CVE-2026-54109 is an Integer Overflow or Wraparound vulnerability in Windows Resilient File System (ReFS) that can lead to Remote Code Execution. Microsoft rates it at CVSS v3.1 7.8 base / 6.8 temporal, with LOCAL attack vector, LOW attack complexity, LOW privileges required, and no user interaction. The reported impact is high across confidentiality, integrity, and availability. Microsoft says it is not publicly disclosed, not exploited, and exploit code maturity is UNPROVEN; exploitation is assessed as unlikely. Because the vector is local rather than network-based, it is less relevant to internet-exposed services but still matters for systems where untrusted local code or users can operate.

Highlights:
- CVSS scores: Base 7.8 - Temporal 6.8
- Exploit status & code maturity: Publicly Disclosed: No; Exploited: No; Exploit Code Maturity: UNPROVEN
- Impact: Remote Code Execution with HIGH confidentiality, integrity, and availability impacts
- Exploitation likelihood: Exploitation Unlikely; LOCAL vector with LOW attack complexity, LOW privileges required, and NO user interaction
- Severity for enterprise: High severity by CVSS, but lower risk for remote enterprise exposure because exploitation is local-only; higher concern on hosts where local access is available to untrusted users or code
- Patch status: available

Affected systems:
- Windows 10 1607
- Windows 10 1809
- Windows 10 21H2
- Windows 10 22H2
- Windows 11 24H2
- Windows 11 25H2
- Windows 11 26H1
- Windows 11 unspecified
- Windows Server 2016
- Windows Server 2019
- Windows Server 2022
- Windows Server 2025