Tuesday, May 3, 2022

Satisfying CMMC IA.L2-3.5.3 MFA requirement with Windows Hello for Business

The Cybersecurity Maturity Model Certification (CMMC) is a set of certification standards produced by the United States Department of Defense and intended to serve as a verification mechanism to ensure that companies bidding on defense contracts have appropriate levels of cybersecurity practices and processes in place. The CMMC is a unified standard for implementing cybersecurity across the defense industrial base (DIB). The CMMC is the DoD's response to significant compromises of sensitive defense information located on contractors' information systems. 


Of particular interest is the following requirement: 


CMMC IA.L2-3.5.3 (NIST 800-171r2 3.5.3) - Use multifactor authentication for local and network access to privileged accounts and for network access to non-privileged accounts.  


  • Local Access - Access to an organizational information system by a user (or process acting on behalf of a user) communicating through a direct connection without the use of a network. 
  • Network Access - Access to an information system by a user (or a process acting on behalf of a user) communicating through a network (e.g., local area network, wide area network, Internet). 
  • Privileged User - A user that is authorized (and therefore, trusted) to perform security-relevant functions that ordinary users are not authorized to perform. 


Breaking down the above requirement means the following: 


  • All users are required MFA for network/remote access.  
  • Only privileged users are required MFA for local access (if regular user accounts have administrative rights only on their computers, they are not considered a “privileged account” and as such do not require MFA authentication for local access). 


Why Windows Hello for Business is a viable MFA authenticator 


Windows Hello for Business provides authentication methods intended to replace passwords, which can be difficult to remember and easily compromised. The Windows Hello provisioning process creates a cryptographic key pair bound to the Trusted Platform Module (TPM) and combines it with additional information to authenticate users. The additional information the user supplies is the activation factor for Windows Hello for Business and can be a PIN value (“something you know”) or, if the system has the necessary hardware, biometric information, such as fingerprint or facial recognition (“something you are”). The TPM constitutes the “something you have” factor for the purpose of MFALearn more about How Windows Hello for Business uses the TPM.  


The idea of TPM as a valid “something you have” factor is not new, and addressed by NIST 800-63B Section back in December 2017 (as captured in the errata) where TPM is recognized as a hardware cryptographic authenticator. Multi-factor cryptographic device authenticators use tamper-resistant hardware to encapsulate one or more secret keys unique to the authenticator and accessible only through the input of an additional factor, either a memorized secret or a biometric. The authenticator operates by using a private key that was unlocked by the additional factor to sign a challenge nonce presented through a direct computer interface (e.g., a USB port). Alternatively, the authenticator could be a suitably secure processor integrated with the user endpoint itself (e.g., a hardware TPM). 


Configuring Windows Hello in a way that adheres to NIST guidance


Now that we unveiled the mystery behind CMMC IA.L2-3.5.3 requirement and explained why Windows Hello for Business is viable MFA authenticator, let us make sure it is configured in a way that adheres to NIST guidance and provides the required strength: 


  • To adhere to NIST 800-63B Section requirementof activation factor (PIN) at least 8 characters long, configure minimum PIN length setting for PIN Complexity to be at least 8 characters (no complexity rules are required, PIN can be digits only). 
  • Make sure Windows Hello for Business cryptographic key are protected using a tamper-resistant hardware by enabling use a hardware security device setting for Windows Hello for Business. 
  • For securing privileged access, restrict privileged users to only access from secure workstations and require MFA for sign-in: 


For those who still do not accept TPM as “something you have” factor for local accesses (sign-in to the endpoint) or for those seeking additional risk mitigationan alternative option is using FIDO2 keys or smart cards. 



Please note that the information cutoff date for this post is February 12, 2021 and that as of the date of this writing, CMMC developments and guidance are in progress. Additionally, as of the date of this writing, the CMMC Accreditation Body (CMMC AB) has not formalized guidance for Cloud Service Providers. As a result, the information herein, including our CMMC related offerings, is provisional and may be enhanced to align with future guidance from the DoD and CMMC AB. Microsoft is closely tracking developments related to the CMMC.


Additional resources  

Achieving National Institute of Standards and Technology Authenticator Assurance Levels with the Mic... 

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