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             AUSCERT External Security Bulletin Redistribution

           K00724442:BIG-IP DNS and GTM DNSSEC security exposure
                             23 December 2019


        AusCERT Security Bulletin Summary

Product:           F5 BIG-IP DNS
Publisher:         F5 Networks
Operating System:  Network Appliance
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Patch/Upgrade

Original Bulletin: 

Revision History:  December 23 2019: Patch released
                   July      5 2019: Initial Release

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K00724442:BIG-IP DNS and GTM DNSSEC security exposure

Security Advisory

Original Publication Date: 04 Jul, 2019

Latest   Publication Date: 21 Dec, 2019

Security Advisory Description

The BIG-IP DNSSEC implementation returns an incorrect NSEC3 record for a DNS
query for a resource record type, which does not exist at given name.

The incorrect record indicates that only one of TXT/HINFO/RP resource record
types exists at given name, even if A or AAAA types actually do exist and are
returned if a client queried for them. This behavior allows attackers to mount
two kinds of attacks:

  o Single-packet replay to a denial-of-service (DoS) arbitrary DNS resolver
  o Single normal-query DoS attack clients behind a DNS resolver

This issue occurs when the following conditions are met:

  o You use DNSSEC on an affected version of BIG-IP DNS (formerly known as
    BIG-IP GTM).
  o The targeted DNS resolver implements negative caching.


A remote attacker may be able to perform a DNS DoS attack.


As a result of this issue, you may encounter the following symptom:

  o You cannot resolve additional records for the attack target, most notably A
    and AAAA records.

Security Advisory Status

F5 Product Development has assigned ID 744937 to this issue. F5 has confirmed
that this issue exists in the products listed in the Applies to (see versions)
box, located in the upper-right corner of this article. For information about
releases, point releases, or hotfixes that resolve this issue, refer to the
following table.

|Type of fix       |Fixes introduced  |Related articles                       |
|                  |in                |                                       |
|                  |15.1.0            |                                       |
|                  |15.0.1            |K2200: Most recent versions of F5      |
|Release           |14.1.2            |software                               |
|                  |13.1.3            |                                       |
|                  |12.1.5            |                                       |
|Point release/    |None              |None                                   |
|hotfix            |                  |                                       |

Important: To implement the product feature in the listed fixed releases, you
must configure the dnssec.nsec3apextypesbitmap and
dnssec.nsec3underapextypesbitmap system database variables. For more
information refer to: K53347459 Configuring the DNS apex and under-apex NSEC3
types bitmap for NODATA.

Security Advisory Recommended Actions


This workaround relies on the fact that BIG-IP DNSSEC signed zone transfers
(AXFR) currently report the correct NSEC3 types bitmaps. By first transferring
the fully signed zone into DNS Express, you can avoid incorrect NSEC3 records,
specifically eliminating the issue of types bitmaps that are missing records
types that are available in the original unsigned zone.

DNSSEC Workaround

Impact of workaround: This workaround may increase disk usage, result in
increased network traffic between the BIG-IP system and the authoritative zone
server, and result in increased memory usage on the BIG-IP system.

This workaround requires changes to both the BIG-IP DNS or GTM system and the
BIND configuration. You must modify and adapt this generic example for your
particular configuration.

In these examples, use the following:

  o BIG-IP example self IP:
  o BIND server IP:
  o Domain: example.com

BIG-IP DNS or GTM system

  o Create three TCP/UDP pairs of DNS Listeners:
       One pair for DNSSEC (example IP:
       One pair for Client facing DNSX (example IP:
       One pair for internal DNSX (example IP:
  o Create two DNS profiles:
       DNSSEC profile with the following settings:
           DNS Express disabled
           DNSSEC enabled
           Zone Transfer enabled
       DNSX profile with the following settings:
           DNS Express enabled
           DNSSEC enabled
           Zone Transfer enabled
  o Create a BIG-IP LTM pool with a member that points at your BIND server's IP
       Assign the pool to the DNSSEC Listeners under DNS > Delivery >
        Listeners > Load Balancing Tab.
  o Create three nameservers under DNS > Delivery > Nameservers:
       self_ns  > BIG-IP system's self IP
       dnssec_ns  > DNSSEC Listener's IP
       client_ns   > The self IP of the dig client if you want to test AXFR
        from a client machine; this client digs at the IP of the DNSX listener.
  o Create a zone example.com in DNS Express under DNS > Zones > Zones.
  o Create DNSSEC Keys under DNS > Delivery > Keys > DNSSEC Keys.
  o Create zone example.com in DNSSEC under DNS > Zones > DNSSEC Zones and
    assign the keys.
  o In the DNS Express zone example.com:
       server  > dnssec_ns nameserver
       Allow NOTIFY From  > BIND server's self IP (
       Zone Transfer Clients  > self_ns, client_ns

BIND server configuration

In named.conf, add also-notify and optionally notify-explicit to every zone
that needs to be DNSSEC-signed by the BIG-IP system, following the named.conf 

zone "example.com" IN {
        type master;
        file "db.external.example.com.";
        allow-transfer {
        also-notify {
        notify explicit;


F5 would like to acknowledge the following researchers for bringing this issue
to our attention and for following the highest standards of responsible

  o Security impact analysis: Jan Vcelak, NS1 company
  o Problem discovery: Petr Spacek, CZ.NIC company

    Note: The link takes you to a resource outside of AskF5, and it is possible
    that the document may be removed without our knowledge.

Supplemental Information

o K51812227: Understanding Security Advisory versioning
  o K41942608: Overview of AskF5 Security Advisory articles
  o K4602: Overview of the F5 security vulnerability response policy
  o K4918: Overview of the F5 critical issue hotfix policy
  o K9502: BIG-IP hotfix and point release matrix
  o K167: Downloading software and firmware from F5
  o K9970: Subscribing to email notifications regarding F5 products
  o K9957: Creating a custom RSS feed to view new and updated documents

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