CVE: CVE-2025-40776
Title: Birthday Attack against Resolvers supporting ECS
Document version: 2.0
Posting date: 16 July 2025
Program impacted: BIND 9
Versions affected:
BIND Supported Preview Edition
- 9.11.3-S1 -> 9.16.50-S1
- 9.18.11-S1 -> 9.18.37-S1
- 9.20.9-S1 -> 9.20.10-S1
Versions NOT affected:
BIND
- 9.0.0 -> 9.20.10
EDNS Client Subnet (ECS) options are only available in the BIND Subscription Edition (-S), so only the -S edition is affected by this CVE. End-of-Life (EOL) versions of BIND -S software are believed to be affected, but were not tested.
Severity: High
Exploitable: Remotely
Description:
A named caching resolver that is configured to send ECS (EDNS Client Subnet) options may be vulnerable to a cache-poisoning attack.
Impact:
A resolver configured to send ECS options to authoritative servers can be compelled to make queries that slightly increase the odds of guessing the source port and other details necessary to bypass the original birthday cache poisoning attack mitigations. As a result of this weakness, a resolver with ECS enabled is more vulnerable to successful cache poisoning via spoofed query responses than one that does not implement this feature.
-
Authoritative services are believed to be unaffected by this vulnerability but it is important to read: https://kb.isc.org/docs/why-does-my-authoritative-server-make-recursive-queries
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Resolvers are affected by this vulnerability.
CVSS Score: 8.6
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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:C/C:N/I:H/A:N&version=3.1.
Workarounds:
Disable ECS in BIND by removing the ecs-zones option from named.conf.
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:
BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.
- 9.18.38-S1
- 9.20.11-S1
Acknowledgments:
ISC would like to thank Xiang Li from AOSP Lab of Nankai University for bringing this vulnerability to our attention.
Document revision history:
- 1.0 Early Notification, 9 July 2025
- 2.0 Public disclosure, 16 July 2025
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-2025-40776 is the complete and official security advisory document.
Legal Disclaimer:
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