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

Michael McNally mcnally at
Wed Apr 28 20:19:47 UTC 2021

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


    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
CVSS Vector:         CVSS:3.1/AV:N/AC:L/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:


    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


    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 security-officer at To report a new issue, please
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    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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