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FreeBSD Security Advisory: FreeBSD-SA-00:14.imap-uw

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Date: Mon, 24 Apr 2000 15:46:34 -0700
From: FreeBSD Security Officer
Subject: FreeBSD Security Advisory: FreeBSD-SA-00:14.imap-uw

FreeBSD-SA-00:14                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          imap-uw contains security vulnerabilities for "closed"
                mail servers

Category:       ports
Module:         imap-uw
Announced:      2000-04-24
Credits:        Michal Zalewski lcamtuf@DIONE.IDS.PL
                Michal Szymanski siva9@CLICO.PL via BugTraq
Affects:        Ports collection.
Corrected:      See below.
Vendor status:  Aware of the problem, no satisfactory solution provided.
FreeBSD only:   NO
I. Background

imap-uw is a popular IMAP4/POP2/POP3 mail server from the University of Washington.

II. Problem Description

There are numerous buffer overflows available to an imap user after they have successfully logged into their mail account (i.e. authenticated themselves by giving the correct password, etc). Once the user logs in, imapd has dropped root privileges and is running as the user ID of the mail account which has been logged into, so the buffer overflow can only allow code to be executed as that user.

Thus, the vulnerability is only relevant on a "closed" mail server, i.e. one which does not normally allow interactive logins by mail users. For a system which allows users to log in or execute code on the system, there is minimal vulnerability.

Note that once a user has successfully exploited the vulnerability to gain access to their user account they may be able to mount further attacks against the local (or a remote) machine to upgrade their privileges.

The imap-uw port is not installed by default, nor is it "part of FreeBSD" as such: it is part of the FreeBSD ports collection, which contains over 3200 third-party applications in a ready-to-install format. The ports collection shipped with FreeBSD 4.0 contains this problem since it was discovered after the release.

FreeBSD makes no claim about the security of these third-party applications, although an effort is underway to provide a security audit of the most security-critical ports.

III. Impact

A user with a mail account on the imap server can execute arbitrary code as themselves on that machine. This is only likely to be a security issue on "closed" mail servers which do not allow interactive shell logins.

Only imapd is known to be vulnerable to this time - the other daemons installed by the imap-uw port (ipop2d/ipop3d) are not known to suffer from the same vulnerability.

If you have not chosen to install the imap-uw port/package, then your system is not vulnerable to this problem.

IV. Workaround

1) Deinstall the imap-uw port/package, if you you have installed it.
2) If you do not specifically require imap functionality (i.e. pop2/pop3 is sufficient) then disable the imap daemon in /etc/inetd.conf and restart inetd (e.g. with the command 'killall -HUP

V. Solution

Unfortunately the vulnerabilities in imapd are quite extensive and no patch is currently available to address them. There is also no "drop-in" replacement for imap-uw currently available in ports, although the mail/cyrus port is another imap server which may be a suitable replacement. Cyrus has different configuration and operational requirements than imap-uw however, which may make it unsuitable for many users.

Until a security audit of the imap-uw source can be completed and the vulnerabilities patched, it is recommended that operators of "closed" imapd servers take steps to minimize the impact of users being able to run code on the server (i.e., by tightening the local security on the machine to minimize the damage an intruding user can cause).

This advisory will be updated once the known vulnerabilities in imap-uw have been addressed.