CVE-2025-40777

A possible assertion failure when 'stale-answer-client-timeout' is set to '0'

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. This issue affects BIND 9 versions 9.20.0 through 9.20.10, 9.21.0 through 9.21.9, and 9.20.9-S1 through 9.20.10-S1.

Remediation

Solution:

  • Upgrade to the patched release most closely related to your current version of BIND 9: 9.20.11, 9.21.10, or 9.20.11-S1.

Workaround:

  • Setting either of `stale-answer-client-timeout off;` or `stale-answer-enable no;` in the configuration file will prevent the assertion.

Category

7.5
CVSS
Severity: High
CVSS 3.1 •
EPSS 0.02%
Vendor Advisory isc.org
Affected: ISC BIND 9
Published at:
Updated at:

References

Link Tags
https://kb.isc.org/docs/cve-2025-40777 vendor advisory

Frequently Asked Questions

What is the severity of CVE-2025-40777?
CVE-2025-40777 has been scored as a high severity vulnerability.
How to fix CVE-2025-40777?
To fix CVE-2025-40777: Upgrade to the patched release most closely related to your current version of BIND 9: 9.20.11, 9.21.10, or 9.20.11-S1.
Is CVE-2025-40777 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2025-40777 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2025-40777?
CVE-2025-40777 affects ISC BIND 9.
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