4r3rul_w1th_f0rmat_str1ngs_4a2bd219}