Table of Contents
Breaking into Linux systems:
Objectives
- To explore the various methods of breaking into any Linux system without FDE (Full Disk Encryption) through physical access.
- To examine the procedure of setting up FDE and other remediations to prevent this.
Prerequisites
- Physical access to the target machine
- ~60 seconds
Procedure
Method 1: Using /sbin/init to execute a shell
Init is a process started during bootup that initializes a system. It starts, stops and monitors essential service processes during bootup and shutdown. I will use it to execute a root shell after booting the target system.
- Power on/reboot the target machine
- Through a graphical login screen: for a clean reboot just use the shutdown/reboot options in the system menu.
- Through a Textual User Interface: switch to a text console with <Ctrl>-<Alt>-<Del>.
- If all else fails, press the Reset button or power cycle the target machine.
- Press/hold the <Escape> key as soon as you see the GRUB splash screen.
- At the grub prompt, press 'e' to edit
- While still inside grub, add “init=/bin/bash” to the end of the “kernel” line:
kernel=/vmlinuz-<version> [...parameters...] init=/bin/bash
- Continue booting.
DONE! THAT'S ALL FOLKS!
After the target machine finishes booting, the kernel will detect the hardware and immediately drop you into a root shell. Since the system initialization script '/etc/rc.d/rc.sysinit' was bypassed and NOT executed,
you need to remount the root file system to make the system more usable:
- Mount the /proc file system:
mount /proc
You will see an error message complaining that it was already mounted. Ignore it.
- Remount the root file system in read-write mode:
mount -o remount,rw /
- Depending on how the target's file system is laid out, you may need to mount some other file systems. Lets view the file system table:
cat /etc/fstab
Mount any other needed file systems (Like '/home', '/usr', etc.).
- Do whatever nefarious things you want:
- Change the root account password:
passwd root
- PROTIP: I don't recommend doing this. The next time the real user logs into the system they will notice that the root password has been changed and it's GAME OVER. We need to be super sneaky, secretive, and surreptitious so here's something a little less noticeable:
- Simply add another user without modifying the original root password:
adduser -D -u 1000 bad-user passwd bad-user
- Now add the newly created user to the sudoers file. This is also not as noticable as changing the actual root password:
vi /etc/sudoers bad-user ALL=(ALL) ALL
- Reboot the target machine to make any changes to the file system persistent.
- Flush any disk I/O to the hardware:
sync
- Unmount any mounted file systems in reverse order:
umount
- Reboot with either <Ctrl><Alt><Del> or the power switch.
Method 2: boot to single-user mode
Single user mode is a start-up mode that boots a multi-user operating system into single superuser. It is often used for diagnoses and triage of a broken or malware-infected system. After booting into single-user mode, a root shell is provided to the user.
- Power on/reboot the target machine
- Through a graphical login screen: for a clean reboot just use the shutdown/reboot options in the system menu.
- Through a Textual User Interface: switch to a text console with <Ctrl>-<Alt>-<Del>.
- If all else fails, press the Reset button or power cycle the target machine.
- Press/hold the <Escape> key as soon as you see the GRUB splash screen.
- At the grub prompt, press 'a' to modify the kernel parameters.
- Add a space and the letter 'S' (lower or upper case) to the end of the kernel parameters line:
kernel=/vmlinuz-version ro root=LABEL=/ [...other-parameters...] S
- Sometimes there may still be some mysterious failures in single-user mode, because of Security-Enhanced Linux policy enforcement. In that case, add another boot parameter before the 'S':
enforcing=0
- Now press <Enter> to boot with the newly added parameter.
Method 3: Boot a LiveCD/USB Key/initramfs OS
LiveCD
- Power off the target machine
- Through a graphical login screen: for a clean reboot just use the shutdown options in the system menu.
- Through a Textual User Interface: switch to a text console with <Ctrl>-<Alt>-<Del>.
- If all else fails, press the Reset button or power cycle the target machine.
- Press/hold the <Escape> key to enter BIOS/UEFI
- Insert any live CD and boot the system.
- Once it boots, login to the LiveCD OS and get a terminal. Become root with
su -
and mount the file systems as needed.
Remediation Methods
Method 1: BIOS Password
- Reboot the system and go into the BIOS. Disable booting from anything other than the main disk.
- Set a BIOS password. This prevents unauthorized changes to the BIOS settings without a password.
- Set a BIOS Power On password. Now the machine will require a password before powering on.
Method 2: GRUB Password
- In one terminal, run:
# grub-md5-crypt
and follow the directions.
- In another terminal, edit the GRUB configuration file inside the '/boot/grub' named either 'menu.lst' or 'grub.conf'.
- Add a new line directly below the 'timeout' line:
# ... comments above ... default=0 timeout=5 password --md5 5f3782baec534bae412c27fc0850fc6d spashimage=(hd0,0)/grub/splash.xpm.gz hiddenmenu ......
- Now Change the file permissions to prevent viewing and recovery of the GRUB password:
- Now if you needed to legitimately break into your own machine, you need to press P while inside GRUB to enter the password to edit the boot parameters.
Method 3: single-user mode sulogin
- Find where your system has its program sulogin with this command:
# which sulogin
- This will force users to enter the root password to get a shell when booting into single-user mode. This is done by requiring sulogin to get into single-user mode.
- To have the system boot up to its default run state (with the login prompt) type <Ctrl-D>
- This remedy depends on what is running; traditional init, Upstart or systemd
- Look at your file a/etc/inittaba and see if it contains a line specifying the sysinit action.
- If that file contains a line similar to:
si::sysinit:/etc/rc.d/rc.sysinit
then you have traditional init.
- In this case, leave that line alone and add a new line:
# System initialization si::sysinit:/etc/rc.d/rc.sysinit ss:S:respawn:/sbin/sulogin # added line
- If that file is mostly comments with just one line specifying initdefault or even missing, and you have a directory /etc/init, then you have Upstart for init. In this case:
- If /etc/sysconfig/init exists, modify '/etc/sysconfig/init' and change:
SINGLE=/sbin/sushell
to this:
SINGLE=/sbin/sulogin
- If there is no '/etc/sysconfig/init', this file (located in /etc/init/rcS.conf) prevents the booting to single-user mode:
start on runlevel S stop on runlevel [!S] console owner script if [ -x /usr/share/recovery-mode/recovery-menu ]; then exec /usr/share/recovery-mode/recovery-menu else exec /sbin/sulogin fi end script [...]
Full Disk Encryption
Loop Device
A small file named crypt will be created and used to store cryptographic keys needed for booting, hdd encryption, ssh, etc.
# create empty file 'crypt' dd if=/dev/zero of=/crypt bs=1M count=256 # create device node losetup /dev/loop0 /crypt # setup LUKS header cryptsetup -c aes-xts-plain64 --key-size 512 \ --hash sha512 --iter-time 5000 \ --use-urandom luksFormat /dev/loop0 # open file cryptsetup open /dev/loop0 crypt # create filesystem mkfs.ext4 /dev/mapper/crypt # create mountpoint mkdir /mnt/crypt # mount file mount -t ext4 /dev/mapper/crypt /mnt/crypt ... # unmount file umount /mnt/crypt # delete mountpoint rmdir /mnt/crypt # close file cryptsetup close crypt # delete device node losetup -d /dev/loop0
Entire Partition
Note: /dev/sdb1 will be used as the test partition, 'private' will be its name.
# Create partition cryptsetup -c aes-xts-plain64 \ --key-size 512 --hash sha512 \ --iter-time 5000 --use-urandom /dev/sdb1 # open volume onto device mapper cryptsetup open /dev/sdb1 private # create filesystem mkfs.ext4 /dev/mapper/private # open mapped device cryptsetup --type luks open /dev/sdb1 private # mount encrypted partition mount -t ext4 /dev/mapper/private /mnt/private ... # unmount umount /dev/sdb1 # close mapped device cryptsetup close private
References
In performing this project, the following resources were referenced:
- Google: Of course Google was used, it knows everything. No particular page from Google was used, It was mainly used for information about the project (Linked Lists in general): http://www.google.com/; If ya don't know, now ya know, and knowing is half the battle.
- Wikipedia: Has a great article on Linked Lists; Tons of information, pretty pictures, and some code: http://en.wikipedia.org/wiki/Linked_list.