CVE: CVE-2026-5947
Title: SIG(0) validation during query flood may lead to undefined behavior
Document version: 2.0
Posting date: 20 May 2026
Program impacted: BIND 9
Versions affected:
BIND
- 9.20.0 -> 9.20.22
- 9.21.0 -> 9.21.21
BIND Supported Preview Edition
- 9.20.9-S1 -> 9.20.22-S1
Versions NOT affected:
BIND
- 9.18.28 -> 9.18.49
BIND Supported Preview Edition
- 9.18.28-S1 -> 9.18.49-S1
(Versions prior to 9.18.28 were not assessed.)
Severity: High
Exploitable: Remotely
Description:
Undefined behavior may result due to a race condition leading to a use-after-free violation. If BIND receives an incoming DNS message signed with SIG(0), it begins work to validate that signature. If, during that validation, the "recursive-clients" limit is reached (as would occur during a query flood), and that same DNS message is discarded per the limit, there is a brief window of time while the SIG(0) validation may attempt to read the now-discarded DNS message.
Impact:
The use of memory after it is freed is undefined ("dangling pointer"). The BIND process may abort with a segmentation violation or similar error. If memory from the discarded message has not been reused or reclaimed, the validation might proceed normally. Any kind of code execution from such an improper data read is unlikely.
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Authoritative servers are affected by this vulnerability.
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Resolvers are affected by this vulnerability.
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:
- 9.20.23
- 9.21.22
BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.
- 9.20.23-S1
Acknowledgments:
ISC would like to thank Naoki Wakamatsu for bringing this vulnerability to our attention.
Document revision history:
- 1.0 Early Notification, 13 May 2026
- 2.0 Public disclosure, 20 May 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 mailed to bind-security@isc.org or 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-2026-5947 is the complete and official security advisory document.
Legal Disclaimer:
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