A race condition was found in the way 'mount.cifs' and 'umount.cifs' utilities performed mount / umount of a particular CIFS share to / from specified mount point (/etc/mtab~ lockfile was created before updating the /etc/mtab file and deleted once the operation completed), when these utilies were setuid root enabled. A local attacker could use this flaw to conduct denial of service attacks (failure of subsequent CIFS share umount / mount requests) by sending termination signal to 'mount.cifs' / 'umount.cifs' processes in the moment of existence of a stale (/etc/mtab~) lockfile.

References:
[1] https://bugzilla.samba.org/show_bug.cgi?id=7179
    (upstream bug report)
[2] http://git.samba.org/?p=cifs-utils.git;a=commitdiff;h=810f7e4e0f2dbcbee0294d9b371071cb08268200
    (upstream patch)
[3] http://www.openwall.com/lists/oss-security/2011/09/27/1
    (CVE request)
[4] http://www.openwall.com/lists/oss-security/2011/09/30/5
    (CVE assignment)

Note: This flaw to be exploitable as described above requires the 'mount.cifs'
      / 'umount.cifs' utilities to be setuid root enabled. These utilities
      are not setuid root enabled on Red Hat Enterprise Linux and Fedora
      distributions, and thus these distributions as such are not vulnerable
      to this flaw.