CVE-2026-40209
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 might be able to cause outgoing TCP connections to backend to be stuck until a timeout occurs instead of being released immediately, by sending IXFR queries. This could be used to cause a denial of service if there is a limit to the number of concurrent connections to this backend, or if the process runs out of file descriptors.
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 confidenceThis is a DNS server vulnerability where sending IXFR (Incremental Zone Transfer) queries causes outgoing TCP connections to backend servers to remain stuck until timeout instead of being released immediately. The improper connection management allows attackers to exhaust connection limits or file descriptors, leading to 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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Identify DNS server software and versionRun 'dnsserver -v' or check the service process (e.g., 'named -v', 'pdns_server --version') or inspect running services for DNS daemon processesAffected if The DNS server version falls within any affected range or is an unpatched version known to contain this vulnerability
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Verify IXFR functionality is enabledCheck the DNS server configuration file for IXFR-related settings (e.g., 'allow-transfer' or 'ixfr-enable' directives in named.conf, pdns.conf, or equivalent)Affected if IXFR is enabled and accessible to network clients (either locally or remotely)
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Check for stuck outgoing TCP connectionsMonitor active TCP connections using 'netstat -tn' or 'ss -tn' looking for connections in ESTABLISHED or CLOSE_WAIT state to backend/authoritative servers on port 53 that persist beyond normal transfer durationAffected if TCP connections to backend DNS servers remain in established or hanging states for extended periods after IXFR queries complete
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Monitor connection pool exhaustionCheck current connection counts using 'cat /proc/sys/fs/file-nr' (Linux) or observe file descriptor usage via process monitoring; compare against configured limitsAffected if Connection counts approach or exceed configured limits, especially during or after IXFR operations
A user is affected if they run a DNS server with IXFR enabled, and stuck TCP connections accumulate to connection/file descriptor limits during normal IXFR operations.
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 dataApply the vendor-provided patch to fix the connection release logic in the DNS server's IXFR handling code. If no patch available, consider rate-limiting IXFR queries or implementing connection pooling with aggressive timeouts.
- Consultation3.0 h
- Implementation6.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-40209 — 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-2026-40209 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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