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ALERT: NTP-Based Distributed Denial of Service Attacks

Prevent your institution from being an unwitting partner in these attacks

This is the CIO version of the Alert.
There is also a TECH version.

Print-friendly copy (pdf)

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March 10, 2014

To:	CIOs
	(A TECH version of this Alert is available [6])

REN-ISAC ALERT: NTP-Based Distributed Denial of Service Attacks -
Prevent your institution from being an unwitting partner in these

The REN-ISAC [1] wants to raise awareness and drive change
concerning common network time protocol (NTP) and network
configurations that fall short of best practices and which, if left
uncorrected, open the door for your institution to be exploited as
an unwitting partner in delivering crippling distributed denial of
service (DDoS) attacks against third parties.

The higher education and research community needs to do its part
to ensure that we are not helping to facilitate these attacks. The
REN-ISAC recommends the following:

=== ACTIONS ===

1. Distribute a copy of the TECH version [6] of this message to
your network administrators, information security staff, system
administrators, and other relevant personnel.

2. Identify hosts on your network with ntpd installed and running.
Disable "monlist" capabilities on those NTP servers. Or, for systems
which cannot be updated or configured to eliminate the monlist risk,
apply network filters to prevent external requests to these insecure

3. Ensure your institutional networks are unable to originate
Internet traffic with spoofed source addresses.


Although DDoS attacks exploiting the NTP and network configuration
weaknesses have been around for a long time, the frequency and
impact of these attacks have grown over the past year. The traffic
sourced by any single system may be small enough to go unnoticed at
an institution, however, the aggregate traffic from many sources, as
experienced at the target, can be crippling. Recent attacks [2,3,4]
average around 7.3 gigabits per second of traffic aimed at the
victim organization - more than the full Internet bandwidth of most
small and medium-sized colleges and universities.

Given the history and success of recent attacks, we expect these
attacks will rise in frequency and magnitude in the months ahead.

There are two issues:

1. NTP is a protocol used to synchronize the clocks of networked
devices. A deprecated command, "monlist", permits a requesting
computer to receive information regarding the last 600 connections
made to the NTP server. A small input request can generate a large

When a malicious actor spoofs the source IP address of a victim
targeted for attack, and repeatedly sends monlist requests to
thousands of insecure NTP servers located all over the Internet, an
avalanche of traffic is directed at the victim, overwhelming the
victim's network capacity.

Monlist has been removed from newer versions of NTP and monlist can
(and should) be disabled in older versions.

The NTP issue is complicated by the fact that not only is NTP
running on upgradeable enterprise servers, NTP is also employed on
infrastructure devices and embedded systems in your enterprise, some
of which can be updated only with great difficulty and expense.
Concerning these "difficult" systems, network firewalls may be one
approach that can be used to mitigate the problem of these
vulnerable-but-uncorrectable devices.

2. The second issue, the network configuration issue, involves the
problem of traffic with forged apparent source addresses. Systems on
your network should not be able to transmit packets that appear to
be from a source IP address that doesn't belong to you -- your
systems should only originate traffic with accurate source IP

In addition to the problem of educational institutions being
leveraged as the sources of attacks against third parties, there
have been incidents of institutions being targeted as the victim of
DDoS attacks. Best practices for mitigating DDoS attacks against
your institution apply: know in advance how to partner with your ISP
to mitigate the effects of an attack; concerning Internet2, refer to
the "DDoS Attacks Policy" [5]. DDoS mitigation services are also
available from commercial organizations.

A TECH version of this Alert with technical depth has been shared to
campus IT security officers, and network and system administrators.
Print-friendly copies of this Alert and the companion TECH version
(with clobber-free long URLs) are available at the REN-ISAC web

We'd appreciate your input on additional means to protect from this
threat, and general feedback concerning this Alert. If you have any
questions, please don't hesitate to e-mail us at

Special thanks go to the members of the REN-ISAC Technical Advisory
Group (TAG) [7].

On behalf of the REN-ISAC team,

Doug Pearson
Technical Director, REN-ISAC
24x7 Watch Desk +1(317)274-7228



[2] Hackers Spend Christmas Break Launching Large Scale NTP- Reflection Attacks

[3] DoS attacks that took down big game sites abused Web's time-sync protocol

[4] NTP reflection attack

[5] Internet2 DDoS Attacks Policy

[6] TECH version of this alert

[7] REN-ISAC Technical Advisory Group