CVE-2025-55687 Overview
CVE-2025-55687 is a race condition vulnerability in the Windows Resilient File System (ReFS) that allows a local, authenticated attacker to elevate privileges. The flaw is classified under [CWE-362], concurrent execution using a shared resource with improper synchronization. An attacker who wins the race can gain higher privileges on the affected host. Microsoft rates the vulnerability HIGH and it affects a broad range of Windows client and server releases, including Windows 10, Windows 11, and Windows Server versions from 2012 through 2025.
Critical Impact
Successful exploitation grants an attacker elevated privileges on the local system, undermining confidentiality, integrity, and availability of the host.
Affected Products
- Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (22H2, 23H2, 24H2, 25H2)
- Microsoft Windows Server 2012, 2016, 2019, 2022, 2022 23H2, and 2025
Discovery Timeline
- 2025-10-14 - CVE CVE-2025-55687 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-55687
Vulnerability Analysis
The vulnerability resides in the Windows Resilient File System (ReFS) driver. ReFS handles concurrent file and metadata operations, and this issue occurs when multiple threads access a shared resource without adequate synchronization. An attacker who repeatedly triggers the affected code path can interleave operations to reach an unintended state.
The attacker must already have valid low-privilege access to the target system, and exploitation requires a local attack vector. The attack complexity is high because the attacker must reliably win a timing window, but a successful exploit yields full impact on confidentiality, integrity, and availability. No user interaction is required.
Root Cause
The root cause is improper synchronization between concurrent operations on a shared ReFS resource, an instance of [CWE-362]. Because the resource state is checked and used across non-atomic operations, an attacker can modify state between the check and the use, producing a Time-of-Check to Time-of-Use (TOCTOU) condition that grants privileged access to structures the caller should not control.
Attack Vector
Exploitation requires local code execution on a Windows host with a ReFS volume in scope. The attacker spawns parallel threads or processes that repeatedly invoke ReFS operations on the shared resource, racing to alter its state during the driver's window of trust. Winning the race allows the attacker to influence kernel-level file system decisions and escalate to a higher privilege context. Because ReFS is used on data and Storage Spaces Direct volumes in many server deployments, domain controllers and file servers with ReFS volumes are viable targets.
No public proof-of-concept exploit is currently available, and Microsoft has not observed exploitation in the wild. Refer to the Microsoft Security Update CVE-2025-55687 advisory for authoritative technical detail.
Detection Methods for CVE-2025-55687
Indicators of Compromise
- Unexpected creation of new SYSTEM-level processes originating from a low-privileged user session on hosts with ReFS volumes.
- Repeated, high-frequency file system operations against the same ReFS path or handle from a single non-administrative process, suggestive of race-condition brute forcing.
- Kernel error events (Ntfs/ReFS System log entries) coinciding with privilege changes on the same user session.
Detection Strategies
- Correlate process creation events (Sysmon Event ID 1) where a child process runs as SYSTEM or an administrator while the parent runs as a standard user.
- Monitor for anomalous concurrency patterns targeting ReFS-mounted volumes, especially rapid open/close or rename cycles on the same object.
- Baseline normal ReFS activity per endpoint and alert on statistical deviations that align with exploit attempts.
Monitoring Recommendations
- Ingest Windows Security, System, and Sysmon logs into a centralized analytics platform to enable cross-event correlation.
- Track token elevation events (Event ID 4672) issued to accounts that do not normally receive privileged tokens.
- Alert on kernel crashes or bugchecks referencing refs.sys or refsv1.sys, which may indicate failed exploitation attempts.
How to Mitigate CVE-2025-55687
Immediate Actions Required
- Apply the Microsoft security update for CVE-2025-55687 to every affected Windows client and server as identified in the MSRC advisory.
- Prioritize patching on servers hosting ReFS volumes, including file servers, backup targets, Hyper-V hosts, and Storage Spaces Direct clusters.
- Restrict interactive and remote logon rights on ReFS-hosting servers to reduce the pool of accounts capable of local exploitation.
Patch Information
Microsoft addressed the flaw through the October 2025 security update cycle. Consult the Microsoft Security Update CVE-2025-55687 guidance for the KB article and package appropriate to each Windows build, and validate deployment by confirming updated refs.sys binaries on target hosts.
Workarounds
- No official workaround eliminates the flaw; only the vendor patch does. Where patching must be delayed, apply the following compensating controls.
- Enforce least-privilege access and remove unnecessary local user accounts from ReFS-hosting systems.
- Enable and enforce application control (Windows Defender Application Control or AppLocker) to block unauthorized binaries capable of triggering the race.
- Isolate ReFS servers on segmented networks and restrict administrative access through jump hosts with multifactor authentication.
# Verify installed updates on a Windows host (PowerShell)
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10
# Confirm the ReFS driver file version after patching
Get-Item C:\Windows\System32\drivers\refs.sys | Select-Object VersionInfo
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
