GLSA 200808-12 Postfix: Local privilege escalation vulnerability
Posted on: 08/15/2008 01:50 AM

A new security update has been released for Gentoo Linux - Postfix: Local privilege escalation vulnerability. Here the announcement:

Gentoo Linux Security Advisory GLSA 200808-12
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http://security.gentoo.org/
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Severity: High
Title: Postfix: Local privilege escalation vulnerability
Date: August 14, 2008
Bugs: #232642
ID: 200808-12

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Synopsis
========

Postfix incorrectly checks the ownership of a mailbox, allowing, in
certain circumstances, to append data to arbitrary files on a local
system with root privileges.

Background
==========

Postfix is Wietse Venema's mailer that attempts to be fast, easy to
administer, and secure, as an alternative to the widely-used Sendmail
program.

Affected packages
=================

-------------------------------------------------------------------
Package / Vulnerable / Unaffected
-------------------------------------------------------------------
1 mail-mta/postfix lt; 2.5.3-r1 *gt;= 2.4.7-r1
gt;= 2.5.3-r1

Description
===========

Sebastian Krahmer of SuSE has found that Postfix allows to deliver mail
to root-owned symlinks in an insecure manner under certain conditions.
Normally, Postfix does not deliver mail to symlinks, except to
root-owned symlinks, for compatibility with the systems using symlinks
in /dev like Solaris. Furthermore, some systems like Linux allow to
hardlink a symlink, while the POSIX.1-2001 standard requires that the
symlink is followed. Depending on the write permissions and the
delivery agent being used, this can lead to an arbitrary local file
overwriting vulnerability (CVE-2008-2936). Furthermore, the Postfix
delivery agent does not properly verify the ownership of a mailbox
before delivering mail (CVE-2008-2937).

Impact
======

The combination of these features allows a local attacker to hardlink a
root-owned symlink such that the newly created symlink would be
root-owned and would point to a regular file (or another symlink) that
would be written by the Postfix built-in local(8) or virtual(8)
delivery agents, regardless the ownership of the final destination
regular file. Depending on the write permissions of the spool mail
directory, the delivery style, and the existence of a root mailbox,
this could allow a local attacker to append a mail to an arbitrary file
like /etc/passwd in order to gain root privileges.

The default configuration of Gentoo Linux does not permit any kind of
user privilege escalation.

The second vulnerability (CVE-2008-2937) allows a local attacker,
already having write permissions to the mail spool directory which is
not the case on Gentoo by default, to create a previously nonexistent
mailbox before Postfix creates it, allowing to read the mail of another
user on the system.

Workaround
==========

The following conditions should be met in order to be vulnerable to
local privilege escalation.

* The mail delivery style is mailbox, with the Postfix built-in
local(8) or virtual(8) delivery agents.

* The mail spool directory (/var/spool/mail) is user-writeable.

* The user can create hardlinks pointing to root-owned symlinks
located in other directories.

Consequently, each one of the following workarounds is efficient.

* Verify that your /var/spool/mail directory is not writeable by a
user. Normally on Gentoo, only the mail group has write access, and
no end-user should be granted the mail group ownership.

* Prevent the local users from being able to create hardlinks
pointing outside of the /var/spool/mail directory, e.g. with a
dedicated partition.

* Use a non-builtin Postfix delivery agent, like procmail or
maildrop.

* Use the maildir delivery style of Postfix ("home_mailbox=Maildir/"
for example).

Concerning the second vulnerability, check the write permissions of
/var/spool/mail, or check that every Unix account already has a
mailbox, by using Wietse Venema's Perl script available in the official
advisory.

Resolution
==========

All Postfix users should upgrade to the latest version:

# emerge --sync
# emerge --ask --oneshot --verbose "gt;=mail-mta/postfix-2.5.3-r1"

References
==========

[ 1 ] CVE-2008-2936
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-2936
[ 2 ] CVE-2008-2937
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-2937
[ 3 ] Official Advisory
http://article.gmane.org/gmane.mail.postfix.announce/110

Availability
============

This GLSA and any updates to it are available for viewing at
the Gentoo Security Website:

http://security.gentoo.org/glsa/glsa-200808-12.xml

Concerns?
=========

Security is a primary focus of Gentoo Linux and ensuring the
confidentiality and security of our users machines is of utmost
importance to us. Any security concerns should be addressed to
security@gentoo.org or alternatively, you may file a bug at
http://bugs.gentoo.org.

License
=======

Copyright 2008 Gentoo Foundation, Inc; referenced text
belongs to its owner(s).

The contents of this document are licensed under the
Creative Commons - Attribution / Share Alike license.

http://creativecommons.org/licenses/by-sa/2.5



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