Vulnerability Note VU#568372

NTP mode 7 (MODE_PRIVATE) is used by the ntpdc query and control utility. In contrast, ntpq uses NTP mode 6 (MODE_CONTROL), while routine NTP time transfers use modes 1 through 5. Upon receipt of an incorrect mode 7 request or a mode 7 error response from an address that is not listed in a "restrict ... noquery" or "restrict ... ignore" segment, ntpd will reply with a mode 7 error response and log a message.

If an attacker spoofs the source address of ntpd host A in a mode 7 response packet sent to ntpd host B, both A and B will continuously send each other error responses, for as long as those packets get through.

If an attacker spoofs an address of ntpd host A in a mode 7 response packet sent to ntpd host A, then host A will respond to itself endlessly, consuming CPU and logging excessively.

A remote, unauthenticated attacker may be able to cause a denial-of-service condition on a vulnerable NTP server.

Apply an update
This issue is addressed in NTP 4.2.4p8. Please check with your vendor for an update, or you may download NTP 4.2.4p8 from ntp.org.

Configure NTP to limit source addresses

By using "restrict ... noquery" or "restrict ... ignore" entries in the ntp.conf file, ntpd can be configured to limit the source addresses to which it will respond.

Filter NTP mode 7 packets that specify source and destination port 123

In most cases, ntpdc mode 7 requests will have either a source or destination port of 123, but not both.

Use anti-spoofing IP address filters

RFC 2827 (BCP 38) describes network ingress filtering, which can prevent UDP traffic claiming to be from a local address from entering your network from an outside source. Some ISPs may employ unicast reverse path filtering (uRPF) to limit the spoofed traffic that can enter your network.