How do you create custom authentication strengths in Microsoft Entra Conditional Access?

What a custom authentication strength actually is

A custom authentication strength is a policy-defined bundle of authentication methods that conditional access treats as acceptable for a given access decision. Instead of relying only on Microsoft’s built-in strength categories, you define which methods satisfy your own trust requirement, then assign that strength to apps, users, or scenarios where the baseline needs to be tighter or more specific.

This is most useful when one-size-fits-all policy is too blunt. A sensitive workload may justify only phishing-resistant methods, while lower-risk access may allow a wider set. The strength becomes the control point, and Conditional Access becomes the enforcement layer that applies it consistently.

Conditional Access Guide covers how policy design, report-only testing, and authentication-strength choices fit together in a working rollout.

How to define the method set without creating policy drift

The practical design step is to decide which authentication methods you will treat as strong enough for the use case, then keep that set intentionally small. A custom strength should reflect a real trust model, not every method your tenant happens to support. If you include too many options, the strength stops telling you anything useful about assurance.

Good design usually starts with the sensitive application or user population, not with the method list. From there, map the methods that you are willing to trust for that context, and exclude weaker or higher-friction methods when the business impact of compromise would be material. Document the exception cases so the policy remains understandable during review and incident response.

Phishing-Resistant MFA Guide is the best companion when your custom strength needs to distinguish truly resistant methods from merely convenient ones.

NIST SP 800-63 Digital Identity Guidelines is the right external reference when you want to align method selection with authenticator assurance and phishing resistance.

How Conditional Access uses the strength in practice

Once defined, the custom strength is assigned in a Conditional Access policy as the acceptable authentication requirement. That means the policy does not merely ask for “MFA”, it asks for the specific set of methods you have approved. This is how you express different trust levels for different apps, groups, or sign-in conditions without duplicating unrelated policy logic.

In practice, the policy should be tested in report-only mode before enforcement, then reviewed for unintended user lockout, fallback behavior, and compatibility with the apps that actually depend on it. If your method set is too narrow, you may create support pressure or exception sprawl. If it is too broad, you dilute the whole point of having a custom strength.

Identity Provider and SSO Security Guide helps when you need the sign-in and session layer around Conditional Access to be hardened as well.

Risk and Threat Considerations

Custom authentication strengths can improve control, but they can also create false confidence if the method set is broad, poorly reviewed, or copied across unrelated applications. The main risk is treating a labelled strength as stronger than the actual methods behind it. If weaker methods remain eligible, an attacker only needs one acceptable path to satisfy the policy.

Failure mechanism: Policy drift, over-permissive method sets, or exception creep lets low-assurance sign-ins continue to satisfy a control that was intended to raise the bar. That weakens protection against phishing, credential replay, and help-desk driven account recovery abuse.

Impact: A compromised account may still pass Conditional Access, which turns the custom strength into a governance label instead of an effective barrier. In sensitive environments, that can preserve access to high-value apps even after the trust model has changed.

Practitioner Guidance

What to verify: Check that each method included in the strength is acceptable for the actual risk of the protected app, and confirm that excluded methods cannot still be used through alternate policy paths or legacy access routes.

Implementation sequence: Define the smallest defensible method set, assign it to one sensitive policy first, run it in report-only mode, then tighten or expand only after reviewing sign-in outcomes and exception volume.

Common mistake: Teams often design the strength around convenience or broad tenant support, then assume the resulting label guarantees high assurance. The label only matters if the underlying methods are consistently strong and periodically revalidated.

Practitioner takeaway: A custom authentication strength is only useful when it expresses a narrow, intentional trust decision, and the value comes from disciplined review of the methods you allow, not from the name of the strength itself.

Related resources from NHI Mgmt Group

• How do you block legacy authentication with Conditional Access in Microsoft Entra ID?
• How do Conditional Access authentication strengths work for external users in Microsoft Entra ID?
• How do you apply Conditional Access policies to AI agents in Microsoft Entra?
• How do you secure AI agent identities with Microsoft Entra Conditional Access?