This indicates an attack attempt against a denial of service vulnerability in MIT's Kerberos.
The vulnerability is caused by a NULL pointer dereference error when the vulnerable software handles a specially crafted TGS request. It allows a remote attacker to cause a denial of service.
MIT Kerberos 5 1.7
Denial of Service: Remote attackers can crash vulnerable systems.
Upgrade to krb5-1.7.1 or apply patch, available from the web site.