Giving "1 LIST .. *" IMAP command allows the user to see all files and
directories under the mbox root's parent directory, so potentially you
could see other users' mailbox names. Nothing can be done with them
though, so it's not possible to read or modify them.

There are also some other less than wanted behaviors that can be done
with "." and ".." directories, such as "DELETE .." command can give
errors. Also with maildir if you store indexes in different directory
than mails this command can be used to delete the index files. Anyway I
don't see any of these as security issues.

Hopefully most people have their mail directory under user's home
directory, so this would only list the home directory's contents.

But if you do have users' mails stored in eg. under /var/mail/%u/
directory, giving that command would list everything under /var/mail,
assuming that the process has permissions to do that (that's why you
should be using different UIDs for users if possible).

This affects 1.0 beta releases, but not 0.99.x. Also the older snapshots
called "1.0 stable" are affected. 1.0 beta8 and later are not affected.

Patch for the fix is in
http://dovecot.org/list/dovecot-cvs/2006-May/005563.html

Thanks for Bill Boebel for noticing "DELETE .." doing weird things with
maildir.