NIST authenticator assurance level 3 by using Azure Active Directory

Article
03/03/2023
5 minutes to read

Use the information in this article for National Institute of Standards and Technology (NIST) authenticator assurance level 3 (AAL3).

Before obtaining AAL2, you can review the following resources:

NIST overview: Understand AAL levels
Authentication basics: Terminology and authentication types
NIST authenticator types: Authenticator types
NIST AALs: AAL components and Azure Active Directory (Azure AD) authentication methods

Permitted authenticator types

Use Microsoft authentication methods to meet required NIST authenticator types.

Azure AD authentication methods	NIST authenticator type
Recommended methods
FIDO 2 security key, or Smart card (Active Directory Federation Services), or Windows Hello for Business with hardware TPM	Multi-factor cryptographic hardware
Additional methods
Password and Hybrid Azure AD joined with hardware TPM or, Azure AD joined with hardware TPM)	Memorized secret and single-factor cryptographic hardware
Password and single-factor one-time password OTP hardware, from an OTP manufacturer and Hybrid Azure AD joined with software TPM or Azure AD joined with software TPM or Compliant managed device)	Memorized secret and single-factor OTP hardware and single-factor cryptographic software

Recommendations

For AAL3, we recommend using a multi-factor cryptographic hardware authenticator. Passwordless authentication eliminates the greatest attack surface, the password. Users have a streamlined authentication method. If your organization is cloud based, we recommend you use FIDO 2 security keys.

Note

Windows Hello for Business is not validated at the required FIPS 140 Security Level. Federal customers should conduct risk assessments and evaluation before accepting this service as AAL3.

For guidance, see Plan a passwordless authentication deployment in Azure Active Directory. See also Windows Hello for Business deployment guide.

FIPS 140 validation

Verifier requirements

Azure AD uses the Windows FIPS 140 Level 1 overall validated cryptographic module for its authentication cryptographic operations, making Azure AD a compliant verifier.

Authenticator requirements

Single-factor and multi-factor cryptographic hardware authenticator requirements.

Single-factor cryptographic hardware

Authenticators are required to be:

FIPS 140 Level 1 Overall, or higher
FIPS 140 Level 3 Physical Security, or higher

Azure AD joined and Hybrid Azure AD joined devices meet this requirement when:

You run Windows in a FIPS-140 approved mode
On a machine with a TPM that's FIPS 140 Level 1 Overall, or higher, with FIPS 140 Level 3 Physical Security
- Find compliant TPMs: search for Trusted Platform Module and TPM on Cryptographic Module Validation Program.

Consult your mobile device vendor to learn about their adherence with FIPS 140.

Multi-factor cryptographic hardware

Authenticators are required to be:

FIPS 140 Level 2 Overall, or higher
FIPS 140 Level 3 Physical Security, or higher

FIDO 2 security keys, smart cards, and Windows Hello for Business can help you meet these requirements.

FIDO2 key providers are in FIPS certification. We recommend you review the list of supported FIDO2 key vendors. Consult with your provider for current FIPS validation status.
Smart cards are a proven technology. Multiple vendor products meet FIPS requirements.
- Learn more on Cryptographic Module Validation Program

Windows Hello for Business

FIPS 140 requires the cryptographic boundary, including software, firmware, and hardware, to be in scope for evaluation. Windows operating systems can be paired with thousands of combinations of hardware. Microsoft can't maintain FIPS certifications for each combination.

Evaluate the following component certifications in your risk assessment of using Windows Hello for Business as an AAL3 authenticator:

Windows 10 and Windows Server use the US Government Approved Protection Profile for General Purpose Operating Systems Version 4.2.1 from the National Information Assurance Partnership (NIAP). This organization oversees a national program to evaluate commercial off-the-shelf (COTS) information technology products for conformance with the international Common Criteria.
Windows Cryptographic Library has FIPS Level 1 Overall in the NIST Cryptographic Module Validation Program (CMVP), a joint effort between NIST and the Canadian Center for Cyber Security. This organization validates cryptographic modules against FIPS standards.
Choose a Trusted Platform Module (TPM) that's FIPS 140 Level 2 Overall, and FIPS 140 Level 3 Physical Security. Your organization ensures hardware TPM meets the AAL level requirements you want.

To determine the TPMs that meet current standards, go to NIST Computer Security Resource Center Cryptographic Module Validation Program. In the Module Name box, enter Trusted Platform Module for a list of hardware TPMs that meet standards.

Reauthentication

For AAL3, NIST requirements are reauthentication every 12 hours, regardless of user activity. Reauthentication is required after a period of inactivity 15 minutes or longer. Presenting both factors is required.

To meet the requirement for reauthentication, regardless of user activity, Microsoft recommends configuring user sign-in frequency to 12 hours.

Use NIST for compensating controls to confirm subscriber presence:

Set a session inactivity time out of 15 minutes: Lock the device at the OS level by using Microsoft Configuration Manager, Group Policy Object (GPO), or Intune. For the subscriber to unlock it, require local authentication.
Set timeout, regardless of activity, by running a scheduled task using Configuration Manager, GPO, or Intune. Lock the machine after 12 hours, regardless of activity.

Man-in-the-middle resistance

Communications between the claimant and Azure AD are over an authenticated, protected channel for resistance to man-in-the-middle (MitM) attacks. This configuration satisfies the MitM resistance requirements for AAL1, AAL2, and AAL3.

Verifier impersonation resistance

Azure AD authentication methods that meet AAL3 use cryptographic authenticators that bind the authenticator output to the session being authenticated. The methods use a private key controlled by the claimant. The public key is known to the verifier. This configuration satisfies the verifier-impersonation resistance requirements for AAL3.

Verifier compromise resistance

All Azure AD authentication methods that meet AAL3:

Use a cryptographic authenticator that requires the verifier store a public key corresponding to a private key held by the authenticator
Store the expected authenticator output by using FIPS-140 validated hash algorithms

For more information, see Azure AD Data Security Considerations.

Replay resistance

Azure AD authentication methods that meet AAL3 use nonce or challenges. These methods are resistant to replay attacks because the verifier can detect replayed authentication transactions. Such transactions won't contain the needed nonce or timeliness data.

Authentication intent

Authentication makes it more difficult for directly connected physical authenticators, like multi-factor cryptographic devices, to be used without the subject's knowledge. For example, by malware on the endpoint.

Use NIST compensating controls for mitigating malware risk. Any Intune-compliant device that runs Windows Defender System Guard and Windows Defender ATP meets this mitigation requirement.