PatriotCTF 2024 Really Only Echo Writeup

Here's my writeup for the PatriotCTF 2024 Really Only Echo challenge. Challenge note Hey, I have made a terminal that only uses echo, can you find the flag? Author: Ryan Wong (shadowbringer007) nc chal.competitivecyber.club 3333 Challenge file Download the challenge server's server.py script with the Wget download tool3: wget --content-disposition \ "https://pctf.competitivecyber.club/files/0140073002c3143d3bb99bf24edbb598/server.py?token=…" Connect to challenge server Connect to the challenge server with Netcat1: nc chal.competitivecyber.club 3333 The challenge server greets you once you connect: This is shell made to use only the echo command. Please input command: Solve the challenge The challenge server receives your input in the backend(req) function. This function filters and executes your input: # server.py def backend(req): # … if filter_check(user_input): output = os.popen(user_input).read() req.sendall((output + '\n').encode()) The backend(reg) function calls the filter…