Dan Rosenberg reported that when MBX locking is enabled in exim, local users could exploit a race condition to change permissions of other non-root users' files. This could lead to a denial of service, to create new files owned by other users in unauthorized locations, or to possibly escalate privileges.
Further information is available from the upstream bug report  and this has
been fixed upstream in exim 4.72 .
While exim is built to support the MBX format, it is not the default for local mail delivery (Unix mailbox support is the default). This will only affect users that use the "mbx_format" option in the appendfile transport.
The Red Hat Security Response Team has rated this issue as having low security impact. While support for the MBX mailbox format is compiled into Exim, it is not used by default. MBX mailboxes are only useful when used with UW-IMAP or the Pine mail client, neither of which are provided with Red Hat Enterprise Linux. If the MBX format is used, this issue can be worked around by specifying "use_fcntl_lock" rather than "use_mbx_lock". 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.