apf_portal/decisions/0011-mfa-enforcement-entra-conditional-access.md

status, date, decision-makers, tags

status	date	decision-makers	tags
accepted	2026-04-29	R&D Lead	security

MFA enforcement — Entra ID Conditional Access baseline, BFF claim sanity-check, step-up hooks designed-in

Context and Problem Statement

Multi-factor authentication is mandatory for the portal's workforce audience. The question is where MFA logic lives, how the BFF gains assurance that an authenticated session has actually been multi-factored, and how the application is prepared for future step-up MFA on sensitive operations — without writing any of it in v1.

Identity is Microsoft Entra ID (ADR-0008) and the auth flow is OIDC Authorization Code + PKCE via MSAL Node (ADR-0009). The v1 portal has no admin UI and no defined "sensitive" operations beyond authentication itself, so step-up MFA is a design concern, not a v1 implementation concern.

Decision Drivers

• MFA is a non-negotiable security baseline for workforce.
• Application code must not implement MFA mechanics — that belongs in the IdP. "Anti-bricolage" applies particularly here.
• The mechanism chosen at sign-in must be auditable in the session (was MFA actually performed?).
• The architecture must support future step-up MFA (a sensitive operation requires fresh MFA, beyond what was done at sign-in) without retrofitting authorization.
• Configuration responsibility must be clearly separated: the IdP (org IT) owns the policy; the BFF owns the verification.

Considered Options

Where MFA is enforced

• Microsoft Entra Conditional Access policy at the tenant level. (Chosen.)
• MFA logic in the BFF (e.g., a second-factor prompt after password).

How the BFF validates that MFA happened

• Sanity-check the amr claim of the id_token at session creation. (Chosen.)
• Require an explicit Authentication Context Class Reference (ACR) value via acr_values/claims request parameter.
• Trust Entra unconditionally without verification.

Step-up MFA in v1

• Designed-in via a @RequireMfa() decorator and a claims-challenge mechanism, but not applied to any v1 route. (Chosen.)
• Built and exercised in v1 — premature, no consumer.
• Deferred entirely (no decorator, no hook) — costly to retrofit.

Decision Outcome

MFA enforcement lives in Microsoft Entra Conditional Access. The org's IT/identity contact configures a Conditional Access policy at the tenant level that requires MFA for any sign-in to the BFF's app registration, for both the prod tenant and the M365 Developer tenant used for non-prod. Recommended settings (org-side, not in our code):

• Assignment. Cloud apps → the BFF app. Users → All users (or "All workforce", depending on how the org structures groups). Excluded: break-glass accounts, by Microsoft's own recommendation.
• Conditions (recommended, not strictly required): device compliance, location, sign-in risk (Entra ID P2 for risk-based — opt-in).
• Grant. Require multi-factor authentication. Prefer phishing-resistant methods (FIDO2 / passkeys / Windows Hello for Business). Fall back to authenticator-app push or TOTP. SMS/voice deprecated, to be avoided.

This policy is out of scope for application code. Our app does not implement MFA — it consumes the MFA outcome.

BFF verification. At session creation (after the OIDC callback exchange and id_token validation), the BFF performs a sanity-check on the amr claim of the id_token. The session is rejected with a 401 and an explicit error if the claim is absent, empty, or contains only password-class values without an MFA-class indicator. The expected normal value emitted by Entra after a CA-enforced MFA sign-in is ["pwd","mfa"] (or equivalent depending on the factor).

This is defense in depth, not the primary control: the primary control is the CA policy. The sanity-check exists so that a misconfigured or disabled CA policy does not silently regress the security posture without anyone noticing — the BFF will simply refuse to grant sessions if MFA assurance is missing.

A small validation table maintained in the BFF (in source, reviewed in PRs) lists the amr values accepted as multi-factor:

// Values Entra emits in 'amr' that we treat as evidence of MFA having occurred.
// Not exhaustive — extended on review when Entra emits new values.
const MFA_AMR_VALUES = ['mfa', 'otp', 'fido', 'wia', 'phr'] as const;

This list is reviewed against Microsoft's documentation as part of the security review cadence.

Step-up MFA — designed-in, dormant. A @RequireMfa() controller-method decorator and an associated guard are implemented in v1, but no v1 route uses them. The guard, when active, will:

1. Inspect the session for a recent MFA assertion (a mfaVerifiedAt timestamp added to the session payload, set to lastSeenAt of the auth callback).
2. If the timestamp is older than a configurable freshness window (MFA_FRESHNESS_SECONDS, default 600 = 10 min), respond with 401 + a WWW-Authenticate header carrying a claims challenge (an opaque blob produced by MSAL Node).
3. The Angular SPA HTTP interceptor catches the 401 + claims challenge, redirects the browser to /auth/login?claims=<challenge>, which forwards the challenge to Entra. Entra re-prompts MFA, returns a fresher amr/mfaVerifiedAt, and the original request is retried.

Authentication Context Classes (acr_values) are not used in v1. The freshness-timestamp approach is sufficient for the baseline. ACR-based step-up (e.g., "this action requires CA policy c2") will be considered later, gated behind a follow-up ADR if specific operations demand it.

Service accounts / app-only tokens. Out of scope. The BFF authenticates end-users via OIDC; service-to-service tokens (if needed for downstream APIs in a future ADR) follow a different flow — On-Behalf-Of or app-only — and have their own MFA story (i.e., none, because no human is involved).

Consequences

• Good, because MFA mechanics live where they belong (the IdP). The application is not in the business of implementing OTP delivery, push notifications, or hardware-key challenges.
• Good, because the IdP configuration can evolve independently — adding device compliance, sign-in risk, location-based rules, or moving to phishing-resistant-only — without code changes.
• Good, because the amr sanity-check in the BFF catches a category of misconfiguration that would otherwise silently regress security.
• Good, because step-up MFA is designed-in via a single decorator. When a sensitive operation appears in a future feature, marking it requires only @RequireMfa() — no architectural change.
• Good, because separating freshness-based step-up (default) from ACR-based step-up (advanced) keeps v1 simple while leaving a clean upgrade path.
• Bad, because the amr value list is implicitly tied to Entra's emitted values; if Microsoft introduces a new factor with a new amr token, the BFF rejects sessions until the list is updated. Mitigated by a documented review cadence.
• Bad, because step-up MFA needs SPA cooperation (the HTTP interceptor that handles 401 + claims challenge); if a future SPA feature bypasses the interceptor, step-up will silently degrade to plain 401. Mitigated by integration tests on the interceptor.
• Bad, because Conditional Access policies — especially advanced ones — require Entra ID P1 (and P2 for risk-based). Already flagged in ADR-0008, restated here.
• Neutral, because v1 does not exercise the step-up path. Hooks must be kept alive by automated tests, not by being used in production traffic.

Confirmation

• apps/portal-bff/src/auth/mfa.ts exports the MFA_AMR_VALUES constant and a wasMultiFactor(claims): boolean helper.
• The OIDC callback path rejects sessions for which wasMultiFactor(idTokenClaims) is false, with a structured 401 (logged as an audit event in the future audit-log ADR).
• Each session payload carries mfaVerifiedAt: number (epoch ms), set at the callback and refreshed on any subsequent step-up auth completion.
• apps/portal-bff/src/auth/require-mfa.guard.ts provides the RequireMfaGuard and the @RequireMfa() decorator (on top of the global AuthGuard from ADR-0009).
• MFA_FRESHNESS_SECONDS is read from env, defaulting to 600. The BFF refuses to start with a value below 60 (to catch misconfiguration).
• The Angular HTTP interceptor in portal-shell handles 401 + WWW-Authenticate: Bearer error="insufficient_user_authentication" (or the claims-challenge equivalent) and redirects to /auth/login with the challenge propagated.
• Integration tests cover: a sign-in without MFA assertion is rejected; a sign-in with valid MFA assertion succeeds; a route protected by @RequireMfa() accepts a fresh session, rejects with claims challenge after MFA_FRESHNESS_SECONDS, and accepts again after re-auth.
• The org-side Conditional Access policy is documented in an operational runbook (out of repo, owned by IT) and reviewed during onboarding of each tenant (prod + dev).

Pros and Cons of the Options

Where MFA is enforced

Conditional Access (chosen)

• Good, because it is the Microsoft-native, audited, IdP-managed mechanism — exactly the kind of "recognised, battle-tested" choice the project values.
• Good, because changes to MFA policy are a tenant-level config, not a deployment.
• Good, because the IdP catalogue of factors evolves without us — passkeys, FIDO2, certificate-based, etc.
• Bad, because it requires Entra ID P1 minimum at the tenant level (cost flagged in ADR-0008).

MFA logic in the BFF

• Bad, because it would re-implement OTP/push/key flows the IdP already provides — exactly the bricolage we are forbidden.
• Bad, because secret material (TOTP shared secrets, etc.) ends up in our Postgres, expanding the attack surface.

How the BFF validates MFA

amr sanity-check (chosen)

• Good, because cheap, immediate, no extra round trip.
• Good, because catches a real failure mode (CA policy disabled, scope misconfigured).
• Bad, because dependent on Entra's amr emission values — list maintenance required.

Required ACR via claims parameter

• Good, because explicit and machine-verifiable per request.
• Bad, because requires Authentication Context Classes to be configured tenant-side and exchanged on every request — heavy for a baseline "MFA at sign-in" need. Re-evaluate when fine-grained step-up arrives.

No verification

• Bad, because a CA policy disabled by mistake would silently regress security with no audit signal until an incident.

Step-up MFA in v1

Designed-in via decorator, dormant (chosen)

• Good, because the cost is minimal: one guard, one decorator, one timestamp in the session.
• Good, because step-up is a one-line annotation on any future sensitive route.
• Bad, because a dormant code path drifts unless covered by tests — addressed in the Confirmation section.

Built and exercised in v1

• Bad, because no v1 route is sensitive enough to require it. Building a feature with no consumer is bricolage by another name.

Deferred entirely

• Bad, because retrofitting step-up after the fact means refactoring guards, sessions, and the SPA interceptor. Cost asymmetric vs. the dormant-hook option.

More Information

• Microsoft Entra Conditional Access overview: https://learn.microsoft.com/entra/identity/conditional-access/overview
• amr claim values (OpenID Connect): https://www.iana.org/assignments/authentication-method-reference-values/
• Microsoft amr documentation: https://learn.microsoft.com/azure/active-directory/develop/access-tokens
• Authentication Context Classes (ACR) and claims challenges: https://learn.microsoft.com/entra/identity-platform/v2-conditional-access-dev-guide
• Phishing-resistant MFA (passkeys / FIDO2): https://learn.microsoft.com/entra/identity/authentication/concept-authentication-passwordless
• Related ADRs: ADR-0008 (identity model — P1 licensing flagged), ADR-0009 (auth flow), ADR-0010 (session — mfaVerifiedAt lives here), and the future ADRs for downstream APIs (On-Behalf-Of), audit trail (rejected-MFA logged events), and the operational runbook for tenant Conditional Access policies.