There are some neat TTPs that I don’t use frequently, and if the time arises, I need to dig up details again. So, I figured to write some of them down, starting with SSH Agent Hijacking.

What is SSH Agent Hijacking?

Short story, if you have keys added to an SSH Agent an adversary with root permissions can use them. If you forward the SSH Agent to another host, an adversary with root permission on that other host can exploit and leverage your keys as well.

Jumpboxes

The typical scenario where this is quite powerful are jumpboxes. One can query the SSH Agent’s socket of others and then connect the ssh client to that socket, and leverage the victim’s SSH private keys that way.

Forwarding is typically done by updating the ssh_config file to include:

    ForwardAgent yes

Manually this can be done by using ssh -A.

SSH_AUTH_SOCK

What makes this all possible is the environment variable SSH_AUTH_SOCK.

On that jumpbox with multiple users logged in using SSH Agent forwarding an adversary can:

1. Query for other user’s processes, e.g. via ps aux ww or pstree -p victim.
2. Take note of a process id (PID) of a shell started by the sshd, like for instance bash
3. grep through the env variables (as root) of the process to find the SSH_AUTH_SOCK
4. Use that socket and pivot to the same or another machine as the victim user

That’s it.

Commands

Command line wise, it looks like this:

1. Find the sshd that launched the users shell (e.g. bash).

$ pstree -p sarah | grep sshd

2. Take note of the PID of the bash process and fill in the PID accordingly when running:

$ sudo cat /proc/{PID}/environ | tr '\0' '\n' | grep SSH_AUTH_SOCK 

3. If all works you can ssh to other machines where sarah has access to.

$ sudo SSH_AUTH_SOCK={value} ssh sarah@localhost

This is a neat TTP that is especially impactful when an adversary gains access to a shared host.

Greetings.

References

• Endpoint Protection - SSH and ssh-agent
• MITRE Attack TTP T1563 - SSH Hijacking

Appendix

Just some random notes around launching ssh-agent

When launching ssh-agent it will return the commands to the set the environment variable accordingly. This is also the reason why SSH Agent must be launched using an eval statement to set the SSH_AUTH_SOCK variable.

eval $(ssh-agent -s)

If you don’t do this, you get Could not open a connection to your authentication agent. as an error message when trying to list or add keys via ssh-add.

ssh-add -l
The agent has no identities.