This indicates an attack attempt to exploit an Authentication Bypass vulnerability in Cisco Secure ACS.
The vulnerability is caused by an improper validation of user supplied data when the vulnerable application handles a maliciously crafted request. An attacker can exploit this to bypass authentication process and gain unauthorized access to the affected application.
Cisco Secure ACS version 5.1
Cisco Secure ACS version 5.2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor