CVE-2026-40378
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 · uneditedMemory allocation with excessive size value in Windows Local Security Authority Subsystem Service (LSASS) allows an unauthorized attacker to deny service over a network.
In the news
Third-party coverage- Microsoft July 2026 Patch Tuesday fixes massive 570 flaws, 3 zero-days
- Microsoft CVE Summary - Bleeping Computer
Surfaced from public web coverage — external links open in a new tab.
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 denial of service vulnerability in Windows LSASS (Local Security Authority Subsystem Service) where memory allocation is attempted with an excessive size value, causing the service to fail. The vulnerability can be exploited remotely by an unauthorized attacker over a network without authentication.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 10.0.14393.9339< 10.0.17763.9020< 10.0.19044.7548< 10.0.19045.7548< 10.0.22631.7376< 10.0.26100.8875< 10.0.26200.8875< 10.0.28000.2269CVSS 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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Check Windows versionRun 'winver' or 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to obtain the exact Windows build numberAffected if The installed version is one of the affected builds listed in the CVE (for example, Windows 10 22h2 with build < 10.0.19045.7548, or Windows 11 23h2 with build < 10.0.22631.7376)
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Verify LSASS service is runningOpen Services console (services.msc) or run 'sc query lsass' to confirm the Local Security Authority Subsystem Service is activeAffected if LSASS service is running - this is required for the vulnerability to be exploitable since the flaw affects memory allocation within this service
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Identify network accessibility of the target systemRun 'netstat -an | findstr :445' or check network adapter bindings to determine if the system is directly exposed to untrusted networksAffected if The system is directly reachable over network by unauthorized attackers (e.g., has unfiltered inbound connectivity on ports used for SMB/rpc, or is in a DMZ or internet-facing segment)
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Confirm system role and exposure contextReview whether this Windows system is a domain controller, member server, or standalone workstation, and check firewall rules applicable to itAffected if The system is a domain controller or has LSASS accessible via network protocols without proper segmentation from untrusted networks
A system is affected if it runs a Windows version with a build number lower than the fixed versions AND has LSASS running with network accessibility from untrusted actors.
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 · scoped10.0.14393.933910.0.17763.902010.0.19044.7548
Apply the relevant Microsoft Windows security update for this vulnerability once released. Until then, network segmentation and restricting unauthorized network access to systems running LSASS may reduce exposure.
Windows 10 1607: 10.0.14393.9339+ | Windows 10 1809: 10.0.17763.9020+ | Windows 10 21h2: 10.0.19044.7548+ | Windows 10 22h2: 10.0.19045.7548+ | Windows 11 23h2: 10.0.22631.7376+ | Windows 11 24h2: 10.0.26100.8875+ | Windows 11 25h2: 10.0.26200.8875+ | Windows 11 26h1: 10.0.28000.2269+
- Identify the current Windows version by running 'winver' or 'systeminfo' in Command Prompt
- Determine which Windows 10 or 11 version family is installed (1607, 1809, 21h2, 22h2, 23h2, 24h2, 25h2, or 26h1)
- Compare the current build number against the fixed version threshold for your Windows version family
- For Windows 10 1607: upgrade to build 10.0.14393.9339 or later
- For Windows 10 1809: upgrade to build 10.0.17763.9020 or later
- For Windows 10 21h2: upgrade to build 10.0.19044.7548 or later
- For Windows 10 22h2: upgrade to build 10.0.19045.7548 or later
- For Windows 11 23h2: upgrade to build 10.0.22631.7376 or later
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 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-2026-40378 — 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-2026-40378 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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- Version or environment caveats, and links to real fixes
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