6.2 - GLSA-200808-12

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.

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