Knowledge Base ISC Main Website Ask a Question/Contact ISC
CVE-2018-5738: Some versions of BIND can improperly permit recursive query service to unauthorized clients
Author: Michael McNally Reference Number: AA-01616 Views: 39227 Created: 2018-06-06 23:07 Last Updated: 2018-06-12 23:08 0 Rating/ Voters

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

Description:

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.

Impact:

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: https://www.first.org/cvss/calculator/3.0#CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Workarounds:

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.

Solution:

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 security-officer@isc.org 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 BIND9 Security Vulnerability Matrix at https://kb.isc.org/article/AA-00913 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 http://www.isc.org/support/.

Do you still have questions?  Questions regarding this advisory should go to security-officer@isc.orgTo report a new issue, please encrypt your message using security-officer@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-00861/164/ISC-Software-Defect-and-Security-Vulnerability-Disclosure-Policy.html

This Knowledge Base article https://kb.isc.org/article/AA-01616 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 time.  A stand-alone copy or paraphrase of the text of this document that omits the document URL is an uncontrolled copy. Uncontrolled copies may lack important information, be out of date, or contain factual errors.


© 2001-2018 Internet Systems Consortium

For assistance with problems and questions for which you have not been able to find an answer in our Knowledge Base, we recommend searching our community mailing list archives and/or posting your question there (you will need to register there first for your posts to be accepted). The bind-users and the dhcp-users lists particularly have a long-standing and active membership.

ISC relies on the financial support of the community to fund the development of its open source software products. If you would like to support future product evolution and maintenance as well having peace of mind knowing that our team of experts are poised to provide you with individual technical assistance whenever you call upon them, then please consider our Professional Subscription Support services - details can be found on our main website.

Feedback
  • There is no feedback for this article
Quick Jump Menu