another way to avoid the ssh agent intrusion is to use TCP tunneling through SSH. Then the intermediate host is only used for TCP connection bridging, but all the authentication is happening outside of your jumphost.
----- Original Message -----
> From: Stanislav Sinyagin <ssinyagin@yaho...>
>> From: Jeroen Massar <jeroen@unfi...>
>> On 2 Jun 2012, at 05:49, Stanislav Sinyagin <ssinyagin@yaho...>
>> When I'm logged in to the VPS, I can do
>>> ssh -A firstname.lastname@example.org
>>> with this command, the server 184.108.40.206 authenticates me through my public
> key, and the VPS acts as the SSH agent proxy. So, if that server has my public
> key in .ssh/authorized_keys, I'm easily in, and no security breach on the
> VPS would affect my security.
>> Unless the attacker is on the jumpbox as root as then they can also forward
> in the same way, but this should not happen ofcourse ;)
> yes, in theory if the attacker is logged in as root, then during my SSH session
> they may make an SSH connection using my credentials. But it would be difficult
> to stay unnoticed, and it's only possible while I'm logged in.