Is NTP secure?

hi is there a risk to open the firewall to the internet for port 123 (TCP and UDP) for synchronisation (NetworkTimeProtokol)? Greetings Chrigu Schweingruber -- Homepage: http://www.kolk.ch/chrigu http://www.catatec.ch PGP key: http://www.kolk.ch/chrigu/Pgp.txt phone: +41 31 330 26 33 priv. +41 31 330 26 30 büro +41 31 330 26 32 FAX icq#: 27628141