Is there any security information about CVE-2013-5211 ?

Solution Verified - Updated -


  • Red Hat Enterprise Linux 4
  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6
  • ntp


  • How to deal with CVE-2013-5211?

  • Why does monlist in NTP allow remote attackers to cause a denial of service (DOS 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.


  • This issue does not affect the default configuration of ntp packages shipped with Red Hat Enterprise Linux, which does not allow remote ntpd control queries.

  • User changing ntpd access control configuration should consider reviewing additional information provided via Bug 1047854 to avoid exposing their systems to this traffic amplification issue.

  • Disable the monitor functionality in ntpd if you had previously changed the default configuration to enable it.

There are two workarounds

  • Firstly, use noquery in your default restrictions to block all status queries. Add the noquery directive to the restrict default line in the system’s /etc/ntp.conf , as shown below:
# vi /etc/ntp/ntp.conf
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
  • Secondly, to disable monlist functionality on a public-facing NTP server that cannot be updated to 4.2.7. Use disable monitor to disable the ntpdc -c monlist command while still allowing other status queries.
# vi /etc/ntp.conf
disable monitor



  2. CVE-2013-5211

  3. NVD definition

  4. MITRE dictionary

Root Cause

  • The NTP service supports a monitoring service that allows administrators to query the server for traffic counts of connected clients. This information is provided via the monlist command.

  • The basic attack technique consists of an attacker sending a get monlist request to a vulnerable NTP server, with the source address spoofed to be the victim’s address.

Diagnostic Steps

  • Entering the following commands can help users verify if the REQ_MON_GETLIST and REQ_MON_GETLIST_1 responses of NTP are currently enabled:
 ntpq -c rv <NTP_SERVER>
 ntpdc -c sysinfo <NTP_SERVER>
 ntpdc -n -c monlist <NTP_SERVER>

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