DEV Community

CTFDojo
CTFDojo

Posted on Originally published at ctfdojo.com

PicoCTF Wave a Flag Writeup — Read a Remote Binary's Help Output

#ctf #security #beginners

This introductory challenge teaches the most basic reflex in offensive security: read a program's help before using it. By connecting to the remote binary and typing ./flag --help, the flag is displayed directly.

  • Platform: picoGym
  • Category: General Skills / Misc
  • Points: 50 pts
  • Difficulty: Beginner
  • Technique: Reading a program's help (--help)

Challenge description

The prompt provides a connection command to a remote shell:

nc saturn.picoctf.net 54981
Enter fullscreen mode Exit fullscreen mode

Once connected, we land in a minimal Linux environment with a binary named flag available. No other hint is given — that's precisely the point of the challenge: learning to explore on your own.

Step 1 — Connect

We open a terminal and connect with netcat to the given host and port:

$ nc saturn.picoctf.net 54981
Enter fullscreen mode Exit fullscreen mode

The connection is established and a plain shell prompt appears, with no particular welcome message.

Step 2 — Explore

We list the contents of the current directory to see what's available:

$ ls
flag
Enter fullscreen mode Exit fullscreen mode

Just one file: an executable named flag. No source code, no further hint in the prompt — we need to interact with the binary directly to understand what it does.

Step 3 — Read the help

Facing an unknown binary, the very first reflex (even before launching a disassembler or debugger) is to check whether it offers built-in help, via --help or -h:

$ ./flag --help
Enter fullscreen mode Exit fullscreen mode

The program immediately responds by printing its usage — and the flag is included directly in the message:

Usage: flag [options]
This program prints the flag and exits.

picoCTF{***************************}
Enter fullscreen mode Exit fullscreen mode

Full netcat session 

$ nc saturn.picoctf.net 54981
$ ls
flag
$ ./flag --help
Usage: flag [options]
This program prints the flag and exits.

picoCTF{***************************}
Enter fullscreen mode Exit fullscreen mode

🚩 picoCTF{ flag intentionally hidden }

The flag is deliberately hidden — follow the method, you've earned it. 💪

Key takeaways

This challenge may seem trivial, but it teaches an essential habit: before diving into a complex analysis (disassembly, reverse engineering, fuzzing…), always check the most obvious options of an unknown binary or command.

  • --help and -h are often the first thing to try when facing an unknown binary
  • A lot of useful information (usage, hidden options, versions, even hints or educational secrets) can be found in help messages before even starting a deeper analysis
  • In pentesting as in CTFs, enumeration always starts with the simplest means before bringing out the heavy artillery

Originally published on CTFdojo — join the CTFdojo Discord to discuss writeups and get notified about new ones.

Top comments (0)