Using the ssh connection command given and the password, I connect to the server:

$ ssh -p 62619 [email protected]
...
[email protected]'s password:
Specialer$ ls
-bash: ls: command not found

We are not restricted by characters, but the binaries don’t exist! We can use . as an alias for source (which both still exist) to execute files.

Specialer$ .
-bash: .: filename argument required
.: usage: . filename [arguments]
Specialer$ . *
-bash: .: abra: is a directory
Specialer$ . */*
-bash: Nothing: command not found

So we can execute a file but when there is a command with a non-existent command, it will fail!

The solution is a simple feature that is so often forgotten, Tab. The tab key does autocompletion and if we do abra/ then press tab twice, we get this:

Specialer$ . abra/cada
cadabra.txt   cadaniel.txt 

We can now explore the directories without ls! We can also see what binaries we have:

Specialer$ /
bin/   home/  lib/   lib64/
Specialer$ /bin/bash

Pressing tab inside /bin only gave us bash, which means that is the only binary available.

Let’s press tab without any input to see what is available to us:

Specialer$
!          ]]         break      command    coproc     done       esac       false      function   if         local      pushd      return     source     times      ulimit     wait
./         alias      builtin    compgen    declare    echo       eval       fc         getopts    in         logout     pwd        select     suspend    trap       umask      while
:          bash       caller     complete   dirs       elif       exec       fg         hash       jobs       mapfile    read       set        test       true       unalias    {
[          bg         case       compopt    disown     else       exit       fi         help       kill       popd       readarray  shift      then       type       unset      }
[[         bind       cd         continue   do         enable     export     for        history    let        printf     readonly   shopt      time       typeset    until

A few features catch my eye, specifically exec and echo.

With exec we could provide the file content and if will execute it to the command line. After trying with a few files we eventually land the correct file!

Specialer$ exec < ala/kazam.txt
Specialer$ return 0 picoCTF{y0u_d0n7_4ppr3c1473_wh47_w3r3_d01ng_h3r3_838b49d1}

Flag: picoCTF{y0u_d0n7_4ppr3c1473_wh47_w3r3_d01ng_h3r3_838b49d1}

Other Solutions

mapfile

We can read the file by using mapfile to take the file contents into a variable a then printing the variable’s contents with echo.

Specialer$ mapfile -t a < ala/kazam.txt
Specialer$ echo $a
return 0 picoCTF{y0u_d0n7_4ppr3c1473_wh47_w3r3_d01ng_h3r3_838b49d1}

echo

We can provide the file content to echo by using a command substitution and the file content as an input.

Specialer$ echo $(<abra/cadaniel.txt )
Yes, I did it! I really did it! I'm a true wizard!
Specialer$ echo $(<ala/kazam.txt )
return 0 picoCTF{y0u_d0n7_4ppr3c1473_wh47_w3r3_d01ng_h3r3_838b49d1}