CVE-2024-21397: Azure File Sync Privilege Escalation

CVE-2024-21397 Overview

Microsoft Azure File Sync contains an Elevation of Privilege vulnerability that allows a local attacker with low-level privileges to potentially escalate their permissions on affected systems. This vulnerability is classified under CWE-59 (Improper Link Resolution Before File Access), also known as a symlink attack, which indicates the flaw involves improper handling of symbolic links that could be exploited for privilege escalation.

Critical Impact

Local attackers with limited privileges can exploit this vulnerability to gain elevated access, potentially compromising system integrity and causing limited availability impact on Microsoft Azure File Sync deployments.

Affected Products

• Microsoft Azure File Sync (all versions prior to patch)
• Microsoft Azure File Sync version 17.0.0.0

Discovery Timeline

• February 13, 2024 - CVE-2024-21397 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2024-21397

Vulnerability Analysis

This vulnerability affects Microsoft Azure File Sync, a service that enables organizations to centralize file shares in Azure Files while maintaining local access and caching capabilities. The flaw stems from improper link resolution before file access, a class of vulnerability that occurs when software incorrectly follows symbolic links during file operations.

The local attack vector requires an attacker to have initial access to the target system with low-level privileges. While the attack complexity is high, indicating that specific conditions must be met for successful exploitation, the potential impact includes high integrity compromise and low availability impact. Notably, this vulnerability does not affect confidentiality.

Azure File Sync installations that synchronize files between on-premises Windows Servers and Azure file shares are at risk. The service runs with elevated privileges to manage file synchronization operations, making it an attractive target for privilege escalation attacks.

Root Cause

The root cause of this vulnerability lies in CWE-59: Improper Link Resolution Before File Access. This class of vulnerability occurs when the Azure File Sync agent fails to properly validate or restrict symbolic links (symlinks) before accessing files. An attacker can craft malicious symbolic links that, when followed by the privileged synchronization process, allow operations on files or directories outside the intended scope.

This type of flaw typically manifests when software running with elevated privileges operates on user-controllable paths without adequate symlink validation, enabling local privilege escalation through file manipulation.

Attack Vector

The attack is conducted locally on a system running Microsoft Azure File Sync. An attacker with low-privilege access must exploit the vulnerability through a multi-step process that involves creating malicious symbolic links in locations monitored by the Azure File Sync service.

When the synchronization agent processes these crafted symlinks with its elevated privileges, the attacker can manipulate the agent into performing unauthorized file operations. This could include overwriting protected system files, modifying configuration files, or escalating privileges through file replacement attacks.

The high attack complexity indicates that successful exploitation requires specific timing, configuration states, or other environmental factors to be present. No user interaction is required once the attacker has local system access.

Detection Methods for CVE-2024-21397

Indicators of Compromise

• Unusual symbolic link creation within Azure File Sync monitored directories
• Unexpected file permission changes or file replacements in system directories
• Anomalous Azure File Sync agent behavior or crash patterns
• Suspicious process activity from FileSyncSvc.exe or related Azure File Sync components

Detection Strategies

• Monitor for symbolic link creation events in directories synchronized by Azure File Sync using Windows Security Event logs
• Implement file integrity monitoring on critical system files that could be targeted for replacement
• Deploy endpoint detection rules to identify privilege escalation attempts associated with Azure File Sync processes
• Analyze Azure File Sync service logs for unusual file access patterns or errors related to symlink resolution

Monitoring Recommendations

• Enable verbose logging for Azure File Sync agents to capture detailed file operation events
• Configure SIEM alerts for privilege escalation indicators on servers running Azure File Sync
• Monitor Windows Event Log channels for security events (Event ID 4663, 4656) related to Azure File Sync directories
• Implement application control policies to restrict unauthorized symbolic link creation in sensitive directories

How to Mitigate CVE-2024-21397

Immediate Actions Required

• Apply the security update from Microsoft as soon as possible
• Review Azure File Sync server configurations and limit local access to authorized administrators
• Audit user accounts with local access to systems running Azure File Sync
• Implement the principle of least privilege for accounts accessing Azure File Sync servers

Patch Information

Microsoft has released a security update to address CVE-2024-21397. Organizations should consult the Microsoft Security Response Center advisory for specific patch details, affected version information, and download links.

The patch should be applied to all systems running Microsoft Azure File Sync. Organizations using Azure File Sync should follow their standard patch management procedures and test updates in a staging environment before production deployment.

Workarounds

• Restrict local user access to Azure File Sync servers to only essential personnel
• Implement strict access controls on directories monitored by Azure File Sync
• Disable symlink support in synchronized directories where feasible using Windows Group Policy
• Consider temporary isolation of affected servers from sensitive network segments until patching is complete

# Verify Azure File Sync agent version on Windows Server
# Run in PowerShell to check current installed version
Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Azure\StorageSync\Agent" | Select-Object Version

# Check Azure File Sync service status
Get-Service -Name FileSyncSvc | Select-Object Status, Name, DisplayName