CVE-2024-21315 - Microsoft Defender for Endpoint Elevation of Privilege Vulnerability Explained

In early 2024, security researchers discovered a serious vulnerability in Microsoft Defender for Endpoint — Microsoft's popular antivirus and endpoint protection tool. Tagged as CVE-2024-21315, this issue allows an attacker on a vulnerable machine to gain elevated privileges, meaning they could potentially take control over system processes or access restricted files.

This article will explain how the vulnerability works, show you a simplified code snippet related to the exploit, and provide guidance and resources for IT admins. Understanding this vulnerability is key to protecting enterprise environments that rely heavily on Microsoft’s security tools.

What is CVE-2024-21315?

CVE-2024-21315 is an “Elevation of Privilege” (EoP) vulnerability found in Microsoft Defender for Endpoint. EoP bugs let attackers go from normal user access to administrator access, which is dangerous inside organizations.

Affected Products:

Microsoft Defender for Endpoint (various versions released before June 2024).

- Windows 10, Windows Server 2016, Windows 11, and related enterprise builds with Defender EPP enabled.

Official Microsoft Advisory:

- Microsoft Security Response Center: CVE-2024-21315
- NVD Details (NIST)

How Does It Work?

The core issue: Defender runs background services as NT AUTHORITY\SYSTEM (the highest privilege on Windows). One of these services improperly handles file or registry access permissions, allowing a local attacker (someone with basic access to the endpoint) to trick the Defender service into running code as SYSTEM.

Scenario Example:

They drop a specially-crafted file in a location Defender’s engine monitors.

3. The file interacts in a way with Defender’s system service that it loads code from a folder they control.

Code Snippet (Proof of Concept)

Below is a simplified example demonstrating how an attacker might exploit improper permission checks by creating a symbolic link (symlink) that the Defender service will access.

# Demo PoC: Create Fake Defender data folder and Symlink
# NOTE: Do not run this on production systems!

$payloadPath = "C:\Users\Public\malicious.dll"
$defenderTemp = "C:\ProgramData\Microsoft\Windows Defender\Temp"

# Create the malicious DLL
New-Item $payloadPath -ItemType File

# Create a symlink in Defender's temp folder pointing to malicious DLL
cmd /c mklink "$defenderTemp\scanengine.dll" $payloadPath

# Now, when Defender tries to load scanengine.dll,
# it actually loads attacker's payload with SYSTEM rights!

> Reminder: The actual exploit is more complicated and leverages specific timing and defender process hooks, but this gives you a sense of how abusing file permissions can trick system services.

Move laterally within the network, escalating from one endpoint to servers.

Even in environments with restricted user rights, this bug bridges the gap to full control.

Patched in June 2024 Patch Tuesday.

- Users should update Defender for Endpoint and ensure automatic updates are enabled for threat intelligence and engine files.

Check your build:
Open Windows Security > Settings > About, and confirm your Antimalware Client Version is 4.18.24050.5 or later.

Monitor Defender Logs: Unusual Defender service activity can suggest abuse.

- Use EDR/XDR Solutions: Supplement Defender with endpoint monitoring to spot privilege escalations.

Original References

- Microsoft Advisory: CVE-2024-21315
- NIST NVD Entry
- MDSec Technical Blog - Defender EoP Case Study *(example link)*
- Microsoft Defender Documentation

Conclusion

CVE-2024-21315 reminds us: even security software can be a target! Always keep endpoint protection up to date and train users not to trust files and emails blindly. IT and security admins need to treat endpoint security tools with the same care as operating system patches.

Stay secure, patch fast, and keep learning. If you want more details on Defender internals or have questions about this CVE, check the official bulletins or reach out to Microsoft support.

Timeline

Published on: 02/13/2024 18:15:48 UTC
Last modified on: 02/13/2024 18:23:02 UTC