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VulnerabilityPublished

CVE-2026-2636: Microsoft Windows OS · Improper Handling of Invalid Use of Special Elements

Affects Microsoft Windows OSSource CVE Database V5

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Not known to be exploited · CVSS 5.5 Medium.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score
5.5Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 40% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 25 Feb 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: evidence found.

    Public exploit code: GitHub proofs of concept

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), GitHub proofs of concept, the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Affected software

ProductAffectedFixed in
MicrosoftWindows OSAffected
  • <25H2
  • <=1123h2
Fixed in–

AI-suggested

To mitigate CVE-2026-2636, organizations should prioritize installing the September 2025 cumulative update or later versions that include the patch, such as Windows 11 25H2 and Windows Server 2025. For environments where immediate patching is not feasible, restrict local user access and privileges to minimize the risk of unprivileged users triggering the vulnerability. Implement strict access controls on shared systems and terminal servers. Monitor system logs for unexpected crashes related to CLFS.sys or bug check events that could indicate attempted exploitation. Employ endpoint detection and response (EDR) solutions to detect anomalous local activity. Regularly review and update patch management processes to ensure timely deployment of security updates. Additionally, educate users about the risks of local access exploitation and enforce policies to limit unnecessary local user accounts. Consider network segmentation to isolate critical systems from less trusted user groups. Finally, maintain up-to-date backups to reduce impact from potential denial of service incidents.

Description

This vulnerability is caused by a CWE‑159: "Improper Handling of Invalid Use of Special Elements" weakness, which leads to an unrecoverable inconsistency in the CLFS.sys driver. This condition forces a call to the KeBugCheckEx function, allowing an unprivileged user to trigger a system crash. Microsoft silently fixed this vulnerability in the September 2025 cumulative update for Windows 11 2024 LTSC and Windows Server 2025. Windows 25H2 (released in September) was released with the patch. Windows 1123h2 and earlier versions remain vulnerable.

References

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