I have a small embedded device that runs Linux and an SSH server. This box is set up with one user (root) and no password whatsoever.
Currently, there is one Windows XP-based laptop that is used to run diagnostics on the embedded device. When attempting to SSH into the device from the laptop, the user is prompted for a password, even though there isn't one associated with root. From any other computer, SSH correctly logs in sans password.
On the laptop, this behavior is exhibited when using putty, cygwin, and a virtual machine. Again, on any other computer, this exact same VM will connect via SSH successfully without a password.
It's clear that I don't have a firm grasp on Linux/SSH's ideas of password requirements.
As I said, though, from any other device, ssh root@DEVICEIP will log in without a password once the device's key is stored. It's only this one laptop (and any method of using SSH on this laptop) that is misbehaving.
For reference, here's passwd and shadow:
root@ULCB-Linux:/etc cat passwd root:x:0:0:root:/home:/bin/sh ftp:x:11:101:ftp user:/home:/bin/false www:x:12:102:www user:/home:/bin/false sshd:x:13:100:SSH Server:/var/run/sshd:/bin/false messagebus:x:103:104:messagebus:/dev/null:/bin/false service:x:500:100:Service User:/home:/bin/sh root@ULCB-Linux:/etc cat shadow root:$1$yoD9VQnv$V9iaWdgnoSnp2ASzsgxgI.:15826:0:99999:7::: sshd:!:0:0:99999:7::: ftp::0:0:99999:7::: service::0:0:99999:7:::