The symmetric-key feature in the receive function in ntp_proto.c in ntpd in NTP 4.x before 4.2.8p2 requires a correct MAC only if the MAC field has a nonzero length, which makes it easier for man-in-the-middle attackers to spoof packets by omitting the MAC.

The receive function in ntp_proto.c in ntpd in NTP before 4.2.8 continues to execute after detecting a certain authentication error, which might allow remote attackers to trigger an unintended association change via crafted packets.

Multiple stack-based buffer overflows in ntpd in NTP before 4.2.8 allow remote attackers to execute arbitrary code via a crafted packet, related to (1) the crypto_recv function when the Autokey Authentication feature is used, (2) the ctl_putdata function, and (3) the configure function.

util/ntp-keygen.c in ntp-keygen in NTP before 4.2.7p230 uses a weak RNG seed, which makes it easier for remote attackers to defeat cryptographic protection mechanisms via a brute-force attack.

The config_auth function in ntpd in NTP before 4.2.7p11, when an auth key is not configured, improperly generates a key, which makes it easier for remote attackers to defeat cryptographic protection mechanisms via a brute-force attack.

The monlist feature in ntp_request.c in ntpd in NTP before 4.2.7p26 allows remote attackers to cause a denial of service (traffic amplification) via forged (1) REQ_MON_GETLIST or (2) REQ_MON_GETLIST_1 requests, as exploited in the wild in December 2013.