CVE-2025-40777: ISC BIND 9

High severity, CVSS 7.5. EPSS: 1% chance of exploitation in the next 30 days.

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.

Affected products

• ISC BIND 9: from 9.20.0, up to and including 9.20.10; from 9.21.0, up to and including 9.21.9; from 9.20.9-S1, up to and including 9.20.10-S1

Published 2025-07-16. Last modified 2026-06-17.