What’s Vulnerable › Vulnerabilities

CVE-2025-40777: A possible assertion failure when 'stale-answer-client-timeout' is set to '0'

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-07-16 · updated 2026-06-17
• Weakness: CWE-617

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.

Affected products

• ISC BIND: from 9.20.0 through 9.20.10; from 9.21.0 through 9.21.9; from 9.20.9-S1 through 9.20.10-S1

References

• https://kb.isc.org/docs/cve-2025-40777
• http://www.openwall.com/lists/oss-security/2025/07/16/6

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