CVE-2025-40777
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIf 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA denial of service vulnerability in BIND 9 named caching resolver occurs when configured with `serve-stale-enable yes` and `stale-answer-client-timeout` set to `0`. When resolving queries involving CNAME chains with specific combinations of cached or authoritative records, the daemon crashes due to an assertion failure.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify BIND 9 versionRun `named -v` or `named -V` to obtain the installed BIND versionAffected if Version is within the affected range (prior to patched version)
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Locate BIND configurationFind the named.conf file (commonly in /etc/, /etc/bind/, or check for -c flag usage)Affected if Configuration file exists and contains stale-answer related options
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Check serve-stale-enable settingSearch the configuration for 'serve-stale-enable' directive and confirm its valueAffected if serve-stale-enable is set to yes
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Verify stale-answer-client-timeout valueSearch for 'stale-answer-client-timeout' directive and note its configured valueAffected if stale-answer-client-timeout is set to 0
Environment is affected if BIND 9 version is vulnerable AND both serve-stale-enable yes AND stale-answer-client-timeout 0 are configured simultaneously
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedAvoid using `stale-answer-client-timeout 0` with `serve-stale-enable yes` in production until a patched version is available, or upgrade BIND to a version beyond the affected ranges.
BIND 9.20.11 or later (stable series), or latest 9.22.x/9.24.x stable release
- 1. Identify current BIND 9 version using `named -v` or `named -V`
- 2. If running 9.20.0-9.20.10, 9.21.0-9.21.9, or 9.20.9-S1-9.20.10-S1, plan for upgrade
- 3. Backup current named configuration files (typically in /etc/named.conf, /etc/bind/, or /var/named/)
- 4. Download the latest stable BIND 9 release from ISC (https://www.isc.org/download/)
- 5. Alternatively, check your distribution's package manager for available bind9 updates
- 6. Install the updated package or compile and install from source
- 7. Verify the new version with `named -v`
- 8. Restart the named service (e.g., `systemctl restart named` or `service named restart`)
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-40777 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-40777 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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