Date: Sat, 1 Jun 2002 16:14:15 +0400
From: 3APA3A <[email protected]>
To: [email protected]Subject: SECURITY.NNOV: Courier CPU exhaustion + bonus on imap-uw
Cc: VULN-DEV <[email protected]>
Original version
http://www.security.nnov.ru/advisories/courier.asp
Title: Courier CPU exhaustion
Author: ZARAZA <[email protected]>
Date: May, 31 2002
Affected: courier-0.38.1
Vendor: Double Precision, Inc.
Risk: Low to average
Remote: Yes
Exploitable: Yes
Vendor notified: May, 20 2002
Product URL: http://www.courier-mta.org
SECURITY.NNOV URL: http://www.security.nnov.ru
Advanced info: http://www.security.nnov.ru/search/news.asp?binid=2055
Introduction:
Courier is widely used suite of e-mail services written with security in
mind.
Problem:
A loop with unchecked iteration counter controlled by user input may
cause courier to freeze for over the minute with 100% CPU usage on
single command or message.
Details:
rfc822_parsedt.c:
unsigned day=0, mon=0, year;
...
unsigned y;
...
if (year < 1970) return (0);
...
for (y=1970; y<year; y++) ...
year may be any unsigned integer.
Vendor:
Sam Varshavchik <[email protected]> was contacted on May, 20.
Problem was patched in CVS version on the same day.
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Bonus on imap-uw:
Imap-uw allows user to access any file he could access locally. It's not
a bug it's insecurity by design (it was not created with security in
mind ;-). According FAQ from vendor's web site (it's not mentioned in a
FAQ inside program distribution):
-=-=-=-=-=-=-
5.1 I see that the IMAP server allows access to arbitary files on the
system, including /etc/passwd! How do I disable this?
You should not worry about this if your IMAP users are allowed shell
access. The IMAP server does not permit any access that the user can
not have via the shell. If, and only if, you deny your IMAP users shell
access, you may want to consider one of three choices. Note that these
choices reduce IMAP functionality, and may have undesirable side
effects. Each of these choices involves an edit to file
src/osdep/unix/env_unix.c
The first (and recommended) choice is to set restrictBox as described
in file CONFIG. This will disable access to the filesystem root, to
other users' home directory, and to superior directory.
The second (and strongly NOT recommended) choice is to set closedBox as
described in file CONFIG. This puts each IMAP session into a so-called
"chroot jail", and thus setting this option is extremely dangerous; it
can make your system much less secure and open to root compromise
attacks. So do not use this option unless you are absolutely certain
that you understand all the issues of a "chroot jail."
The third choice is to rewrite routine mailboxfile() to implement
whatever mapping from mailbox name to filesystem name (and
restrictions) that you wish. This is the most general choice. As a
guide, you can see at the start of routine mailboxfile() what the
restrictBox choice does.
-=-=-=-=-=-
It should be noted that restrictBox/closedBox is not described in
neither CONFIG nor any other document from program distribution at all
(as for imap-2001a)... And even if you smart enough to check the FAQ on
the web site after you red the FAQ in source distribution restrictBox
can be bypassed in case of any Windows builds (for example
http://sourceforge.net/projects/uw-imap-cygwin/) because '\\' symbol is
never checked. Hope nobody uses UW under NT or a version from OS ports
distribution in production environment because as far as I can see port
maintainers do not change the value of closedBox :).
I'm not sure if there are utilities to access file system via imap-uw,
a created a small set of tools you can download imaptools.tgz from
http://www.security.nnov.ru/search/news.asp?binid=2063
it includes:
imapget.c - to retrieve file via imap-uw, usage example:
imapget imap.host.name /etc/passwd > passwd
it should work for both text and binary files.
imapls.c - to get a file listing, usage example:
imapls imaphostname /tmp/\* > ls-tmp
imaprm.c, imapmkdir.c - hope you catch the idea.
it's also possible to create file with any name in mailbox format.
--
http://www.security.nnov.ru
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