Generate Passwords with Hydra Brute-Force

Introduction

In this lab, we will explore password generation and brute-force techniques using Hydra. The focus is on leveraging Hydra's -x option to create passwords based on defined criteria, such as length and character sets.

The lab guides you through generating passwords with the -x option, targeting a dummy FTP server on localhost. You'll learn how to specify minimum and maximum password lengths, and how to use character sets like lowercase letters. The lab also covers disabling symbols with the -y option and analyzing the output of a brute-force attack.

Skills Graph

Generate Passwords with -x Option

In this step, we will explore how to use the -x option in Hydra to generate passwords based on a specified format. This is particularly useful when you have some knowledge about the potential password structure, such as a common prefix or suffix.

Hydra's -x option allows you to define a minimum and maximum length for the generated passwords, along with a character set to use. The syntax is -x min:max:charset.

Let's start with a simple example. Suppose we want to generate passwords that are between 6 and 8 characters long, using only lowercase letters.

First, navigate to the ~/project directory:

cd ~/project

Now, let's use Hydra to generate passwords and attempt to crack an FTP server. For demonstration purposes, we'll use a dummy FTP server that always accepts any username and password. We'll use localhost as the target, assuming an FTP server is running locally (though it doesn't need to be for this password generation step).

Execute the following command:

hydra -l user -P /dev/null -vV -x 6:8:lc localhost ftp

Let's break down this command:

• hydra: The Hydra command-line tool.
• -l user: Specifies the username to use. We're using "user" here, but it could be any username.
• -P /dev/null: Tells Hydra not to use a password list file. We're using -x to generate passwords instead. /dev/null is a special file that discards any data written to it.
• -vV: Enables verbose output, showing each login attempt.
• -x 6:8:lc: This is the key part. It tells Hydra to generate passwords with:
  • 6: Minimum length of 6 characters.
  • 8: Maximum length of 8 characters.
  • lc: Use only lowercase letters (a-z). Other options include uc (uppercase), 12 (digits), sc (symbols). You can combine them, e.g., lcuc12 for lowercase, uppercase, and digits.
• localhost: The target host (in this case, our dummy FTP server).
• ftp: The service to attack (FTP).

You will see Hydra attempting logins with generated passwords like "aaaaaa", "aaaaaaa", "aaaaaaaa", "aaaaab", and so on. Since we're using /dev/null as the password file, Hydra will only use the passwords generated by the -x option.

Important Note: This example uses a dummy FTP server for demonstration. Do not attempt to crack FTP servers without explicit permission.

Now, let's try a more complex example. Suppose we know that the passwords might start with "prefix" and end with a digit, with 4-6 characters in between that are alphanumeric.

We can't directly specify a prefix and suffix with -x. However, we can generate the middle part and then prepend and append the known parts using other tools (like sed or awk) to create a password list file, which we would then use with Hydra. For simplicity, we'll stick to the basic -x usage in this step. We'll explore more advanced password list manipulation in later steps.

For now, let's generate passwords that are 8-10 characters long, using lowercase letters and digits:

hydra -l user -P /dev/null -vV -x 8:10:lc12 localhost ftp

This will generate passwords like "aaaaaaaa", "aaaaaaa1", "aaaaaa11", and so on.

This step demonstrated how to use the -x option to generate passwords with specific length and character set constraints. This is a powerful technique when you have some prior knowledge about the potential password structure.

Attack FTP with Generated Passwords

In this step, we will build upon the previous step and use the password generation capabilities of Hydra to attack a (dummy) FTP server. We'll focus on using a password list generated "on-the-fly" with the -x option.

As a reminder, it is crucial to emphasize that attacking systems without explicit permission is illegal and unethical. This lab is for educational purposes only, and you should only use these techniques on systems you own or have permission to test.

For this step, we'll assume you have a basic FTP server running (even if it's a dummy server that accepts any credentials). If you don't have one, you can quickly set up a simple one using Python:

sudo apt update
sudo apt install python3-pyftpdlib

Then, create a directory for the FTP server to serve:

mkdir ~/project/ftp_root

And start the FTP server:

python3 -m pyftpdlib -p 2121 -w ~/project/ftp_root

This starts an FTP server on port 2121, serving files from the ~/project/ftp_root directory. The -w option allows write access. You can stop the server with Ctrl+C. Note that this is a very basic server for testing purposes and is not suitable for production use.

Now, let's use Hydra to attack this FTP server. We'll use the -x option to generate passwords and attempt to log in as the user "testuser".

hydra -l testuser -P /dev/null -vV -x 4:6:lc 127.0.0.1 ftp

Let's break down this command again:

• hydra: The Hydra command-line tool.
• -l testuser: Specifies the username to use: "testuser".
• -P /dev/null: Tells Hydra not to use a password list file. We're using -x to generate passwords instead.
• -vV: Enables verbose output, showing each login attempt.
• -x 4:6:lc: This tells Hydra to generate passwords with:
  • 4: Minimum length of 4 characters.
  • 6: Maximum length of 6 characters.
  • lc: Use only lowercase letters (a-z).
• 127.0.0.1: The target host (localhost). We use 127.0.0.1 instead of localhost to avoid potential DNS resolution issues within the Docker container.
• ftp: The service to attack (FTP).

You should see Hydra attempting logins with generated passwords. Since our dummy FTP server accepts any credentials, Hydra will likely find a valid password quickly.

Now, let's try a slightly more complex example, combining lowercase letters and digits:

hydra -l testuser -P /dev/null -vV -x 5:7:lc12 127.0.0.1 ftp

This will generate passwords like "aaaaa", "aaaaa1", "aaaa11", and so on.

Important Considerations:

• Rate Limiting: Real-world FTP servers often have rate limiting in place to prevent brute-force attacks. Hydra has options to control the number of concurrent connections and the delay between attempts.
• Password Complexity: The effectiveness of a brute-force attack depends heavily on the complexity of the passwords. Longer and more complex passwords (using a mix of uppercase, lowercase, digits, and symbols) are much harder to crack.
• Legal and Ethical Implications: Always obtain explicit permission before attempting to test the security of any system.

This step demonstrated how to use Hydra with the -x option to generate passwords and attack an FTP server. Remember to use these techniques responsibly and ethically.

Disable Symbols with -y Option

In this step, we will learn how to use the -y option in Hydra to disable the use of symbols when reading passwords from a file. This is useful when you suspect that the target system might have issues handling special characters in passwords, or when you want to specifically exclude symbols from your brute-force attempts.

The -y option tells Hydra to ignore any symbols present in the password list file. It doesn't affect password generation with the -x option; it only applies when reading passwords from a file.

First, let's create a password list file containing some passwords with symbols. Navigate to the ~/project directory:

cd ~/project

Now, create a file named passwords.txt using nano:

nano passwords.txt

Add the following passwords to the file:

password123
P@sswOrd
secret!
12345
qwerty

Save the file and exit nano (Ctrl+X, then Y, then Enter).

Now, let's use Hydra to attack our dummy FTP server (assuming you still have it running from the previous step) using this password list, without the -y option:

hydra -l testuser -P passwords.txt -vV 127.0.0.1 ftp

Hydra will attempt to log in using each password in the passwords.txt file, including those with symbols.

Now, let's use the -y option to disable symbols:

hydra -l testuser -P passwords.txt -y -vV 127.0.0.1 ftp

With the -y option, Hydra will effectively treat passwords like "P@sswOrd" as "PsswOrd" and "secret!" as "secret". It removes the symbols before attempting the login.

Important Note: The -y option modifies the passwords before they are sent to the target service. This means that if the actual password requires a symbol, using -y will prevent you from finding it.

To illustrate this further, let's create a scenario where the correct password is "secret!". If we use -y, Hydra will try "secret" and fail.

When to Use -y:

• When you suspect the target system has issues with symbols in passwords.
• When you want to specifically exclude symbols from your brute-force attempts (e.g., if you know the password policy doesn't allow them).
• When you are troubleshooting connection issues and suspect symbols might be the cause.

This step demonstrated how to use the -y option in Hydra to disable symbols when reading passwords from a file. Remember to consider the potential impact on your brute-force attempts, as it can prevent you from finding passwords that contain symbols.

Analyze Brute-Force Output

In this step, we will focus on analyzing the output generated by Hydra during a brute-force attack. Understanding the output is crucial for identifying successful logins, troubleshooting issues, and optimizing your attack strategy.

Hydra provides different levels of verbosity, controlled by the -v option. We've been using -vV in the previous steps, which provides a good balance of information. Let's examine the different output messages and what they mean.

First, let's run a simple Hydra command against our dummy FTP server (make sure it's still running):

hydra -l testuser -P passwords.txt -vV 127.0.0.1 ftp

Assuming you have the passwords.txt file from the previous step, Hydra will attempt to log in using each password in the file.

Here's a breakdown of the common output messages you might see:

• [DATA] 127.0.0.1:21 - banner: 220 pyftpdlib 1.5.6: This shows the banner received from the FTP server. The banner provides information about the server software and version.
• [STATUS] 127.0.0.1:21 - 1/5 [5/5 pp]: This indicates the progress of the attack.
  • 1/5: Means Hydra has tried 1 password out of a total of 5.
  • [5/5 pp]: Shows the number of passwords per process.
• [DEBUG] ...: Debug messages provide more detailed information about the connection and authentication process. These are helpful for troubleshooting.
• [21][ftp] host: 127.0.0.1 login: testuser password: password123: This shows the credentials being attempted.
• [21][ftp] host: 127.0.0.1 login: testuser password: P@sswOrd: This shows the next credentials being attempted.
• [ATTEMPT][ftp] 127.0.0.1:21 - login: 'testuser' password: 'password123': This line indicates that Hydra is attempting to log in with the specified username and password.
• [21][ftp] 127.0.0.1: Login incorrect: This indicates a failed login attempt. The exact message depends on the FTP server's configuration.
• [21][ftp] 127.0.0.1: Invalid username or password: This also indicates a failed login attempt.
• [21][ftp] 127.0.0.1: ftp:Login OK: This is the most important message! It indicates a successful login. Hydra has found a valid username and password combination.

If Hydra finds a valid login, it will display the "Login OK" message and then stop the attack (by default).

Saving Output to a File:

You can save Hydra's output to a file for later analysis using the -o option:

hydra -l testuser -P passwords.txt -vV -o output.txt 127.0.0.1 ftp

This will save all the output messages to a file named output.txt in the ~/project directory. You can then use tools like grep to search for specific messages, such as "Login OK".

For example, to find successful logins in the output.txt file:

grep "Login OK" output.txt

Interpreting the Output:

• No "Login OK" messages: This means Hydra was unable to find a valid username and password combination. This could be due to several reasons:
  • The correct password is not in your password list.
  • The username is incorrect.
  • The target service is not vulnerable to brute-force attacks (e.g., due to account lockout policies).
  • There are network connectivity issues.
• Many "Login Incorrect" messages: This indicates that Hydra is attempting many invalid username and password combinations. This is expected during a brute-force attack.
• "Connection refused" or "Timeout" errors: These indicate network connectivity issues. Make sure the target service is running and accessible from your machine.

By carefully analyzing Hydra's output, you can gain valuable insights into the security of the target system and refine your attack strategy.

Summary

In this lab, we explored using Hydra to generate passwords for brute-force attacks, specifically focusing on the -x option. This option allows for the creation of passwords based on defined parameters such as minimum and maximum length, and character sets (lowercase, uppercase, digits, symbols).

The lab demonstrated how to use -x with specific length constraints and the lc (lowercase) character set to generate passwords and attempt to crack a dummy FTP server running on localhost. The command hydra -l user -P /dev/null -vV -x 6:8:lc localhost ftp was used to illustrate this process, highlighting the importance of understanding the syntax and available character set options for effective password generation.