CVE-2021-25214: A broken inbound incremental zone update (IXFR) can cause named to terminate unexpectedly
  • 28 Apr 2021
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CVE-2021-25214: A broken inbound incremental zone update (IXFR) can cause named to terminate unexpectedly

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Article summary

CVE: CVE-2021-25214

Document version: 2.0

Posting date: 28 April 2021

Program impacted: BIND

Versions affected: BIND 9.8.5 -> 9.8.8, 9.9.3 -> 9.11.29, 9.12.0 -> 9.16.13, and versions BIND 9.9.3-S1 -> 9.11.29-S1 and 9.16.8-S1 -> 9.16.13-S1 of BIND 9 Supported Preview Edition, as well as release versions 9.17.0 -> 9.17.11 of the BIND 9.17 development branch

Severity: Medium

Exploitable: Remotely, if a target server accepts zone transfers from a potential attacker.


Incremental zone transfers (IXFR) provide a way of transferring changed portion(s) of a zone between servers. An IXFR stream containing SOA records with an owner name other than the transferred zone's apex may cause the receiving named server to inadvertently remove the SOA record for the zone in question from the zone database. This leads to an assertion failure when the next SOA refresh query for that zone is made.


When a vulnerable version of named receives a malformed IXFR triggering the flaw described above, the named process will terminate due to a failed assertion the next time the transferred secondary zone is refreshed.

CVSS Score: 6.5


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


Disabling incremental zone transfers (IXFR) by setting request-ixfr no; in the desired configuration block (options, zone, or server) prevents the failing assertion from being evaluated.

Active exploits:

We are not aware of any active exploits.


Upgrade to the patched release most closely related to your current version of BIND:

  • BIND 9.11.31
  • BIND 9.16.15
  • BIND 9.17.12

BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.

  • BIND 9.11.31-S1
  • BIND 9.16.15-S1

Acknowledgments: ISC would like to thank Greg Kuechle of SaskTel for bringing this vulnerability to our attention.

Document revision history:

1.0 Early Notification, 14 April 2021
1.1 Disclosure schedule change, 15 April 2021
2.0 Public disclosure, 28 April 2021

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 To 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 at

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

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