CVE-2026-40211
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 · uneditedAn attacker can send crafted DNS over HTTP/3 queries, triggering an exception that prevents some buffer from being freed right away. The buffer will be freed at the end of the QUIC connection, but on some setups it might be possible to open enough concurrent DoH3 streams to trigger an out-of-memory condition, resulting in a denial of service.
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 confidenceA vulnerability in DNS over HTTP/3 (DoH3) implementation allows attackers to send crafted queries that trigger an exception, preventing immediate buffer deallocation. The buffer is only freed when the QUIC connection closes, enabling attackers to exhaust memory by opening many concurrent DoH3 streams, causing denial of service.
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
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
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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Confirm DoH3 is enabledReview your DNS server configuration for DNS-over-HTTP/3 (DoH3) settings. Look for QUIC/HTTP3 protocol configurations, typically found in named.conf (BIND), unbound.conf, or similar DNS resolver config files. Check for lines enabling quic, http3, or doh3 protocols.Affected if DoH3 is actively enabled and processing queries
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Identify the DNS software versionRun the version command for your DNS resolver software (e.g., named -v, unbound -v, or your specific DoH3 implementation). Compare the installed version against any published version ranges for your product that include this CVE.Affected if The installed version falls within a vulnerable version range for this CVE
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Check concurrent stream limitsReview DNS/DoH3 configuration for any settings controlling maximum concurrent streams per client. Look for parameters like max-concurrent-streams, max-streams-per-connection, or similar QUIC stream limit settings.Affected if No limits are configured on concurrent DoH3 streams per client
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Monitor for memory anomaliesObserve memory usage patterns on the DNS server, particularly during periods of high DoH3 query volume. Use tools like top, htop, or process-specific memory monitoring to identify unusual growth.Affected if Memory usage grows abnormally and correlates with DoH3 traffic patterns
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Review exception/error logsExamine DNS server logs for exceptions, errors, or warnings related to buffer allocation, QUIC connections, or DoH3 query handling. Check system logs and application-specific logs.Affected if Exception errors related to DoH3 query processing appear in logs
You are affected if DoH3 is enabled and your DNS software version is within a vulnerable range, especially if no concurrent stream limits are configured.
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 dataImplement proper exception handling to ensure buffer cleanup occurs even when exceptions are thrown, and impose limits on concurrent DoH3 streams per client to prevent memory exhaustion.
- Consultation4.0 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 $5,888.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-40211 — 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-2026-40211 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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