The busybox NTP implementation doesn't check the NTP mode of packets received on the server port and responds to any packet with the right size. This includes responses from another NTP server. An attacker can send a packet with a spoofed source address in order to create an infinite loop of responses between two busybox NTP servers. Adding more packets to the loop increases the traffic between the servers until one of them has a fully loaded CPU and/or network.
It seems this bug was actually inherited from openntpd, on which the busybox implementation was based on. In openntpd it was fixed in:
Busybox upstream patch:
Name: Miroslav Lichvar (Red Hat)
Created busybox tracking bugs for this issue:
Affects: fedora-all [bug 1363711]