Windows environments depend on tight security, especially when it comes to protecting user credentials. Microsoft Defender Credential Guard is supposed to help, by isolating and protecting secrets with virtualization-based security. But sometimes even the most robust security features can be poked full of holes — and CVE-2022-21921 is a clear example of just that.

In this long read, we’ll break down CVE-2022-21921 in simple terms, explain how it works, show what an exploit could look like, and what you can do to defend your systems. Along the way, we’ll include links and references for further reading. Let’s dive in.

What is Windows Defender Credential Guard?

Credential Guard is a Windows security feature that leverages virtualization-based security (VBS) to isolate NTLM, Kerberos, and other secrets so that only privileged system software can access them. This helps prevent credential theft attacks like *Pass-the-Hash* and *Pass-the-Ticket* by attackers who have already gained a foothold on a system.

Prevents direct access to credential material like domain hashes or Kerberos TGTs.

You can learn more about Credential Guard on the official Microsoft docs:
https://learn.microsoft.com/en-us/windows/security/identity-protection/credential-guard/credential-guard

Description (in plain English)

CVE-2022-21921 is a security feature bypass discovered in Windows Defender Credential Guard. It allows a local attacker to break out of the isolation that protects credential secrets. Essentially, this means that an attacker with the right privileges could grab sensitive credential material that was supposed to be locked away by Credential Guard.

Why is this scary? Because if attackers can get their hands on hashes or tickets, *they can move laterally across the network*, impersonate users, and potentially gain access to high-value targets. In the worst-case scenario, this flaw could help attackers elevate their control from a single compromised machine to a broader enterprise domain.

Microsoft’s advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2022-21921

How Does the Exploit Work?

*Please note: The information here is for educational purposes only.*

Microsoft was light on details in their advisories, but code analysis and community research highlighted that the bypass was possible because of improper validation when handling VBS-protected objects. This allowed a standard user or a malicious process to convince the system to leak credential data.

The attacker already needs code execution on the victim machine.

- Through a crafted system call or misuse of a privileged API, the attacker tricks Windows into accessing protected secrets outside the Credential Guard container.

Here’s a generalized (and *safe*) PowerShell example for checking if Credential Guard is enabled

# PowerShell to check if Credential Guard is enabled
$key = "HKLM:\SYSTEM\CurrentControlSet\Control\Lsa"
(Get-ItemProperty -Path $key -Name LsaCfgFlags).LsaCfgFlags

If the value is 1 or 2, Credential Guard is enabled. Before CVE-2022-21921 was fixed, certain crafted system calls could bypass this protection.

A *hypothetical* C-based snippet for attempting to access LSASS process memory (which should be blocked by Credential Guard) might look like:

#include <windows.h>

int main() {
    HANDLE hProcess = OpenProcess(PROCESS_ALL_ACCESS, FALSE, /* target LSASS PID */);
    if (hProcess == NULL) {
        printf("Access denied (as expected with Credential Guard).\n");
    } else {
        printf("Access allowed! Potential credential theft possible.\n");
    }
    return ;
}

If an exploit to CVE-2022-21921 is successful, the above code could unexpectedly open a handle, allowing an attacker to dump credentials.

Proof-of-Concept and Exploit Discussion

Exploit details were reportedly shared privately with Microsoft and are typically not discussed in full detail.
However, security researchers noted that the root cause involved a kernel-level mishap with how "guarded" processes were handled.

For a deeper technical dive, see this third-party writeup:
https://ssd-disclosure.com/ssd-advisory-microsoft-windows-defender-credential-guard-security-feature-bypass/

And tweets by @jonasLyk, who originally reported the vulnerability to Microsoft.

Mitigation and Patch Info

Microsoft patched the flaw in January 2022 Patch Tuesday.

Audit endpoints to make sure Credential Guard works as expected after updating.

> Patch link:
> https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2022-21921

Additionally, monitor event logs for LSASS access attempts and restrict local admin access whenever possible.

Final Thoughts

Credential Guard is a strong protection—but it’s not magic. CVE-2022-21921 shows how a determined attacker with enough system knowledge can bypass even advanced Windows defenses. Always patch quickly, monitor your endpoints, and know that no security layer is ever truly bulletproof.

Further References

- Microsoft’s official note: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2022-21921
- Original discovery: https://twitter.com/jonasLyk/status/1479583081770481666
- SSD Disclosure: https://ssd-disclosure.com/ssd-advisory-microsoft-windows-defender-credential-guard-security-feature-bypass/
- Microsoft’s Defense-in-depth: https://learn.microsoft.com/en-us/windows/security/threat-protection/windows-defender-credential-guard

Stay Secure!
Knowledge is your best defense — so keep up to date, patch regularly, and never underestimate what a crafty attacker can do.

Timeline

Published on: 01/11/2022 21:15:00 UTC
Last modified on: 05/23/2022 17:29:00 UTC