CVE: CVE-2026-81563
Title: SVCB AliasMode additional-data error leaks qpcache references
Document version: 2.0
Posting date: 16 September 2026
Program impacted: BIND 9
Versions affected:
BIND
- 9.18.0 -> 9.18.50
- 9.20.0 -> 9.20.27
- 9.21.0 -> 9.21.25
BIND Supported Preview Edition
- 9.18.11-S1 -> 9.18.50-S1
- 9.20.9-S1 -> 9.20.27-S1
Severity: High
Exploitable: Remotely
Description:
A BIND resolver encountering an SVCB/HTTPS AliasMode record referencing 14 or more SVCB/HTTPS ServiceMode records may fail to properly deallocate internal resources. If this happens repeatedly, resource exhaustion will eventually prevent the resolver from performing new recursive lookups.
Impact:
Repeated successful attacks can result in memory use beyond configured limits until the system memory is exhausted or named is terminated by the operating system. Either case results in a denial-of-service attack.
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: https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1.
Workarounds:
No workarounds known.
Active exploits:
We are not aware of any active exploits.
Solution:
Upgrade to the patched release most closely related to your current version of BIND 9:
- 9.20.29
- 9.21.26
BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.
- 9.20.29-S1
Acknowledgments:
ISC would like to thank Samy Medjahed/Ap4sh for bringing this vulnerability to our attention.
Document revision history:
- 1.0 Early Notification, 9 September 2026
- 1.1 Revised the list of fixed versions, 11 September 2026
- 2.0 Public disclosure, 16 September 2026
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 posted as confidential GitLab issues at:
https://gitlab.isc.org/isc-projects/bind9/-/issues/new?issue[confidential]=true
Note:
ISC patches only currently supported versions. When possible we indicate EOL versions affected. For current information on which versions are actively supported, please see https://www.isc.org/download/.
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 https://kb.isc.org/docs/aa-00861.
The Knowledgebase article https://kb.isc.org/docs/cve-2026-81563 is the complete and official security advisory document.
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