An exploitable configuration modification vulnerability exists in the control mode (mode 6) functionality of ntpd. A specially crafted control mode packet can set ntpd traps, providing information disclosure and DDoS amplification, and unset ntpd traps, preventing legitimate monitoring. A remote, unauthenticated, network attacker can trigger this vulnerability.
CVSSv2: 6.4 - (AV:N/AC:L/Au:N/C:P/I:P/A:N)
CVSSv3: 6.5 - CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
ntpd provides a
trap functionality that sends asynchronous
notifications to a number of
trap receivers whenever an event of
interest occurs. Example events of interest include: association
mobilization and demobilization, authentication failures,
reachability changes, etc.
Since at least ntp-4.0.94 (July 21, 1999), ntpd has allowed traps to
be configured via control (mode 6) and private (mode 7) NTP modes.
Though private mode requires messages modifying trap settings to be
be authenticated, control mode allows unauthenticated packets to
modify trap settings using the
This vulnerability can be used to achieve several goals:
Time Shifting: If an attacker controls a host that is allowed to
receive traps (i.e. not restricted by
restrict noquery or
restrict notrap), the attacker can instruct a victim ntpd
instance to send traps to the attacker's host. Whenever a
reportable event occurs for some peer, the victim ntpd will send a
trap to the attacker leaking all the peer variables associated
with that peer. The information leaked includes the peer's org
and rec variables allowing the attacker to bypass TEST2 and
impersonate said peer in a manner similar to CVE-2015-8139 and
The attacker can force the victim ntpd to leak the information for any peer at any time by triggering a reportable event for said peer. There are multiple methods to trigger a reportable event for a peer, among them spoofing an invalid crypto-NAK or incorrectly authenticated packet from the peer.
NOTE: With ntp-4.2.8p8 and earlier the 0rigin attack (CVE-2015-8138)  already allows impersonation of reachable peers. In those ntpd versions, this vulnerability provides another method for impersonating unreachable peers.
DDoS Amplification: An attacker can use an ntpd instance as a DDoS amplifier to DDoS hosts that are allowed to receive traps from the ntpd instance using the following technique. The amplification factor is 12-13x.
The attacker forges a
SETTRAP packet from the
victim to the
amplifier, causing the
amplifier to set a trap for the
victim. The attacker then repeatedly triggers reportable
events causing trap messages to be sent to the victim. E.g. the
attacker rapidly forges invalid crypto-NAKs and/or bad_auth
packets from the
ntpd attempts to limit the number of consecutive traps sent for events of a single type. To maximize effect, the attacker can alternate between events of different types.
ntpd will periodically time out old traps when a new one is set.
Therefore, for a long-term attack, the attacker may need to
periodically refresh the trap on the
Evading Monitoring: In an environment where dynamically configured
traps are used to modify an ntpd instance, an unauthenticated
attacker can remove traps set by legitimate monitoring systems by
spoofing the source address of the
trap receiver in an
Authentication should be required in order to modify trap configuration.
Several mitigations can lessen the impact of this vulnerability.
Unauthorized hosts can be prevented from receiving traps using
restrict default notrap restriction. This setting is the
default on many modern Linux systems.
This mitigation has no effect on the "Evading Monitoring" impact described above because the alleged sender of the packet is an authorized trap receiver.
Block NTP control mode trap configuration commands using a firewall or IPS. It does not appear that support for configuring control mode traps was ever implemented in ntpq, the reference NTP control mode client. As such, on most networks blocking control mode trap configuration commands should have no effect on legitimate traffic. Specifically, firewalls should block packets with the following characteristics:
Traps specified in ntp.conf cannot be modified using this vulnerability.
2016-09-20 - Vendor Disclosure
2016-11-21 - Public Release
Discovered by Matthew Van Gundy of Cisco ASIG.