[ GLSA 200808-12 ] Postfix: Local privilege escalation vulnerability

[Raphael Marichez <falco () gentoo org>]

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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     < 2.5.3-r1                      *>= 2.4.7-r1
                                                           >= 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 ">=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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