Is SSH safer or more vulnerable with password auth?

Question

I've been having a fight with the university IT people about SSH being unsafe because of the possibility of a brute-force password attack. Of course (as I explain to them) there are myriad ways to thwart this, some of which I had already taken before the fight began (only allow a short time to connect successfully, for example). (Although, I haven't been able to figure out if SSH can simply decide to refuse a connection after a certain number of failed passwords, but that's another issue..). On the other hand, I have seen a few sites in my SSH googling that hint that the RSA key authentication is less secure than password authentication.
So, my question I would like to submit for discussion is this: Is a passwordless RSA key authentication more or less secure than password authentication, and why? Or, if you would rather, under what circumstances are each method more vulnerable?

Answer 1

I think it really depends on the attack vector you are looking at.
Assuming mutually exclusive for the sake of this discussion (either key-based auth with password login disabled, or password login and key based auth disabled). A password-less ssh key is likely more vulnerable to an endpoint exploit -- as if an attacker has your sshkey without a password, he has access. Password-over-ssh is likely more vulnerable to a server-side exploit -- opens the password-guess vector, and if you aren't paying attention to the 'fingerprint doesn't match' message and someone hijacks your dns, you could attempt to login to a compromised system, thus giving away your password. Key-based auth would fail if they did not have your public key on the compromised server (you would still see the fingerprint difference message though).
You can do things to increase the security of the above vectors, from using a passphrase on your ssh-key and using ssh-agent (so you only have to auth once per session and it simply 'unlocks' your key, and doesn't leave it laying around open)..to using something like knockd or fail2ban on the server side.
Personally, I use a passphrase protected ssh key (along with ssh-agent), and disable interactive (password) authentication on my boxes anytime they are exposed to a public network (along with adding root to the denyusers ssh list).
Last edited by cactus (2009-07-08 01:52:11)