CVE: CVE-2026-11721
Title: Cache poisoning possible with label count discrepancy, RRSIG, and wildcards
Document version: 2.0
Posting date: 22 July 2026
Program impacted: BIND 9
Versions affected:
BIND
- 9.11.0 -> 9.18.50
- 9.20.0 -> 9.20.24
- 9.21.0 -> 9.21.23
BIND Supported Preview Edition
- 9.11.3-S1 -> 9.18.50-S1
- 9.20.9-S1 -> 9.20.24-S1
Severity: High
Exploitable: Remotely
Description:
It is possible for an attacker's zone to respond to a query with an RRSIG that has a smaller number of labels than the zone in which the RRSIG is contained. This causes named to produce a wildcard name for a zone that is shorter than the attacker's zone, which can result in cache poisoning. For this attack to have any effect, the resolver under attack must have set synth-from-dnssec yes; (which is the default).
Impact:
Potential cache poisoning.
CVSS Score: 7.5
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
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:H/A:N&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:
- 9.20.26
- 9.21.24
BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.
- 9.20.26-S1
Acknowledgments:
ISC would like to thank Qifan Zhang of Palo Alto Networks for bringing this vulnerability to our attention.
Document revision history:
- 1.0 Early Notification, 15 July 2026
- 1.1 Revised the list of fixed versions, 20 July 2026
- 2.0 Public disclosure, 22 July 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-2026-11721 is the complete and official security advisory document.
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