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 

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
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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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