CVE-2018-5736: Multiple transfers of a zone in quick succession can cause an assertion failure in rbtdb.c
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CVE-2018-5736: Multiple transfers of a zone in quick succession can cause an assertion failure in rbtdb.c

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Article Summary


Document Version: 2.0

Posting date: 18 May 2018

Program ImpactedBIND

Versions affected: 9.12.0 and 9.12.1

Severity: Medium

Exploitable: Remotely, if an attacker can trigger a zone transfer


An error in zone database reference counting can lead to an assertion failure if a server which is running an affected version of BIND attempts several transfers of a slave zone in quick succession.

This defect could be deliberately exercised by an attacker who is permitted to cause a vulnerable server to initiate zone transfers (for example: by sending valid NOTIFY messages), causing the named process to exit after failing the assertion test.


Authoritative servers that serve slave zones are vulnerable to potential denial of service if all of the following are true:

  • they are running an affected version of BIND (BIND 9.12.0 or 9.12.1)
  • at least one of the zones for which they are providing service is of type "slave"
  • they permit NOTIFY messages from any source.

CVSS Score:   5.3

CVSS Vector:   CVSS:3.0/AV:N/AC:H/PR:L/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:


For servers which must receive notifies to keep slave zone contents current, no complete workarounds are known although restricting BIND to only accept NOTIFY messages from authorized sources can greatly mitigate the risk of attack.

Active exploits:

No known active exploits.


The reference counting error which can be exploited in this vulnerability is present in only two public release versions of BIND, 9.12.0 and 9.12.1.

If you are running an affected version, then upgrade to BIND 9.12.1-P1.


ISC would like to thank SWITCH for informing us of this vulnerability.

Document Revision History:

1.0 Advance Notification, 09 May 2018
2.0 Public Disclosure, 18 May 2018

Related Documents:

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

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:


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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