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Category:   Application (Generic)  >   ntp Vendors:
(FreeBSD Issues Fix) ntpd Flaw Lets Remote Users Modify Time on the Target ntp Service in Certain Cases
SecurityTracker Alert ID:  1034674
SecurityTracker URL:
CVE Reference:   CVE-2015-5300   (Links to External Site)
Date:  Jan 14 2016
Impact:   Modification of user information
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 4.0 to 4.2.8p5, 4.3.0 to 4.3.78
Description:   A vulnerability was reported in ntp. A remote user that can conduct a man-in-the-middle attack can modify time on the target ntp service.

When ntpd is started with the '-g' option, a remote user that can conduct a man-in-the-middle attack can exploit a threshold limitation flaw to cause the target ntp client to make multiple steps larger than the panic threshold. This can be exploited to set the time to an arbitrary value.

The original advisory is available at:

Aanchal Malhotra, Isaac E. Cohen, Erik Brakke, and Sharon Goldberg of Boston University reported this vulnerability.

Impact:   A remote user that can conduct a man-in-the-middle attack can modify time on the target ntp service.
Solution:   FreeBSD has issued a fix.

The FreeBSD advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Access control error
Underlying OS:  UNIX (FreeBSD)
Underlying OS Comments:  9.3, 10.1, 10.2

Message History:   This archive entry is a follow-up to the message listed below.
Jan 14 2016 ntpd Flaw Lets Remote Users Modify Time on the Target ntp Service in Certain Cases

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-16:02.ntp

Hash: SHA512

FreeBSD-SA-16:02.ntp                                        Security Advisory
                                                          The FreeBSD Project

Topic:          ntp panic threshold bypass vulnerability

Category:       contrib
Module:         ntp
Announced:      2016-01-14
Credits:        Network Time Foundation
Affects:        All supported versions of FreeBSD.
Corrected:      2016-01-11 01:09:50 UTC (stable/10, 10.2-STABLE)
                2016-01-14 09:10:46 UTC (releng/10.2, 10.2-RELEASE-p9)
                2016-01-14 09:11:16 UTC (releng/10.1, 10.1-RELEASE-p26)
                2016-01-11 01:48:16 UTC (stable/9, 9.3-STABLE)
                2016-01-14 09:11:26 UTC (releng/9.3, 9.3-RELEASE-p33)
CVE Name:       CVE-2015-5300

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:>.

I.   Background

The ntpd(8) daemon is an implementation of the Network Time Protocol (NTP)
used to synchronize the time of a computer system to a reference time

II.  Problem Description

The ntpd(8) daemon has a safety feature to prevent excessive stepping of
the clock called the "panic threshold".  If ever ntpd(8) determines the
system clock is incorrect by more than this threshold, the daemon exits.
There is an implementation error within the ntpd(8) implementation of this
feature, which allows the system time be adjusted in certain circumstances.

III. Impact

When ntpd(8) is started with the '-g' option specified, the system time will
be corrected regardless of if the time offset exceeds the panic threshold (by
default, 1000 seconds).  The FreeBSD rc(8) subsystem allows specifying the
'-g' option by either including '-g' in the ntpd_flags list or by enabling
ntpd_sync_on_start in the system rc.conf(5) file.

If at the moment ntpd(8) is restarted, an attacker can immediately respond to
enough requests from enough sources trusted by the target, which is difficult
and not common, there is a window of opportunity where the attacker can cause
ntpd(8) to set the time to an arbitrary value.

IV.  Workaround

No workaround is available, but systems not running ntpd(8), or running
ntpd(8) but do not use ntpd_sync_on_start="YES" or specify the '-g' option in
ntpd_flags are not affected.  Neither of these are set by default.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

The ntpd service has to be restarted after the update.  A reboot is
recommended but not required.

2) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

The ntpd service has to be restarted after the update.  A reboot is
recommended but not required.

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

[FreeBSD 10.1 and 10.2]
# fetch
# fetch
# gpg --verify ntp-10.patch.asc

[FreeBSD 9.3]
# fetch
# fetch
# gpg --verify ntp-9.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:>.

Restart the applicable daemons, or reboot the system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/9/                                                         r293652
releng/9.3/                                                       r293896
stable/10/                                                        r293650
releng/10.1/                                                      r293894
releng/10.2/                                                      r293893
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://

Or visit the following URL, replacing NNNNNN with the revision number:


VII. References



The latest revision of this advisory is available at

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