Talos Vulnerability Report


Network Time Protocol Broadcast Mode Replay Prevention Denial of Service Vulnerability

November 21, 2016
CVE Number



An exploitable denial of service vulnerability exists in the broadcast mode replay prevention functionality of ntpd. To prevent replay of broadcast mode packets, ntpd rejects broadcast mode packets with non-monotonically increasing transmit timestamps. Remote unauthenticated attackers can send specially crafted broadcast mode NTP packets to cause ntpd to reject all broadcast mode packets from legitimate NTP broadcast servers.

Tested Versions

NTP 4.2.8p6

Product URLs


CVSS Scores

CVSSv2: 5.0 - (AV:N/AC:L/Au:N/C:N/I:N/A:P)
CVSSv3: 5.3 - CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L


In response to the NTP Deja Vu vulnerability (CVE-2015-7973), ntp-4.2.8p6 introduced several new integrity checks on incoming broadcast mode packets. Upon receipt of a broadcast mode packet, before authentication is enforced, ntpd will reject the packet if any of the following conditions hold:

  1. The packet poll value is out of bounds for the broadcast association, i.e.

     pkt->ppoll < peer->minpoll || pkt->ppoll > peer->maxpoll
  2. The packet was received before a full poll interval has elapsed since the last broadcast packet was received from the packet’s sender. i.e. A server cannot ingress packets more frequently than peer->minpoll.

  3. The packet transmit timestamp is less than the last seen broadcast packet transmit timestamp from the packet’s sender. i.e. Broadcast packet transmit timestamps must be monotonically increasing.

The following logic is used to ensure that packet transmit timestamps are monotonically increasing:

/* ntp-4.2.8p6 ntpd/ntp_proto.c */
1305  if (MODE_BROADCAST == hismode) {
1351      tdiff = p_xmt;
1352      L_SUB(&tdiff, &peer->bxmt);
1353      if (tdiff.l_i < 0) {
1354          msyslog(LOG_INFO, "receive: broadcast packet from %s contains non-monotonic timestamp: %#010x.%08x -> %#010x.%08x",
1355              stoa(&rbufp->recv_srcadr),
1356              peer->bxmt.l_ui, peer->bxmt.l_uf,
1357              p_xmt.l_ui, p_xmt.l_uf
1358              );
1359          ++bail;
1360      }
1362      peer->bxmt = p_xmt;
1364      if (bail) {
1365          peer->timelastrec = current_time;
1366          sys_declined++;
1367          return;
1368      }
1369  }

If the packet transmit timestamp is less than the transmit timestamp on the last received broadcast packet from this association (p_xmt - peer->bxmt < 0), the packet will be discarded.

Unfortunately, line 1362 updates the saved transmit timestamp for alleged sender of the packet (peer->bxmt) before the packet is discarded. The update takes place even if the packet is unauthenticated and fails the previous integrity checks.

This leads to a trivial denial of service attack. The attacker:

  1. Discovers the IP address of the victim’s broadcast server. e.g. Send the victim a client mode NTP packet and discover the broadcast server from the refid field of the victim’s reply.

  2. Every poll period, send the victim a spoofed broadcast mode packet from the broadcast server with a transmit timestamp in the future. This will move peer->bxmt forward so that any legitimate packet will be rejected by the non-monotonic timestamp check.

    • The attacker does not need to be on the same subnet as the victim. The attacker can address the spoofed broadcast NTP packet directly to the victim’s IP address.

    • The attacker can choose any reasonably small estimate for the poll period. Because the peer->bxmt update happens even when a packet fails the poll period checks, there is no penalty for sending packets too frequently.

To prevent this vulnerability, peer->bxmt should only be updated when a packet authenticates correctly. This is the approach taken in the patch below.


There is no workaround for this issue. Because the vulnerable logic is executed before authentication is enforced, authentication and the restrict notrust ntpd.conf directive have no effect. An attacker can bypass notrust restrictions by sending incorrectly authenticated packets.

In order to succeed in an attack, the attacker must send at least one spoofed packet per poll period. Therefore observing more than one NTP broadcast packet from the same sender address per poll period indicates a possible attack.

The following patch can be used to fix this vulnerability:

From 097fd4dae9ac4927d7cfa8011fd42f704bd02c45 Mon Sep 17 00:00:00 2001
From: Matthew Van Gundy <mvangund@cisco.com>
Date: Tue, 26 Jan 2016 15:00:28 -0500
Subject: [PATCH] Fix unauthenticated broadcast mode denial of service (peer->bxmt)

 include/ntp_fp.h |  1 +
 ntpd/ntp_proto.c | 22 ++++++++++++++++------
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/include/ntp_fp.h b/include/ntp_fp.h
index 7806932..ad7a01d 100644
--- a/include/ntp_fp.h
+++ b/include/ntp_fp.h
@@ -242,6 +242,7 @@ typedef u_int32 u_fp;
 #define  L_ISGTU(a, b) M_ISGTU((a)->l_ui, (a)->l_uf, (b)->l_ui, (b)->l_uf)
 #define  L_ISHIS(a, b) M_ISHIS((a)->l_ui, (a)->l_uf, (b)->l_ui, (b)->l_uf)
 #define  L_ISGEQ(a, b) M_ISGEQ((a)->l_ui, (a)->l_uf, (b)->l_ui, (b)->l_uf)
+#define  L_ISGEQU(a, b)  L_ISHIS(a, b)
 #define  L_ISEQU(a, b) M_ISEQU((a)->l_ui, (a)->l_uf, (b)->l_ui, (b)->l_uf)

diff --git a/ntpd/ntp_proto.c b/ntpd/ntp_proto.c
index ad45409..ac469ce 100644
--- a/ntpd/ntp_proto.c
+++ b/ntpd/ntp_proto.c
@@ -1305,7 +1305,6 @@ receive(
    if (MODE_BROADCAST == hismode) {
      u_char poll;
      int bail = 0;
-     l_fp tdiff;

      DPRINTF(2, ("receive: PROCPKT/BROADCAST: prev pkt %ld seconds ago, ppoll: %d, %d secs\n",
            (current_time - peer->timelastrec),
@@ -1348,9 +1347,8 @@ receive(

-     tdiff = p_xmt;
-     L_SUB(&tdiff, &peer->bxmt);
-     if (tdiff.l_i < 0) {
+     /* Use L_ISGEQU() to ensure unsigned comparison */
+     if (!L_ISGEQU(&p_xmt, &peer->bxmt)) {
        msyslog(LOG_INFO, "receive: broadcast packet from %s contains non-monotonic timestamp: %#010x.%08x -> %#010x.%08x",
          peer->bxmt.l_ui, peer->bxmt.l_uf,
@@ -1359,8 +1357,6 @@ receive(

-     peer->bxmt = p_xmt;
      if (bail) {
        peer->timelastrec = current_time;
@@ -1563,6 +1559,14 @@ receive(
  peer->xmt = p_xmt;

+  * Now that we know the packet is correctly authenticated,
+  * update peer->bxmt if needed
+  */
+ if (MODE_BROADCAST == hismode) {
+   peer->bxmt = p_xmt;
+ }
+ /*
   * Set the peer ppoll to the maximum of the packet ppoll and the
   * peer minpoll. If a kiss-o'-death, set the peer minpoll to
   * this maximum and advance the headway to give the sender some
@@ -2400,6 +2404,7 @@ peer_clear(
  u_char  u;
+ l_fp bxmt = peer->bxmt;

 #ifdef AUTOKEY
@@ -2436,6 +2441,11 @@ peer_clear(
  peer->flash = peer_unfit(peer);
  peer->jitter = LOGTOD(sys_precision);

+ /* Don't throw away our broadcast replay protection */
+ if (peer->hmode == MODE_BCLIENT) {
+   peer->bxmt = bxmt;
+ }
   * If interleave mode, initialize the alternate origin switch.


2016-09-12 - Vendor Disclosure
2016-11-21 - Public Release


Discovered by Matthew Van Gundy of Cisco ASIG.