How should organisations use PKI with MFA and biometrics to reduce unauthorized access?

Why This Matters for Security Teams

PKI, MFA, and biometrics solve different parts of the access problem, and organizations often fail when they treat any one of them as sufficient on its own. A certificate can prove that a device or system key is trusted, but it does not automatically prove that the person using it is authorised. MFA raises the bar by requiring more than one factor, and biometrics can add stronger user assurance when implemented with proper liveness and anti-spoofing checks. For security teams, the real objective is to prevent a stolen credential, cloned certificate, or compromised endpoint from becoming a single point of entry.

This matters most in telecom and other high-value environments where access decisions must reflect both device trust and user trust. Current guidance suggests mapping each control to a distinct purpose: PKI for device or workload identity, MFA for session initiation, and biometrics where user presence needs to be verified with higher assurance. That is consistent with the control intent in NIST SP 800-53 Rev 5 Security and Privacy Controls, which treats authentication and access control as layered safeguards rather than isolated products.

In practice, many security teams encounter certificate trust failures only after a stolen endpoint or replayed credential has already been used to gain access, rather than through intentional design.

How It Works in Practice

The most effective pattern is to separate device authentication from user authentication. PKI validates the endpoint, service, or workload through a certificate chain anchored in trusted infrastructure. MFA then verifies the human session with a second factor, and biometrics can serve as one factor where the business risk justifies stronger identity assurance. The key design principle is that certificate possession should not automatically grant interactive access.

Operationally, teams should define where each control sits in the flow:

• Use certificates for device registration, mutual TLS, or workload authentication.
• Require MFA at login, privileged action, or step-up access points.
• Use biometrics only where the matching process includes anti-spoofing, secure template storage, and fallback paths.
• Bind session policy to device posture, user risk, and transaction sensitivity.

Where biometrics are involved, governance matters as much as technology. Biometric data is sensitive personal data in many jurisdictions, so storage, retention, purpose limitation, and user notice must be designed carefully under the EU General Data Protection Regulation (GDPR). Where digital identity assurance is regulated, eIDAS 2.0 also signals that identity proofing and authentication strength should be proportionate to the use case, especially for cross-border trust.

For non-human and machine identities, the same logic applies. Certificates, tokens, and secrets used by systems should be inventoried and governed so they do not become silent bypass paths. That concern aligns closely with the OWASP Non-Human Identity Top 10, which highlights the risk of over-privileged or poorly governed machine credentials. These controls tend to break down when legacy applications cannot separate device trust from user trust because the authentication stack was built around a single sign-on decision.

Common Variations and Edge Cases

Tighter authentication often increases friction, recovery overhead, and privacy obligations, requiring organisations to balance stronger assurance against operational usability. Not every environment can use biometrics as a primary factor, and best practice is evolving on when biometrics should be mandatory versus optional. There is no universal standard for this yet, especially where workforce populations, customer populations, and regulated access paths have different risk profiles.

In some deployments, PKI is used only for device trust while MFA remains the main user control. That is often the right choice when certificates can be rotated reliably and biometric enrolment would create unnecessary legal or accessibility issues. In high-assurance settings, biometrics may be appropriate for privileged actions, but only if a secure fallback exists for failed matches, injured users, or degraded sensors. Organisations should avoid forcing biometric systems to carry the entire assurance burden, because a spoofed face, weak liveness check, or insecure template store can undermine the intended gain.

The practical takeaway is to treat PKI, MFA, and biometrics as complementary controls within a broader access policy, not as interchangeable substitutes. Where device identity, human identity, and non-human identity all coexist, policy should define which trust signal is authoritative for each action and which signals only raise confidence. That is the difference between layered assurance and control overlap that looks strong on paper but leaves the real access path exposed.

Related resources from NHI Mgmt Group

• Should organisations use PKI or FIDO for passwordless access?
• How should organisations use MFA and SSO together for enterprise access?
• Should organisations use zero-knowledge biometrics for passwordless access?
• How can organisations reduce the blast radius of agentic access?