CVE: CVE-2025-40777
Document version: 2.0
Posting date: 16 July 2025
Program impacted: BIND
Versions affected:
BIND
- 9.20.0 -> 9.20.10
- 9.21.0 -> 9.21.9
(Versions prior to 9.18.0 were not assessed.)
BIND Supported Preview Edition
- 9.20.9-S1 -> 9.20.10-S1
(Versions prior to 9.18.11-S1 were not assessed.)
Severity: High
Exploitable: Remotely
Description:
If a named caching resolver is configured with serve-stale-enable yes, and with stale-answer-client-timeout set to 0 (the only allowable value other than disabled), and if the resolver, in the process of resolving a query, encounters a CNAME chain involving a specific combination of cached or authoritative records, the daemon will abort with an assertion failure.
Impact:
If an attacker is able to make queries that yield the necessary combination of circumstances, they can force named to exit.
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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: 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:
Setting either of stale-answer-client-timeout off; or stale-answer-enable no; in the configuration file will prevent the assertion.
Active exploits:
This flaw was discovered in internal testing. We are not aware of any active exploits.
Solution:
Upgrade to the patched release most closely related to your current version of BIND:
- BIND 9.20.11
- BIND 9.21.10
BIND Supported Preview Edition is a special feature preview branch of BIND provided to eligible ISC support customers.
- BIND 9.20.11-S1
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-2025-40777 is the complete and official security advisory document.
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