Understanding the `nxc ssh` Command (with Examples)

The nxc ssh command is a powerful tool designed primarily for penetration testers and security analysts. It facilitates various methods of interacting with SSH servers, making it easier to test for vulnerabilities and assess the security posture of network environments. The command supports multiple use cases, from password spraying to checking for sudo privileges, and acts as an extension to popular security tools like hydra. Here’s an exploration of the different ways you can leverage the nxc ssh command’s capabilities.

Spray a Password Against a List of Usernames

Code:

nxc ssh 192.168.178.2 -u path/to/usernames.txt -p password

Motivation:

This use case is vital for penetration testers who aim to identify unauthorized access potential by checking if a single password is valid across a plethora of user accounts. This method is known as password spraying, and it’s often used to avoid account lockouts when facing robust login mechanisms that enforce such restrictions after multiple failed login attempts.

Explanation:

• 192.168.178.2: This is the target IP address where the SSH server resides.
• -u path/to/usernames.txt: This argument specifies the path to a file containing a list of usernames to be tested against the password provided.
• -p password: This single password will be systematically attempted against each username from the list.

Example Output:

Trying username: admin, password: password -> Success
Trying username: user1, password: password -> Failed
Trying username: guest, password: password -> Failed

Search for Valid Credential Combinations

Code:

nxc ssh 192.168.178.2 -u path/to/usernames.txt -p path/to/passwords.txt

Motivation:

This use case is particularly useful when you need to uncover valid username-password pairs, especially when faced with accounts having weaker passwords. The brute-force mechanism enables an extensive search through provided lists to find combinations that grant access to the SSH server, aiding security assessments.

Explanation:

• 192.168.178.2: The server IP address targeted for authentication checks.
• -u path/to/usernames.txt: Path to a file containing usernames to test.
• -p path/to/passwords.txt: Path to another file containing passwords to try for each username.

Example Output:

Trying username: admin, password: 123456 -> Success
Trying username: root, password: admin1234 -> Failed
Trying username: guest, password: guest2019 -> Success

Use a Private Key for Authentication

Code:

nxc ssh 192.186.178.2 -u path/to/usernames.txt -p password --key-file path/to/id_rsa

Motivation:

When SSH key-based authentication is enabled, testing the security of SSH servers involves using private keys. This method is more secure compared to password-based authentication, and penetration testers need to ensure that such keys are securely managed and not vulnerable to unauthorized use.

Explanation:

• 192.186.178.2: Denotes the target IP address hosting the SSH server.
• -u path/to/usernames.txt: File path containing potential usernames, matching the key’s capabilities.
• -p password: Passphrase tied to the private key, necessary for validating its use.
• --key-file path/to/id_rsa: Specifies the actual private key file path required for this authentication.

Example Output:

Using private key -> Successful login for username: admin
Using private key -> Failed login for username: guest

Test Username-Password Combos on Multiple Targets

Code:

nxc ssh 192.168.178.0/24 -u username -p password

Motivation:

This is a brilliant approach when conducting network-wide scans to determine weak authentication points across servers. It’s essential for evaluating credentials across different machines in a larger network to ensure consistent security policies.

Explanation:

• 192.168.178.0/24: Represents a subnet range spanning multiple devices or servers to engage.
• -u username: A single username to be used for authentication attempts across the specified IP range.
• -p password: The corresponding password matching the specified username.

Example Output:

Trying 192.168.178.1 -> Failed
Trying 192.168.178.2 -> Failed
Trying 192.168.178.3 -> Success

Check for sudo Privileges Post-Login

Code:

nxc ssh 192.168.178.2 -u username -p path/to/passwords.txt --sudo-check

Motivation:

Determining sudo privileges post-authentication is crucial as it checks if a user has administrative rights once logged in. This information is indispensable during security audits to recognize potential privilege escalation risks.

Explanation:

• 192.168.178.2: The selected IP of the SSH server where credentials will be validated.
• -u username: Indicates the testing username that follows authentication checks.
• -p path/to/passwords.txt: User passwords list to attempt with the specified username.
• --sudo-check: Enabling this switch initiates a check for sudo permissions immediately after a successful login.

Example Output:

Login successful for username: admin, checking sudo access...
Sudo privileges found -> Admin rights confirmed

Conclusion:

The nxc ssh command demonstrates its effectiveness in quickly assessing SSH server security through various penetration testing methodologies. From password spraying to scanning multi-host environments for valid credentials, this tool efficiently supports security practitioners in identifying potential vulnerabilities. To realize its full potential responsibly, always ensure its use aligns with ethical and legal guidelines.