CVE-2023-2911: Exceeding the recursive-clients quota may cause named to terminate unexpectedly when stale-answer-client-timeout is set to 0
  • 21 Jun 2023
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CVE-2023-2911: Exceeding the recursive-clients quota may cause named to terminate unexpectedly when stale-answer-client-timeout is set to 0

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Article summary

CVE: CVE-2023-2911

Document version: 2.0

Posting date: 21 June 2023

Program impacted: BIND 9

Versions affected:


  • 9.16.33 -> 9.16.41
  • 9.18.7 -> 9.18.15

BIND Supported Preview Edition

  • 9.16.33-S1 -> 9.16.41-S1
  • 9.18.11-S1 -> 9.18.15-S1

(BIND 9.11-S versions that support the stale-answer-client-timeout option are not vulnerable.)

Severity: High

Exploitable: Remotely


If the recursive-clients quota is reached on a BIND 9 resolver configured with both stale-answer-enable yes; and stale-answer-client-timeout 0;, a sequence of serve-stale-related lookups could cause named to loop and terminate unexpectedly due to a stack overflow.


By sending specific queries to the resolver, an attacker can cause named to terminate unexpectedly.

CVSS Score: 7.5


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


Setting stale-answer-client-timeout to off or to a non-zero value prevents the issue.

Users of versions 9.18.10, 9.16.36, 9.16.36-S1 or older who are unable to upgrade should set stale-answer-client-timeout to off; using a non-zero value with these older versions leaves named vulnerable to CVE-2022-3924.

Although it is possible to set the recursive-clients limit to a high number to reduce the likelihood of this scenario, this is not recommended; the limit on recursive-clients is important for preventing exhaustion of server resources. The limit cannot be disabled entirely.

Active exploits:

This flaw was discovered in internal testing. We are not aware of any active exploits.


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

  • 9.16.42
  • 9.18.16

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

  • 9.16.42-S1
  • 9.18.16-S1

Document revision history:

  • 1.0 Early Notification, 14 June 2023
  • 2.0 Public disclosure, 21 June 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 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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