This indicates an attack attempt to exploit a Denial of Service vulnerability in MIT Kerberos.
The vulnerability is due to a NULL pointer dereference error when vulnerable module handles an invalid token inside SPNEGO related packets. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system by sending an empty token during negotiation process.
MIT Kerberos prior to 5 1.12.2
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.