Dan Rosenberg reported that when exim is used with a world-writable mail directory, with the sticky-bit set, local users could create hard-links to other non-root users' files in the mailbox storage directory, causing files to be overwritten upon mail delivery. This could be used to create a denial of service condition or potentially escalate privileges to those of targeted users.
Further information is available from the upstream bug report  and this has been fixed upstream in exim 4.72 .
The /var/spool/mail directory on Red Hat Enterprise Linux and Fedora is mode 0755 and owned root:mail. As a result, this is not exploitable by default.
The Red Hat Security Response Team has rated this issue as having low security impact. By default, /var/spool/mail/ is not provided with permissions to make an attack scenario possible, and there is no reason for permissions to be relaxed in such a way as to make it possible. We therefore have no plans to fix this flaw in Red Hat Enterprise Linux 4 or 5.
exim-4.72-1.fc12 has been submitted as an update for Fedora 12.
exim-4.72-1.fc13 has been submitted as an update for Fedora 13.
exim-4.72-1.fc12 has been pushed to the Fedora 12 stable repository. If problems still persist, please make note of it in this bug report.
exim-4.72-1.fc13 has been pushed to the Fedora 13 stable repository. If problems still persist, please make note of it in this bug report.