CVE-2015-4620: Specially Constructed Zone Data Can Cause a Resolver to Crash when Validating
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CVE-2015-4620: Specially Constructed Zone Data Can Cause a Resolver to Crash when Validating

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Article Summary

An attacker who can cause a validating resolver to query a zone containing specifically constructed contents can cause that resolver to fail an assertion and terminate due to a defect in validation code.

CVE: CVE-2015-4620

Document version: 2.0

Posting date: 7 July 2015

Program impacted: BIND

Versions affected: BIND 9.7.1 -> 9.7.7, 9.8.0 -> 9.8.8, 9.9.0 -> 9.9.7, 9.10.0 -> 9.10.2-P1.  

Severity: Critical

Exploitable: Remotely


A very uncommon combination of zone data has been found that triggers a bug in BIND, with the result that named will exit with a "REQUIRE" failure in name.c when validating the data returned in answer to a recursive query. 

This means that a recursive resolver that is performing DNSSEC validation can be deliberately stopped by an attacker who can cause the resolver to perform a query against a maliciously-constructed zone.


A recursive resolver that is performing DNSSEC validation can be deliberately terminated by any attacker who can cause a query to be performed against a maliciously constructed zone. This will result in a denial of service to clients who rely on that resolver.

DNSSEC validation is only performed by a recursive resolver if it has "dnssec-validation auto;" in its configuration or if it has a root trust anchor defined and has "dnssec-validation yes;" set (either by accepting the default or via an explicitly set value of "yes").  By default, ISC BIND recursive servers will not validate. (However, ISC defaults may have been changed by your distributor.)

CVSS Score: 7.8

CVSS Vector: (AV:N/AC:L/Au:N/C:N/I:N/A:C)

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


Disabling DNSSEC validation prevents exploitation of this defect but is not generally recommended. The recommended solution is to upgrade to a patched version.

Active exploits: There are no known active exploits at this time.

Solution: Upgrade to the patched release most closely related to your current version of BIND. These are all available at

  • BIND 9 version 9.9.7-P1
  • BIND 9 version 9.10.2-P2

Acknowledgements: ISC would like to thank Breno Silveira Soares of Serviço Federal de Processamento de Dados (SERPRO) for discovering and reporting this defect.

Document Revision History:

1.0 Phase One: Advance Notification, 23 June 2015
1.1 Phase Two: Notification to BIND Packagers, 29 June 2015
1.2 Revised public release date (to 7 July 2015) and re-notified Phase One and Phase Two recipients, 30 June 2015
2.0 Phase Three: Public Disclosure, 7 July 2015

Related Documents:

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See our BIND 9 Security Vulnerability Matrix for a complete listing of security vulnerabilities and versions affected.

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

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