[Dovecot] Security hole #3: zlib plugin allows opening any gziped mboxes

[Timo Sirainen]

zlib plugin allows opening gzipped mboxes as read-only mailboxes.
However when using it, the mailbox name checks are bypassed so it's
possible to open for example "../otheruser/somefile.gz". Only valid
gzipped mbox files can be opened, and only if their name ends with
".gz".

You can fix this by upgrading to v1.0.rc29 (available soon) or with this
patch: http://dovecot.org/list/dovecot-cvs/2007-March/008488.html

I don't think this matters much though. zlib plugin is rarely used, and
those who do use it are probably using Dovecot with systems users
(per-user UIDs), so the imap process wouldn't have access to other
users' mbox files anyway.

I found this problem when I was cleaning up the code in CVS HEAD.
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