Linux System Hardening

Table of Contents

Note:

The questions are shortened for a cleaner view
Questions that are straightforward will only have answers attached. Such questions can be easily answered by reading the documentation given in the room.

Task 1 - Introduction
#

No answer needed

Task 2 - Physical Security
#

1) What command can you use to create a password for the GRUB bootloader?
#

Ans: grub2-mkpasswd-pbkdf2

2) What does PBKDF2 stand for?
#

PBKDF2 (Password-Based Key Derivation Function 2) is ==a cryptographic standard that securely turns a password or passphrase into a long, strong cryptographic key, primarily by applying a pseudorandom function (like HMAC) iteratively with a unique salt and a high number of rounds (iterations)==. Its purpose is to make brute-force attacks computationally expensive and slow, protecting stored passwords and creating encryption keys, and is defined in RFC 2898.

Ans: Password-Based Key Derivation Function 2

Task 3 - Filesystem Partitioning and Encryption
#

1) What does LUKS stand for?
#

Ans: Linux Unified Key Setup

2) What is the flag in the secret vault?
#

Step 1: Decrypt/Open the Encrypted Image with cryptsetup
#

Use cryptsetup to decrypt the image file.

sudo cryptsetup open /home/tryhackme/secretvault.img myvault

open: Opens/decrypts the LUKS container
/home/tryhackme/secretvault.img: The encrypted file
myvault: The name for the decrypted device mapping
When prompted, enter the password: 2N9EdZYNkszEE3Ad

After this command, the decrypted device will be available at /dev/mapper/myvault

Step 2: Mount the Decrypted Device
#

Now mount the decrypted device

sudo mount /dev/mapper/myvault myvault

Step 3: Access the Files
#

Now you can read the files:

ls myvault

Ans: THM{LUKS_not_LUX}

Task 4 - Firewall
#

1) It is allowing another TCP port; what is it?
#

Run the following command:

sudo ufw status

Status: active

To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       Anywhere                  
14298/udp                  ALLOW       Anywhere                  
12526/tcp                  ALLOW       Anywhere                  
22/tcp (v6)                ALLOW       Anywhere (v6)             
14298/udp (v6)             ALLOW       Anywhere (v6)             
12526/tcp (v6)             ALLOW       Anywhere (v6) 

Ans: 12526

2) What is the allowed UDP port?
#

Ans: 14298

Task 5 - Remote Access
#

1) What flag is hidden in the `sshd_config` file?
#

I used grep to search through the file to make it easier:

grep "THM" /etc/ssh/sshd_config
# THM{secure_SEA_shell}

Ans: THM{secure_SEA_shell}

Task 6 - Securing User Accounts
#

1) What is the suggested value to use for the shell?
#

Ans: /sbin/nologin

2) What is the name of the RedHat and Fedora systems sudoers group?
#

Ans: wheel

3) What is the name of the sudoers group on Debian and Ubuntu systems?
#

Ans: sudo

4) Other than `tryhackme` and `ubuntu`, what is the username…?
#

The following command lists all users in sudo group:

getent group sudo

Ans: blacksmith

Task 7 - Software and Services
#

1) Besides FTPS, what is another secure replacement for TFTP and FTP?
#

Instead of Telnet, the SSH protocol is now widely available. For example, the Secure File Transfer Protocol (SFTP) protocol provides a great alternative to the TFTP protocol. The critical point is that a secure alternative is selected and used.

Ans: SFTP