I'm trying to provide access to a local folder for someone over the internet. After googling around a bit, I've come to the conclusion that SFTP is the safest thing to expose through the firewall to the chaotic and evil world of the Internet. I'm planning to use the openssh-server to this end. Even though I trust that openssh will stop a random attacker, I'm not so sure about the security of my computer once someone is connected through ssh.
In particular, even if I don't give that person's user account any privileges whatsoever, he might just be able to "su" to, say, "nobody". And since I was never worried about such things before, I might have given some moderate privileges to nobody at some point (not sudo rights surely!).
I would of course value your comments about giving privileges to nobody in the first place, but that's not the point, really. My aim is to give SFTP access to someone in such a sandboxed state that I shouldn't need to worry about such things (at least not more so than I should have done before).
Is this really possible? Am I speaking nonsense or worried in vain?