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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
=================
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Package / Vulnerable / Unaffected
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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@xxxxxxxxxx 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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