CVE-2025-29973: Azure File Sync Privilege Escalation Flaw

CVE-2025-29973 Overview

CVE-2025-29973 is an improper access control vulnerability [CWE-284] in Microsoft Azure File Sync. An authorized local attacker can exploit weaknesses in the agent's access controls to elevate privileges on the host system. Microsoft published the advisory on May 13, 2025, and the issue affects Azure File Sync agent versions 19.0.0.0 and 20.0.0.0.

The vulnerability requires local access and low privileges, but successful exploitation grants high impact to confidentiality, integrity, and availability. No public exploit code or proof-of-concept has been observed, and the flaw is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
A local attacker with a valid account on a server running the Azure File Sync agent can escalate to SYSTEM-level privileges, gaining full control over the affected file server and its synchronized cloud endpoints.

Affected Products

Microsoft Azure File Sync agent version 19.0.0.0
Microsoft Azure File Sync agent version 20.0.0.0
Windows Server hosts running vulnerable Azure File Sync agents

Discovery Timeline

2025-05-13 - CVE-2025-29973 published to NVD by Microsoft
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-29973

Vulnerability Analysis

CVE-2025-29973 stems from improper access control within the Azure File Sync agent installed on Windows Server. Azure File Sync centralizes file services in Azure Files while maintaining performance and compatibility of an on-premises file server. The agent runs privileged services and manipulates file system objects to synchronize data between the server and Azure Files shares.

The flaw allows a low-privileged local user to interact with resources that should be restricted to higher-privileged contexts. When the agent processes these resources, the attacker inherits elevated privileges. This maps to [CWE-284] Improper Access Control, indicating missing or misconfigured authorization checks on securable objects owned by the service.

The attack requires local access and low privileges. The high attack complexity indicates that exploitation depends on specific timing or environmental conditions rather than a straightforward invocation.

Root Cause

The root cause is insufficient enforcement of access controls on objects managed by the Azure File Sync agent. Microsoft's advisory does not disclose the specific object class, but improper access control in file-synchronization agents commonly involves service pipes, scheduled tasks, file paths, or registry keys with permissive ACLs that a standard user can influence before a SYSTEM-level process consumes them.

Attack Vector

An attacker authenticated to the file server with a low-privileged account manipulates a resource under the agent's control. When the SYSTEM-privileged Azure File Sync service acts on that resource, the attacker's payload executes or the attacker inherits elevated permissions. The attack does not require user interaction. Microsoft has not published exploitation code, and no proof-of-concept is available in public repositories.

See the Microsoft Security Update for CVE-2025-29973 for vendor guidance.

Detection Methods for CVE-2025-29973

Indicators of Compromise

Unexpected child processes spawned by FileSyncSvc.exe or associated Azure File Sync agent binaries
Modifications to Azure File Sync installation directories or registry keys by non-administrative accounts
New scheduled tasks, services, or persistence artifacts created by SYSTEM shortly after low-privileged user activity on a file server
Anomalous privilege assignments or token manipulation events on servers running Azure File Sync

Detection Strategies

Correlate process ancestry to identify SYSTEM processes spawned as a result of low-privileged user actions on Azure File Sync hosts
Monitor Windows Security event ID 4672 (special privileges assigned) for accounts that should not receive elevated tokens
Baseline expected agent behavior and alert on deviations such as unusual DLL loads or file writes outside the sync working directories

Monitoring Recommendations

Enable command-line and process creation auditing (event ID 4688) on all servers running the Azure File Sync agent
Forward endpoint telemetry to a centralized SIEM for cross-host correlation of privilege escalation patterns
Track version inventory of the Azure File Sync agent and alert when hosts remain on 19.0.0.0 or 20.0.0.0

How to Mitigate CVE-2025-29973

Immediate Actions Required

Apply the Azure File Sync agent update referenced in the Microsoft Security Update for CVE-2025-29973 to all affected servers
Inventory file servers running versions 19.0.0.0 and 20.0.0.0 and prioritize patching those exposed to standard user logons
Restrict interactive and remote logon rights on Azure File Sync hosts to administrative personnel only
Review audit logs on affected hosts for signs of prior local privilege escalation attempts

Patch Information

Microsoft released a security update addressing CVE-2025-29973 on May 13, 2025. Administrators should download the latest Azure File Sync agent from the Microsoft Update Catalog or through the Azure portal and apply it to every registered server. Refer to the Microsoft Security Update for CVE-2025-29973 for supported update paths and file hashes.

Workarounds

Limit local logon rights on Azure File Sync servers to trusted administrators until the patch is deployed
Enforce the principle of least privilege for service accounts and interactive users on file servers participating in sync groups
Apply application control policies to prevent execution of unauthorized binaries by non-administrative users on affected hosts

bash

# Verify installed Azure File Sync agent version on Windows Server
Get-ItemProperty -Path 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*' |
  Where-Object { $_.DisplayName -like 'Azure File Sync Agent*' } |
  Select-Object DisplayName, DisplayVersion, InstallDate

# List local users with interactive logon rights that should be reviewed
secedit /export /cfg C:\Temp\secpol.cfg
Select-String -Path C:\Temp\secpol.cfg -Pattern 'SeInteractiveLogonRight'