CVE-2021-25215: An assertion check can fail while answering queries for DNAME records that require the DNAME to be processed to resolve itself

Michael McNally mcnally at
Wed Apr 28 20:20:01 UTC 2021

CVE:                 CVE-2021-25215
Document version:    2.0
Posting date:        28 April 2021
Program impacted:    BIND
Versions affected:   BIND 9.0.0 -> 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 Supported Preview Edition, as well as release
                      versions 9.17.0 -> 9.17.11 of the BIND 9.17 development
Severity:            High
Exploitable:         Remotely


    DNAME records, described in RFC 6672, provide a way to redirect
    a subtree of the domain name tree in the DNS. A flaw in the way
    named processes these records may trigger an attempt to add the
    same RRset to the ANSWER section more than once. This causes an
    assertion check in BIND to fail.

    DNAME records are processed by both authoritative and recursive
    servers. For authoritative servers, the DNAME record triggering
    the flaw can be retrieved from a zone database. For servers
    performing recursion, such a record is processed in the course
    of a query sent to an authoritative server.


    When a vulnerable version of named receives a query for a record
    triggering the flaw described above, the named process will
    terminate due to a failed assertion check.

    The vulnerability affects all currently maintained BIND 9 branches
    (9.11, 9.11-S, 9.16, 9.16-S, 9.17) as well as all other versions
    of BIND 9.

CVSS Score:          7.5
CVSS Vector:         CVSS:3.1/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:


    No workarounds known.

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

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