Super SSH

Platform: PicoCTF Category: General Skills Operating system: Linux Environment: Standalone

Goal: the challenge page hands over a host, a port, a username and a password. Connect, and the flag arrives in what the login prints.

Approach

Nothing has to be found here. Every parameter is printed on the challenge page, and -p is the only flag the invocation needs, because the instance does not listen on 22. The one decision the session actually contains arrives before the password does: this host has never been seen before, so OpenSSH prints its key fingerprint and asks whether to go on.

Bash

┌──(Idan@Kali)-[~/Super_SSH]
└─> ssh -p 49808 ctf-player@titan.picoctf.net
The authenticity of host '[titan.picoctf.net]:49808 ([3.139.174.234]:49808)' can't be established.
ED25519 key fingerprint is SHA256:4S9EbTSSRZm32I+cdM5TyzthpQryv5kudRP9PIKT7XQ.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[titan.picoctf.net]:49808' (ED25519) to the list of known hosts.
ctf-player@titan.picoctf.net's password:
Welcome ctf-player, here's your flag: <flag>

Flag picoCTF{s3cur3_c0nn3ct10n_5d09a462} Reveal

picoCTF assigns the port when the instance is launched, so a reader following along is given a different one.

Why it works

SSH authenticates in both directions, and this challenge sits on the half that gets clicked through. The password proves the client to the server. The host key proves the server to the client, and nothing vouches for it in advance the way a certificate authority does for a web server, so a first connection can only print a fingerprint and put the decision to the operator.

This key is not known by any other names is OpenSSH reporting that it went looking: it searches the known hosts for this same key filed under any other name or address, and says so when it finds none. Launch the challenge again and it prints the opposite: picoCTF issues a fresh port but the same host key, so the first visit’s entry turns up under the old address. Answering yes wrote that entry, and it is what would catch a substituted server on a later visit, never on the first. The prompt takes a third answer for that reason, a pasted fingerprint the client checks for you, but picoCTF publishes none and the challenge text simply says to accept with yes.