CVE-2023-4236: named may terminate unexpectedly under high DNS-over-TLS query load
  • 20 Sep 2023
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CVE-2023-4236: named may terminate unexpectedly under high DNS-over-TLS query load

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Article summary

CVE: CVE-2023-4236

Document version: 2.0

Posting date: 20 September 2023

Program impacted: BIND 9

Versions affected:


  • 9.18.0 -> 9.18.18

BIND Supported Preview Edition

  • 9.18.11-S1 -> 9.18.18-S1

Severity: High

Exploitable: Remotely


A flaw in the networking code handling DNS-over-TLS queries may cause named to terminate unexpectedly due to an assertion failure. This happens when internal data structures are incorrectly reused under significant DNS-over-TLS query load.


A named instance vulnerable to this flaw may terminate unexpectedly when subjected to significant DNS-over-TLS query load.

This flaw does not affect DNS-over-HTTPS code, as that uses a different TLS implementation.

CVSS Score: 7.5


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


Disabling listening for DNS-over-TLS connections (by removing listen-on ... tls ... { ... }; statements from the configuration) prevents the affected code paths from being taken, rendering exploitation impossible. However, there is no workaround for this flaw if DNS-over-TLS support is required.

Active exploits:

We are not aware of any active exploits.


Upgrade to the patched release most closely related to your current version of BIND 9:

  • 9.18.19

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

  • 9.18.19-S1


ISC would like to thank Robert Story from the USC/ISI DNS root server operations team for bringing this vulnerability to our attention.

Document revision history:

  • 1.0 Early Notification, 13 September 2023
  • 2.0 Public disclosure, 20 September 2023

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 be mailed to or posted as confidential GitLab issues at[confidential]=true.


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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