Operational Notification: "update-policy local" was named misleadingly and could permit non-local DDNS updates

[Michael McNally]

Posting date:        14 March 2018
Program Impacted:    BIND
Versions affected:   9.0.x -> 9.8.x, 9.9.0->9.9.11-P1,
                     9.10.0->9.10.6-P1, 9.11.0->9.11.2-P1,
                     9.12.0->9.12.0-P1
Description:

   "update-policy local;", which is a permission cluster provided
   as a shortcut for operators who use Dynamic DNS (DDNS), was
   misleadingly named in that its original implementation did not
   actually enforce a requirement that the updates it allows originate
   locally.

   A full description of "update-policy local;" is included in
   Section 6.2 of the BIND Administrator Reference Manual, but to
   briefly summarize:

      When "update-policy local;" is set for a zone in named.conf,
      named will create and use an automatically generated session
      key (named "local-ddns" by default and stored in local storage
      on the server) and will permit updates to the zone to any
      client successfully authenticating using that key.

      Since the key is generated and stored locally, in usual cases
      this should equate to only allowing local updates unless an
      operator deliberately copies the local-ddns key elsewhere.

   However, in June 2017 disclosed CVE-2017-3143, a flaw in TSIG
   authentication which enabled an attacker who was able to send
   and receive messages to an authoritative DNS server and who had
   knowledge of a valid TSIG key name for the zone and service being
   targeted to manipulate BIND into accepting an unauthorized dynamic
   update.  In our disclosure for CVE-2017-3143 we warned of its
   potential interaction with the behavior of update-policy local.
   By policy, however, ISC prefers security releases to contain
   only the minimal fix required to prevent the exploitable condition.
   Therefore, security patch releases for CVE-2017-3143 fixed only
   the TSIG authentication flaw without changing the behavior of
   the "update-policy local;" feature.

   Beginning with the March 2018 maintenance releases of BIND
   (9.9.12, 9.10.7, 9.11.3, and 9.12.1) the behavior of "update-policy
   local" is now changed so that updates are permitted under the
   policy only when they are received from locally configured
   addresses AND use the local session key.

Impact:

   We think it is unlikely that many operators are deliberately
   relying on the non-local option of the previous behavior (and
   we recommend against it) but if any are, please see the "Workarounds"
   section of this advisory for advice on how to replicate the
   previous behavior.  For all other operators (those who were not
   relying on the non-local side-effect of the previous behavior)
   the new behavior should represent an improvement in DDNS security
   if you use the local update policy.

Workarounds:

   The change in "update-policy local;" behavior which debuts in
   the March 2018 maintenance releases should improve security by
   properly restricting updates to only those that are received
   from locally configured addresses AND are authenticated using
   the local session key.  However, in the event that an operator
   was deliberately relying on the non-local option of the previous
   behavior, behavior equivalent to the previous behavior of
   "update-policy local;" can be produced by using this syntax:
   "update-policy { grant local-ddns zonesub any; };"

Solution:

   Software versions which enforce the corrected, more restrictive
   behavior are now available from our downloads page,
   http://www.isc.org/downloads/all.

   - BIND 9 version 9.9.12
   - BIND 9 version 9.10.7
   - BIND 9 version 9.11.3
   - BIND 9 version 9.12.1

Do you still have questions?  Questions regarding this advisory
should go to security-officer at isc.org.  To report a new issue,
please encrypt your message using security-officer at isc.org's PGP
key which can be found here:
   https://www.isc.org/downloads/software-support-policy/openpgp-key/.
If you are unable to use encrypted email, you may also report new
issues at: https://www.isc.org/community/report-bug/.

Note:

   ISC patches only currently supported versions. When possible we
   indicate EOL versions affected.  (For current information on
   which versions are actively supported, please see
   http://www.isc.org/downloads/).

ISC Security Vulnerability Disclosure Policy:

   Details of our current security advisory policy and practice can
   be found here: https://kb.isc.org/article/AA-00861

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