Cisco Security Advisory – Cisco IOS Software with support for Network Time Protocol (NTP) version (v4) contains a vulnerability processing specific NTP packets that will result in a reload of the device. This results in a remote denial of service (DoS) condition on the affected device.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/81597/cisco-sa-20090923-ntp.txt
Source: https://packetstormsecurity.com/files/81597/Cisco-Security-Advisory-20090923-ntp.html