Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Local authenticated low-priv access with no user interaction (AV:L/PR:L/UI:N); heap overflow in a kernel driver yields SYSTEM control, so full C/I/A impact.
Primary rating from NVD.
CVSS VectorNVD
Lifecycle Timeline
2DescriptionCVE.org
Heap-based buffer overflow in Windows Common Log File System Driver allows an authorized attacker to elevate privileges locally.
AnalysisAI
Local privilege escalation in the Windows Common Log File System (CLFS) driver lets an already-authenticated low-privileged user corrupt heap memory to gain SYSTEM-level control across a broad range of Windows client and server releases (Windows 10 1607 through Windows 11 26H1, Server 2012 through Server 2025). Exploitation is CVSS 7.8 with total technical impact per CISA SSVC, but SSVC records no observed exploitation and EPSS is very low (0.04%), and no public exploit was identified at time of analysis. A vendor patch is available from Microsoft (MSRC).
Technical ContextAI
The Common Log File System (clfs.sys) is a kernel-mode general-purpose logging subsystem shipped with all supported Windows versions, used by applications and the OS to create and manage transactional log files. The flaw is CWE-122 (heap-based buffer overflow): malformed input processed by the driver writes past an allocated kernel heap buffer, corrupting adjacent kernel memory. Because CLFS runs in kernel context (Ring 0), controlled heap corruption can be leveraged to overwrite kernel structures and hijack execution as SYSTEM. The CPE set confirms the affected component spans the Microsoft Windows platform itself (Windows 10 1607/1809/21H2/22H2, Windows 11 22H3/23H2/24H2/25H2/26H1, and Windows Server 2012 through 2025), consistent with CLFS being a core OS driver rather than an optional feature.
RemediationAI
Apply the Microsoft security update for CVE-2026-40407 via Windows Update or WSUS; this is the primary and complete fix - Vendor-released patch confirmed per Microsoft MSRC (https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40407). Install at minimum the fixed build for your edition (e.g. 10.0.26100.8457 for Windows 11 24H2, 10.0.22631.7079 for 23H2, 10.0.19045.7291 for Windows 10 22H2, 6.2.9200.26079 for Server 2012, 10.0.20348.5139 for Server 2022). CLFS cannot be safely disabled as a general workaround because the OS and many applications depend on it, so there is no clean feature-toggle mitigation; until patched, reduce risk by limiting who can obtain local/interactive or low-privileged code execution on the host - enforce least privilege, restrict local logon and RDP on shared/multi-user systems, and monitor EDR for anomalous CLFS/.blf activity and kernel-integrity alerts, accepting that these controls only shrink the attacker pool rather than close the flaw. Additional references: https://vuldb.com/vuln/363198 and https://nvd.nist.gov/vuln/detail/CVE-2026-40407.
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Same weakness CWE-122 – Heap-based Buffer Overflow
View allSame technique Buffer Overflow
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External POC / Exploit Code
Leaving vuln.today
EUVD-2026-29667
GHSA-6p5f-2v3f-hh8g