I have a machine which is used mostly as a mail server:
$ uname -a Linux myhost.com 2.6.32-279.19.1.el6.x86_64 #1 SMP Wed Dec 19 07:05:20 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
and I would like users who do not actually log into the machine (but use it to authenticate and get their mail) to be able to change their own passwords. If I put /usr/bin/passwd in /etc/shells (so the passwd command is a valid shell), and change the shell entry for the users as in:
then if they ssh to the host, they'll get something like this:
$ ssh myhost.com
email@example.com's password: <type their current password>
Last login: Wed Sep 25 16:07:35 2013 from some-ip
Changing password for user someuser.
Changing password for someuser.
(current) UNIX password: <type their current password again>
New password: <type their new password>
Retype new password: <type their new password again>
-passwd: all authentication tokens updated successfully.
Connection to myhost.com closed.
that works great... but is it safe? Is their some way to exploit that and break into a real shell?
P.S. In my environment, it is reasonable to assume that the users have ssh already--but if there is an alternative for password changing that is simply "better", I'd like to hear it :)