How to Resolve BadHostKeyException: Host Key Changes and Man-in-the-Middle Attacks in Paramiko

BadHostKeyException occurs when a host key changes. This can indicate a man-in-the-middle attack. This tutorial explains how to handle this exception.

What is a Host Key?

A host key uniquely identifies an SSH server. It is used to verify server identity. This prevents unauthorized access.

Understanding BadHostKeyException

BadHostKeyException means the server’s key has changed. This could be due to a server reinstall. It could also signal a security breach.

Handling the Exception

Catch the BadHostKeyException in your code. This allows you to handle the error gracefully. It prevents program crashes.

import paramiko

try:
    client = paramiko.SSHClient()
    client.load_system_host_keys() # Or load known_hosts
    client.connect("your_hostname", username="your_username", password="your_password")
    # ... your SSH operations ...
    client.close()
except paramiko.ssh_exception.BadHostKeyException as e:
    print(f"Bad Host Key: {e}")
    # Handle the exception, e.g., prompt user, update known_hosts
except Exception as e:
    print(f"Other error: {e}")

Updating the Known Hosts File

If the key change is legitimate, update your known hosts file. This allows future connections to succeed. Verify the new key carefully.

import paramiko

try:
    client = paramiko.SSHClient()
    client.set_missing_host_key_policy(paramiko.AutoAddPolicy()) # Only for initial connection/trusted env
    client.connect("your_hostname", username="your_username", password="your_password")
    # ... your SSH operations ...
    client.close()
except paramiko.ssh_exception.BadHostKeyException as e:
    print(f"Bad Host Key: {e}")
    # Manually verify the key and update known_hosts if legitimate
except Exception as e:
    print(f"Other error: {e}")

Man-in-the-Middle Attacks

A BadHostKeyException may indicate a MITM attack. Do not blindly update the known hosts file. Investigate the key change thoroughly.

Verifying the New Key

If a key change is expected, verify the new key. Obtain the correct key through a secure channel. Compare it with the received key.

Security Best Practices

Use strong passwords and key-based authentication. Regularly audit your SSH server configurations. This enhances overall security posture greatly.