ntpq is a monitoring and control program for ntpd. decodearr() is an internal function of ntpq that is used to -- wait for it -- decode an array in a response string when formatted data is being displayed. This is a problem in affected versions of ntpq if a maliciously-altered ntpd returns an array result that will trip this bug, or if a bad actor is able to read an ntpq request on its way to a remote ntpd server and forge and send a response before the remote ntpd sends its response. It's potentially possible that the malicious data could become injectable/executable code.

References:

http://support.ntp.org/bin/view/Main/NtpBug3414