It was reported [1],[2] that rpc.gssd in nfs-utils is vulnerable to DNS spoofing due to it depending on PTR resolution for GSSAPI authentication.  Because of this, if a user where able to poison DNS to a victim's computer, they would be able to trick rpc.gssd into talking to another server (perhaps with less security) than the intended server (with stricter security).  If the victim has write access to the second (less secure) server, and the attacker has read access (when they normally might not on the secure server), the victim could write files to that server, which the attacker could obtain (when normally they would not be able to).  To the victim this is transparent because the victim's computer asks the KDC for a ticket to the second server due to reverse DNS resolution; in this case Krb5 authentication does not fail because the victim is talking to the "correct" server.

A patch that prevents this issue has been posted [3].

To workaround this issue, set the IP/host pair in /etc/hosts so that it cannot be spoofed.

A good explanation is also available here [4].

[1] http://marc.info/?l=linux-nfs&m=136491998607561&w=2
[2] http://marc.info/?l=linux-nfs&m=136500502805121&w=2
[3] http://marc.info/?l=linux-nfs&m=136493115612397&w=2
[4] http://ssimo.org/blog/id_015.html