CVE-2026-40012
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 · uneditedECS zero scoped answers are stored in the packet cache while they should not. This impacts only configurations that have ECS enabled;
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 · moderate confidenceIn DNS servers supporting EDNS Client Subnet (ECS), zero-scoped ECS answers (responses that should not be cached based on client subnet) are incorrectly being stored in the packet cache. This can cause incorrect geographically-specific DNS responses to be served to clients with different ECS scopes, as cached responses from one scope are improperly reused for other clients.
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
- Low
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
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 your DNS server software and versionRun 'dnsserver -v' or check the package manager (e.g., 'named -v' for BIND, 'knotd -v' for Knot Resolver, 'unbound -v' for Unbound). Compare the version to any vendor-advisored affected ranges.Affected if The server version falls within an affected range but the vendor has not yet released a patch for this issue.
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Determine if EDNS Client Subnet (ECS) is enabledReview the DNS server configuration files (e.g., named.conf for BIND, knot.conf for Knot, unbound.conf for Unbound) for 'edns-client-subnet' or 'ecs' directives. For BIND: check 'options { allow-query { ...; };' and 'ecs' settings. For other servers, search for 'subnet', 'ecs', or 'client-subnet' keywords.Affected if ECS is explicitly enabled or configured with any scope prefixes (such as '/0' or '/24').
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Verify packet caching is activeCheck the DNS server configuration for cache-related settings. In BIND, look for 'max-cache-size' and 'max-cache-ttl' in the 'options' block. In other servers, look for 'cache', 'packet-cache', or 'response-cache' directives.Affected if Packet caching is enabled (the default in most DNS servers), and ECS is also enabled.
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Inspect cache behavior for zero-scoped ECS responsesEnable query logging or packet captures. Make two DNS queries from different source IP subnets for the same domain name while watching cached responses. Look for responses with ECS scope 0 (zero-scoped) being returned to clients in different geographic scopes.Affected if Zero-scoped ECS answers appear in packet cache dumps or logs, or clients in different ECS scopes receive identical cached responses for the same query.
You are affected if your DNS server has ECS enabled, uses packet caching, and is running a version where zero-scoped ECS answers are incorrectly being stored in the cache.
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.
From vendor dataDisable ECS if not needed, or upgrade to the version where the packet cache correctly excludes zero-scoped ECS answers from being stored. Implement ECS scope validation before cache storage.
- Consultation3.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA3.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,568.
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Free · runs locallyCheck whether your project pulls in CVE-2026-40012 — 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-2026-40012 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
- No weaponised exploit code, or anything meant to cause harm
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