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Gentoo: GLSA 200808-12 High: Postfix Local Escalation Threat

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. [More...]. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Gentoo Linux Security Advisory GLSA 200808-12 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - https://security.gentoo.org/ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Severity: High Title: Postfix: Local privilege escalation vulnerability Date: August 14, 2008 Bugs: #232642 ID: 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. 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 canlead 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 Postfixdelivery 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 https://www.cve.org/CVERecord?id=CVE-2008-2936 [ 2 ] CVE-2008-2937 https://www.cve.org/CVERecord?id=CVE-2008-2937 [ 3 ] Official Advisory Availability =========== This GLSA and any updates to it are available for viewing at the Gentoo Security Website: https://security.gentoo.org/glsa/200808-12 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 This email address is being protected from spambots. You need JavaScript enabled to view it. or alternatively, you may file a bug at https://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. https://creativecommons.org/licenses/by-sa/2.5/ . The Gentoo Security Advisory GLSA 200808-12 reveals vulnerabilities in Postfix, highlighting potential risks for local privilege escalation along with recommended resolutions.. Postfix Local Escalation, Gentoo Security Advisory, Mail Risk Alert. . LinuxSecurity.com Team

Aug 15, 2008 Gentoo