CVE-2018-5740: A flaw in the "deny-answer-aliases" feature can cause an assertion failure in named
  • 31 Oct 2018
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CVE-2018-5740: A flaw in the "deny-answer-aliases" feature can cause an assertion failure in named

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Article summary

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

CVE: CVE-2018-5740

Document Version: 2.0

Posting date: 08 August 2018

Program ImpactedBIND

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

Severity: High (but only for servers on which the "deny-answer-aliases" feature is explicitly enabled)

Exploitable: Remotely

Description: "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 assertion failure in name.c. 

Impact: Accidental or deliberate triggering of this defect will cause a REQUIRE 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:

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

Active exploits: No known active exploits.

Solution: 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

Acknowledgements: 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
2.1 Initial text incorrectly described the failed assertion as an INSIST (rather than REQUIRE) assertion 09 August, 2018

Related Documents: See our BIND 9 Security Vulnerability Matrix for a complete listing of Security Vulnerabilities and versions affected.

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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's PGP key which can be found here:  If you are unable to use encrypted email, you may also report new issues at:

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 

ISC Security Vulnerability Disclosure Policy: Details of our current security advisory policy and practice can be found in the ISC Software Defect and Security Vulnerability Disclosure Policy.

This Knowledgebase article is the complete and official security advisory document.

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