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

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Article Summary


Document Version: 2.0

Posting date: 12 June 2018

Program ImpactedBIND

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 nameserver.

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 "allow-query" (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, including:

  • 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 default.

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.

Acknowledgements: 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 BIND 9 Security Vulnerability Matrix for a complete listing of Security Vulnerabilities and versions affected.

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