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Category:   Application (Generic)  >   BIND Vendors:   ISC (Internet Software Consortium)
BIND Recursion Access Control Flaw Lets Remote Users Obtain Potentially Sensitive Information About Previous Nameserver Queries
SecurityTracker Alert ID:  1041115
SecurityTracker URL:
CVE Reference:   CVE-2018-5738   (Links to External Site)
Date:  Jun 13 2018
Impact:   Disclosure of system information, Disclosure of user information
Vendor Confirmed:  Yes  
Version(s): 9.9.12, 9.10.7, 9.11.3, 9.12.0 - 9.12.1-P2
Description:   A vulnerability was reported in BIND. A remote user can obtain potentially sensitive information on the target system.

A remote user can bypass intended recursion access controls and make a recursive query to a BIND nameserver in certain cases.

When configured with "recursion yes;" and match list values are not provided for "allow-query-cache" or "allow-query", the "allow-recursion" setting may permit all hosts to perform recursion.

As a result, a remote user can examine the results of queries answered from the cache to determine which queries a server has previously responded to.

Releases after change #4777 (from October 2017) are affected.

Andrew Skalski reported this vulnerability.

Impact:   A remote user can obtain potentially sensitive information about queries a server has previously responded to.
Solution:   The vendor has described workarounds in their advisory.

The vendor plans to issue a fix as part of an upcoming maintenance release (future versions 9.9.13, 9.10.8, 9.11.4, and 9.12.2).

The vendor advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Access control error
Underlying OS:  Linux (Any), UNIX (Any)

Message History:   This archive entry has one or more follow-up message(s) listed below.
Jun 13 2018 (Ubuntu Issues Fix) BIND Recursion Access Control Flaw Lets Remote Users Obtain Potentially Sensitive Information About Previous Nameserver Queries
Ubuntu has issued a fix for Ubuntu Linux 18.04 LTS.

 Source Message Contents

Subject:  CVE-2018-5738: Some versions of BIND can improperly permit recursive query service to unauthorized clients

Please note that for this vulnerability ISC is not issuing replacement
software releases.  Safe, effective, and side-effect-free configuration
workarounds are available for those whose configurations are affected.
The vulnerability will be fixed in upcoming maintenance releases so that
it cannot recur, but until maintenance releases containing the fix are
released we recommend use of any of the options listed in the workarounds

If using the configuration workarounds is impractical for you, you may
request a patch diff containing an advance version of the fix by sending
e-mail to


CVE:                 CVE-2018-5738
Document Version:    2.0
Posting date:        12 June 2018
Program Impacted:    BIND
Versions affected:   9.9.12, 9.10.7, 9.11.3, 9.12.0->9.12.1-P2,
                     the development release 9.13.0, and also
                     releases 9.9.12-S1, 9.10.7-S1, 9.11.3-S1,
                     and 9.11.3-S2 from BIND 9 Supported Preview Edition.
Severity:            Medium
Exploitable:         Remotely


   Change #4777 (introduced in October 2017) introduced an unforeseen
   issue in releases which were issued after that date, affecting
   which clients are permitted to make recursive queries to a BIND

   The intended (and documented) behavior is that if an operator
   has not specified a value for the "allow-recursion" setting, it
   SHOULD default to one of the following:

   +  none, if "recursion no;" is set in named.conf, or

   +  a value inherited from the "allow-query-cache" or "allow-query"
      settings IF "recursion yes;" (the default for that setting) AND
      match lists are explicitly set for "allow-query-cache" or
      (see the BIND9 Administrative Reference Manual section 6.2 for more
      details), or

   +  the intended default of "allow-recursion {localhost; localnets;};"
      if "recursion yes;" is in effect and no values are explicitly set
      for "allow-query-cache" or "allow-query".

   However, because of the regression introduced by change #4777,
   it is possible when "recursion yes;" is in effect and no match
   list values are provided for "allow-query-cache" or "allow-query"
   for the setting of "allow-recursion" to inherit a setting of all
   hosts from the "allow-query" setting default, improperly permitting
   recursion to all clients.


   There are several potential problems which can be caused by
   improperly permitting recursive service to unauthorized clients,

   +  Additional queries from unauthorized clients may increase the
      load on a server, possibly degrading service to authorized clients.

   +  Allowing queries from unauthorized clients can potentially allow
      a server to be co-opted for use in DNS reflection attacks.

   +  An attacker may be able to deduce which queries a server has
      previously serviced by examining the results of queries answered
      from the cache, potentially leaking private information about what
      queries have been performed.

CVSS Score:          5.3
CVSS Vector:         CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

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


   A number of configuration workarounds are available which
   completely avoid the problem.

   If an operator has not chosen to specify some other permission,
   explicitly specifying "allow-query {localnets; localhost;};" in
   named.conf will provide behavior equivalent to the intended

   If the default setting is not appropriate (because the operator
   wants a different behavior) then depending on which clients are
   intended to be able to receive service for recursive queries,
   explicitly setting a match list value for any of:

   +  allow-recursion
   +  allow-query
   +  allow-query-cache

   will prevent the "allow-recursion" control from improperly
   inheriting a setting from the allow-query default.  If a value
   is set for any of those values the behavior of allow-recursion
   will be set directly or inherited from one of the other values
   as described in the BIND Adminstrator Reference Manual section 6.2

   Servers which are not intended to perform recursion at all may
   also effectively prevent this condition by setting "recursion no;"
   in named.conf

Active exploits:

   We are not aware of any exploits deliberately targeting this
   specific defect but it is not uncommon for scanners to search
   for open resolvers for use in reflection attacks and other
   mischief.  We have at least one report from an operator who
   discovered that unauthorized clients were successfully making
   queries to his server and it is reasonable to assume that other
   servers with similar configurations may be currently affected
   although their operators are unaware.


   Future maintenance releases of BIND will correct the regression
   which introduced this issue but ISC does not believe that
   replacement security releases of BIND are required, given that
   several easy, safe, and completely effective configuration
   workarounds are available for any operators with affected
   configurations.  However, an advance version of the patch diff
   which will be applied to future versions is available upon request
   to and a correction for the behavior
   in question will debut in the release candidates for BIND 9.9.13,
   BIND 9.10.8, BIND 9.11.4, and BIND 9.12.2.


   ISC would like to thank Andrew Skalski for reporting this issue.

Document Revision History:

   1.0 Advance Notification 06 June 2018
   2.0 Public disclosure 12 June 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
   referred to in this notice, including, without limitation, any
   implied warranty of merchantability, fitness for a particular
   purpose, absence of hidden defects, or of non-infringement. Your
   use or reliance on this notice or materials referred to in this
   notice is at your own risk. ISC may change this notice at any
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