dbcve.org
Windows 10 1809Operating system · Microsoft

CVE-2026-21240

HIGH · 7.0 CVSS v3.1 Published 2026-02-10
Fix available
A fix is available. Upgrade to 10.0.17763.8389 / 10.0.19044.6937 or later.
See remediation →
72/100
Remediation priority · Elevated
Zero-click

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 · unedited
Time-of-check time-of-use (toctou) race condition in Windows HTTP.sys allows an authorized attacker to elevate privileges locally.

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 confidence

A time-of-check time-of-use (TOCTOU) race condition in Windows HTTP.sys (the HTTP protocol stack kernel driver) allows an authorized attacker to achieve local privilege escalation. The vulnerability stems from a gap between when a security check is performed and when the result is used, enabling an attacker to manipulate the race window.

MitigationApply the relevant Microsoft security update for CVE-2026-21240 when released. Prioritize patching systems with local user access. As a compensating control, limit local user permissions and monitor for suspicious processes interacting with HTTP.sys.

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
Windows 10 1809Operating system
Affected:< 10.0.17763.8389
Windows 10 21h2Operating system
Affected:< 10.0.19044.6937
Windows 10 22h2Operating system
Affected:< 10.0.19045.6937
Windows 11 23h2Operating system
Affected:< 10.0.22631.6649
Windows 11 24h2Operating system
Affected:< 10.0.26100.7781
Windows 11 25h2Operating system
Affected:< 10.0.26200.7781
Windows Server 2019Operating system
Affected:< 10.0.17763.8389
Windows Server 2022Operating system
Affected:< 10.0.20348.4711

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
Local
Complexity
High
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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 checks

Work through these to decide whether this CVE applies to you.

  1. Check Windows build version
    Open Command Prompt or PowerShell and run 'winver' or 'systeminfo | findstr /B /C:"OS Version"' to see the full build number
    Affected if The build number is lower than the version specified for your Windows release (10.0.17763.8389 for 1809/Server 2019, 10.0.19044.6937 for 21h2, 10.0.19045.6937 for 22h2, 10.0.22631.6649 for 11 23h2, 10.0.26100.7781 for 11 24h2, 10.0.26200.7781 for 11 25h2, or 10.0.20348.4711 for Server 2022)
  2. Identify Windows release name
    Run 'winver' to see the release name (such as '1809', '21H2', '22H2', '23H2') or check systeminfo for the version number
    Affected if The release does not match a version in the affected list or the version falls within an affected range but the build is at or above the fixed version numbers listed
  3. Verify HTTP.sys driver is present
    Run 'sc query http' in Command Prompt or check driver existence with 'driverquery /v | findstr -i http'
    Affected if The HTTP.sys driver service exists on the system (this is the vulnerable component)
  4. Confirm local user access exists
    Run 'net user' to list local user accounts or check Local Users and Groups via 'lusrmgr.msc'
    Affected if There are local user accounts on the system (the vulnerability enables local privilege escalation for an authorized local user)

You are affected if your Windows build number is lower than the fixed version for your specific Windows release and you have local user accounts on the system where HTTP.sys is present.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 10.0.17763.8389 / 10.0.19044.6937 / 10.0.19045.6937 or later
Fixed in 10.0.17763.838910.0.19044.693710.0.19045.6937
Interim mitigation

Apply the relevant Microsoft security update for CVE-2026-21240 when released. Prioritize patching systems with local user access. As a compensating control, limit local user permissions and monitor for suspicious processes interacting with HTTP.sys.

Recommended fix High confidence

Windows 10 1809: 10.0.17763.8389 | Windows 10 21h2: 10.0.19044.6937 | Windows 10 22h2: 10.0.19045.6937 | Windows 11 23h2: 10.0.22631.6649 | Windows 11 24h2: 10.0.26100.7781 | Windows 11 25h2: 10.0.26200.7781 | Windows Server 2019: 10.0.17763.8389 | Windows Server 2022: 10.0.20348.4711

  1. Open Settings on the affected Windows system
  2. Navigate to Windows Update (Settings > Update & Security > Windows Update)
  3. Click 'Check for updates' to download and install the latest security updates
  4. Verify the update was installed by checking the installed updates history
  5. Ensure the build number matches or exceeds the fixed version for your Windows edition (e.g., 10.0.17763.8389 for Windows 10 1809/Server 2019, 10.0.19044.6937 for Windows 10 21h2, etc.)
Caveat Standard Windows security update with no expected breaking changes; patch is cumulative and includes all prior security fixes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Windows 10 1809 Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,280
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Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21240 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data