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Date: Wed, 8 Aug 2018 21:45:43 -0800
From: Michael McNally <>
Cc: "" <>
Subject: CVE-2018-5740: A flaw in the "deny-answer-aliases" feature can cause
 an INSIST assertion failure in named

Earlier today ISC disclosed CVE-2018-5740, involving a potential
denial-of-service vector in an uncommonly-used feature of BIND.

Our official security advisory document is posted in the ISC Knowledge Base
at this URL: but for your convenience
the text is provided below.

Patched versions of BIND are available from

Michael McNally
ISC Security Officer


A rarely-used feature in BIND has a flaw which can cause named to
exit with an INSIST assertion failure.

CVE:                  CVE-2018-5740
Document Version:     2.0
Posting date:         08 August 2018
Program Impacted:     BIND
Versions affected:    9.7.0->9.8.8, 9.9.0->9.9.13, 9.10.0->9.10.8,
                      9.11.0->9.11.4, 9.12.0->9.12.2, 9.13.0->9.13.2
                      and also versions of BIND 9 Supported Preview Edition
Severity:             High (but only for servers on which the
                      feature is explicitly enabled)
Exploitable:          Remotely


   "deny-answer-aliases" is a little-used feature intended to help
   recursive server operators protect end users against DNS rebinding
   attacks, a potential method of circumventing the security model
   used by client browsers.  However, a defect in this feature makes
   it easy, when the feature is in use, to experience an INSIST
   assertion failure in name.c.


   Accidental or deliberate triggering of this defect will cause
   an INSIST assertion failure in named, causing the named process
   to stop execution and resulting in denial of service to clients.
   Only servers which have explicitly enabled the "deny-answer-aliases"
   feature are at risk and disabling the feature prevents exploitation.

CVSS Score:          7.5
CVSS Vector:         CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

For more information on the Common Vulnerability Scoring System and
to obtain your specific environmental score please visit:


   This vulnerability can be avoided by disabling the "deny-answer-aliases"
   feature if it is in use.

Active exploits:

   No known active exploits.


   Most operators will not need to make any changes unless they are
   using the "deny-answer-aliases" feature (which is described in
   the BIND 9 Adminstrator Reference Manual section 6.2.)
   "deny-answer-aliases" is off by default; only configurations
   which explicitly enable it can be affected by this defect.

   If you are using "deny-answer-aliases", upgrade to the patched
   release most closely related to your current version of BIND.

   -  9.9.13-P1
   -  9.10.8-P1
   -  9.11.4-P1
   -  9.12.2-P1

   BIND Supported Preview Edition is a special feature preview
   branch of BIND provided to eligible ISC support customers.

   -  9.11.3-S3


   ISC would like to thank Tony Finch of the University of Cambridge
   for reporting this issue.

Document Revision History:

   1.0 Advance Notification 31 July, 2018
   2.0 Public Disclosure 08 August, 2018

Related Documents:

   See our BIND9 Security Vulnerability Matrix at for a complete listing of
   Security Vulnerabilities and versions affected.

If you'd like more information on ISC Subscription Support and
Advance Security Notifications, please visit

Do you still have questions?  Questions regarding this advisory
should go to  To report a new issue,
please encrypt your message using's PGP
key which can be found here:

If you are unable to use encrypted email, you may also report new
issues at:


   ISC patches only currently supported versions. When possible we
   indicate EOL versions affected.  (For current information on
   which versions are actively supported, please see;.

ISC Security Vulnerability Disclosure Policy:

   Details of our current security advisory policy and practice can
   be found here:

This Knowledge Base article is
the complete and official security advisory document.

Legal Disclaimer:

   Internet Systems Consortium (ISC) is providing this notice on
   an "AS IS" basis. No warranty or guarantee of any kind is expressed
   in this notice and none should be implied. ISC expressly excludes
   and disclaims any warranties regarding this notice or materials
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   Uncontrolled copies may lack important information, be out of
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(c) 2001-2018 Internet Systems Consortium

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