Interesting. Does ssh-agent(1) also make you afraid?
http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man1/...
With the ssh-agent defaults, if I have root/sudo access on a server you are logged into with ssh on, I practically have free use of your private SSH key.
I use gpg's ssh-agent emulation (See the man page for gpg-agent under --enable-ssh-support), and store my ssh private key on my yubikey. With this setup, the yubikey's light flashes when something wants to use my private key, and you can just touch the button on it to approve.
Yes, root can also simply `su - user` and then do all stuff you do.
But you should also not put YOUR private keys on a system that has a root user who should not have access YOUR keys.
> The problem with ssh-agent defaults is that someone with root on the server you're SSH'd into can use your SSH private key (which is on your local machine).
> ssh-agent is like temporarily uploading your private key to each remote server logged into.
That is very different (and much more common) than an attacker having root on your local machine.
Some time read up on SSH Multiplexing. The default is MaxSessions 10. You auth once, my phishing exercise can utilize your session to connect anywhere you have connected. Nothing will show up in syslog. I can take care of `lastlog` entries since folks cache sudoer privs for so long; or in some cases, don't even require a pw.
I will set up a github with a working example of how to set up a backdoor leveraging ssh multiplexing. I got the idea from a coworker (whitehat) that used a simple ruby script to completely pwn my system. The cool part is, it doesn't require exploiting anything beyond the helpful developer or sysadmin.
ssh-agent is like temporarily uploading your private key to each remote server logged into.
The GP is mixing together SSH agent with agent forwarding, and private keys with key signing by agent.
There is last week's CVE-2016-0777, which is a vulnerability that enables the exploit you are describing, but in a properly patched configuration, without agent forwarding (nb: this feature is off by default), this is not the case. Same with connection multiplexing: off by default (for good reasons). In other words, either a) please explain how that would work, or b) please stop spreading FUD.