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CVE-2026-21240

Windows HTTP.sys Elevation of Privilege Vulnerability

No known exploitation. EPSS puts it in the 19th percentile. Fixed versions are out.

CVSS 3.17.8HighEPSS percentile19th0.3% chance in 30 daysCISA KEVNot listedNo confirmed exploitation
No public exploit knownFix5 products10 fixed builds listed below

Published Updated Sources: CVE.org, NVD, BDU

NVD CVE.org

What to do

Find your product and version, then upgrade to the build on the right.

  • Microsoft Windows 10 Version 1809Affected: ≥ 10.0.17763.0, < 10.0.17763.8389Upgrade to: 10.0.17763.8389Affected: ≥ 10.0.19044.0, < 10.0.19044.6937Upgrade to: 10.0.19044.6937Affected: ≥ 10.0.19045.0, < 10.0.19045.6937Upgrade to: 10.0.19045.6937Stated by: CVE record
  • Microsoft Windows 11 version 22H3Affected: ≥ 10.0.22631.0, < 10.0.22631.6649Upgrade to: 10.0.22631.6649Affected: ≥ 10.0.26100.0, < 10.0.26100.7840Upgrade to: 10.0.26100.7840Affected: ≥ 10.0.26200.0, < 10.0.26200.7840Upgrade to: 10.0.26200.7840Affected: ≥ 10.0.28000.0, < 10.0.28000.1575Upgrade to: 10.0.28000.1575Stated by: CVE record
  • Microsoft Windows Server 2019Affected: ≥ 10.0.17763.0, < 10.0.17763.8389Upgrade to: 10.0.17763.8389Stated by: CVE record
  • Microsoft Windows Server 2022Affected: ≥ 10.0.20348.0, < 10.0.20348.4773Upgrade to: 10.0.20348.4773Affected: ≥ 10.0.25398.0, < 10.0.25398.2149Upgrade to: 10.0.25398.2149Stated by: CVE record
  • Microsoft Windows Server 2025Affected: ≥ 10.0.26100.0, < 10.0.26100.32370Upgrade to: 10.0.26100.32370Stated by: CVE record

Versions as the sources state them. Full version matrix after sign-in

Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

What it is

From the CVE record

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

In plain language

No plain-language summary for this CVE yet.

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS19th
CISA KEV

Not in the catalog. CISA has not confirmed exploitation.

Public exploits

No public exploit or proof of concept found in the sources we track.

EPSS

0.3% chance of exploitation activity in the next 30 days, which ranks it in the 19th percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Lifecycle

6 events over 135 days, from the signal feeds we watch.

  1. OpenVAS check added
  2. Patch availablerecord updated
  3. Publishedweakness classified, att&ck mapped

Affected products

And 34 more affected products. See all after sign-in

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

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

Scored 7.8 by NVD.

How it is reached

  • Attack Vector LocalRequires local access to the vulnerable system (e.g. local login, malicious file)
  • Attack Complexity LowNo special conditions — the attack can be reliably reproduced
  • Privileges Required LowRequires basic user-level privileges
  • User Interaction NoneNo user interaction needed — fully automated exploitation

Scope

  • Scope UnchangedImpact is limited to the vulnerable component itself

Impact if exploited

  • Confidentiality HighTotal information disclosure — all data in the component is compromised
  • Integrity HighTotal loss of integrity — attacker can modify any data in the component
  • Availability HighTotal denial of service — the component is completely unavailable

Weaknesses

ATT&CK techniques

Mapped from the weaknesses above (CWE to ATT&CK), not observed in attacks.

Sources

References in the record

In the news

Watch the software you run.

My Stack ranks new CVEs for your products by real-world exploitation, so the next serious one reaches you without reading every advisory.

We'll flag the next CVE, public exploit or patch for Windows 10 Version 1809, not every advisory.