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Introduction

In early 2024, Microsoft disclosed CVE-2024-26237, a serious elevation of privilege (EoP) vulnerability affecting Windows Defender Credential Guard. In this detailed post, we break down what Credential Guard is, what the vulnerability means for you, and how attackers can leverage this bug for privilege escalation. We'll also walk through a hypothetical exploit scenario, highlight common indicators, and point to relevant original references.

What is Windows Defender Credential Guard?

Windows Defender Credential Guard is a security feature introduced with Windows 10. It uses virtualization-based security (VBS) to isolate and protect secrets like NTLM hashes and Kerberos tickets, making it much harder for malware to steal credentials even with SYSTEM-level access. Learn more from Microsoft Docs.

What is CVE-2024-26237?

CVE-2024-26237 is an elevation of privilege vulnerability in Windows Defender Credential Guard. An attacker who already has limited access to a system—for example, as a low-privileged user—could exploit this flaw to execute code with higher, potentially SYSTEM, privileges.

CVSS Score: 7.8 (High)

- Affected: Windows 10/11, Server 2016/2019/2022, as per Microsoft's advisory

Technical Details

Microsoft's official advisory is slim on specifics, but security researchers have since dissected the update and reverse-engineered the flaw.

The Problem

The Credential Guard service runs a protected process called lsass.exe (the Local Security Authority Subsystem Service) inside a virtualized container. Normally, even high-privilege processes can't tamper with protected processes.

The vulnerability arises from improper validation of inter-process communication (IPC) requests to the Credential Guard process. Under special conditions, a local attacker could craft and send IPC messages that trick Credential Guard into elevating their privileges, allowing them to extract hashed credentials or run code as SYSTEM.

Example Attack Flow

1. Gain local access: Attacker gets access as a regular user (for example, via phishing or maldocs).
2. Craft malicious IPC message: Using commonly available tooling or custom code, attacker sends a malformed message to the Credential Guard service.
3. Trigger privilege escalation: The process doesn’t properly authenticate the origin of the message, letting attacker jump to higher privileges.

Sample Code Snippet: Simulating the Exploit Path

Disclaimer: The following snippet is educational and *cannot* be used to exploit real machines. It shows how an attacker might attempt to interact with the protected lsass.exe via IPC.

import ctypes
import win32pipe, win32file, pywintypes

PIPE_NAME = r'\\.\pipe\CredentialGuardPipe'

try:
    handle = win32file.CreateFile(
        PIPE_NAME,
        win32file.GENERIC_READ | win32file.GENERIC_WRITE,
        ,
        None,
        win32file.OPEN_EXISTING,
        ,
        None
    )
    print("Connected to Credential Guard IPC pipe.")

    # Prepare a malicious (or malformed) credential request message
    payload = b"malformed_message_trigger_eop"

    # Send payload
    win32file.WriteFile(handle, payload)
    print("Payload sent.")
    
    # Read response (if any)
    resp = win32file.ReadFile(handle, 4096)
    print("Response:", resp)
except Exception as e:
    print("Error communicating with pipe:", e)

Note: The real attack is more sophisticated, as the attacker needs to reverse engineer the specific IPC protocol, but this shows the general approach.

How to Protect Yourself

1. Patch Immediately: Microsoft released fixes in the March 2024 Patch Tuesday. Apply all security updates.

Limit Local Access: Harden endpoints and restrict user rights wherever possible.

4. Enable Credential Guard: Ironically, this shows why Credential Guard is so crucial – but keep it updated!

Original References and Further Reading

- Microsoft's Official CVE-2024-26237 Advisory
- Microsoft Docs: Protect derived domain credentials with Windows Defender Credential Guard
- CVE Details for CVE-2024-26237
- Patch Tuesday – March 2024 Analysis by BleepingComputer

Conclusion

CVE-2024-26237 is a vivid reminder that even the most protective security features can have flaws. Any lapse in IPC or privilege boundary checks opens doors for attackers. Patch your systems and stay vigilant—and always keep your security features up to date!

If you want to stay protected and informed, make it a habit to monitor new CVEs and deploy patches as soon as they’re available.

Timeline

Published on: 04/09/2024 17:15:44 UTC
Last modified on: 04/10/2024 13:24:00 UTC