apf_portal/docs/decisions/0009-auth-flow-oidc-pkce-msal-node.md

---
status: accepted
date: 2026-04-29
decision-makers: R&D Lead
tags: [security, backend]
---

# Authentication flow — OIDC Authorization Code + PKCE via MSAL Node, BFF session pattern

## Context and Problem Statement

[ADR-0008](0008-identity-model-entra-workforce-dual-audience.md) fixed the identity model: multi-tenant Entra ID for workforce, dual-audience design, M365 Developer tenant for non-prod. We now need to fix the _technical flow_: which OAuth/OIDC mechanism, which library, how tokens are obtained and held, how the session cookie is shaped, how CSRF is handled, how logout works.

The SPA must never hold tokens — that is the BFF security pattern, recommended by Microsoft and by the OAuth 2.0 BCP for browser-based apps (`draft-ietf-oauth-browser-based-apps`). The BFF holds the tokens server-side; the browser only carries an opaque session cookie.

## Decision Drivers

- Conform to the current IETF best current practice for browser-based apps (BFF pattern).
- Conform to Microsoft's recommended path for multi-tenant Entra workforce apps.
- Tokens never leave the server — the SPA cannot leak them via XSS.
- Support a future On-Behalf-Of flow when the BFF needs to call downstream Entra-protected APIs (covered by a later ADR).
- Defense in depth: hardened cookies, CSRF protection, refresh-token rotation.
- Consistent route shape so frontend, backend, and operators speak the same vocabulary.

## Considered Options

### Library

- **`@azure/msal-node`** (`ConfidentialClientApplication`). (Chosen.)
- `openid-client` — generic OIDC client.
- `passport-azure-ad` — deprecated, in maintenance only.

### OAuth flow

- **Authorization Code Flow with PKCE.** (Chosen.)
- Implicit Flow — deprecated by IETF, rejected.
- Hybrid Flow — legacy, rejected.
- Resource Owner Password Credentials — forbidden by Microsoft for production scenarios.

### Token storage

- **Tokens held in the BFF session, never sent to the browser.** (Chosen — the BFF pattern itself.)
- Tokens in browser memory or localStorage / sessionStorage — XSS-exfiltrable, rejected.

### Session cookie shape

- **`__Host-portal_session`** with `HttpOnly`, `Secure`, `SameSite=Lax`. (Chosen.)
- Same attributes without the `__Host-` prefix — weaker (subdomain bleed, downgrade attacks).

### CSRF

- **Double-submit cookie pattern.** (Chosen.) `__Host-portal_csrf` cookie (readable by the SPA) + matching `X-CSRF-Token` header on every state-changing request.
- Synchronizer token (server-side store) — heavier, requires a dedicated CSRF token store.
- No CSRF protection — unacceptable.

### Logout

- **RP-initiated logout.** (Chosen.) BFF invalidates the session, then redirects the browser to Entra's `end_session_endpoint`, which clears the Entra SSO session and redirects back.
- Local-only logout — the Entra SSO session stays live, the user can be silently re-authenticated without intent.

## Decision Outcome

The end-to-end flow at a glance:

```mermaid
sequenceDiagram
  autonumber
  actor U as User (browser)
  participant SPA as portal-shell
  participant BFF as portal-bff
  participant E as Microsoft Entra ID
  participant R as Redis (session store)

  U->>SPA: clicks "Sign in"
  SPA->>BFF: GET /auth/login?returnTo=…
  BFF->>BFF: generate state + nonce<br/>+ code_verifier + code_challenge
  BFF-->>U: 302 → Entra /authorize<br/>(client_id, code_challenge, state, nonce, scope)
  U->>E: authenticate<br/>(Conditional Access enforces MFA)
  E-->>U: 302 → BFF /auth/callback?code&state
  U->>BFF: GET /auth/callback?code&state

  BFF->>BFF: verify state matches<br/>(CSRF defence on the OIDC roundtrip)
  BFF->>E: POST /token<br/>(code + code_verifier + client_secret/cert)
  E-->>BFF: id_token + access_token + refresh_token

  BFF->>BFF: validate id_token<br/>(sig, iss in allowlist, aud, exp/nbf)<br/>+ sanity-check amr (ADR-0011)<br/>+ map audience claim → Audience enum
  BFF->>R: SET session:{opaque_id}<br/>payload incl. tokens encrypted AES-256-GCM (ADR-0010)
  BFF-->>U: 302 → returnTo<br/>+ Set-Cookie __Host-portal_session<br/>+ Set-Cookie __Host-portal_csrf
  U->>SPA: render destination (cookies attached on subsequent calls)
```

The numbered steps line up with the prose below: library / flow / tokens / token validation / refresh / cookies / CSRF / routes / auth layer.

**Library.** `@azure/msal-node`, instance of `ConfidentialClientApplication`, configured with the multi-tenant authority and the tenant allowlist from ADR-0008. PKCE is used despite the confidential-client setup, per IETF current BCP.

**Flow.** OAuth 2.0 Authorization Code Flow with PKCE, executed entirely on the BFF. The SPA never sees `code`, `code_verifier`, or any token.

**Tokens.** ID/access/refresh tokens are stored in the server-side session (storage backend covered by the next ADR). The browser only holds an opaque session identifier in `__Host-portal_session`.

**Token validation.** On callback, MSAL Node validates the `id_token` (signature against JWKS, `iss`, `aud`, `exp`, `nbf`). The BFF additionally enforces:

- `iss` belongs to the tenant allowlist (workforce tenants accepted by ADR-0008);
- `aud` matches our app's `client_id`;
- the audience claim is mapped to our `Audience` enum (workforce tokens → `audience: 'workforce'`; any other classification fails).

**Refresh.** Refresh-token rotation is enabled. MSAL Node's `acquireTokenSilent` is used to refresh access tokens transparently. When the refresh token is itself expired or revoked, the BFF returns 401 to the SPA, which redirects to `/auth/login`.

**Cookies.**

| Cookie                  | Purpose                     | Attributes                                                                                   |
| ----------------------- | --------------------------- | -------------------------------------------------------------------------------------------- |
| `__Host-portal_session` | opaque session id           | `HttpOnly`, `Secure`, `SameSite=Lax`, `Path=/` (forced by the `__Host-` prefix), no `Domain` |
| `__Host-portal_csrf`    | CSRF token (readable by JS) | `Secure`, `SameSite=Lax`, `Path=/`, no `HttpOnly` (the SPA must read it)                     |

`SameSite=Lax` (not `Strict`) is required because the Entra → BFF callback is a cross-site top-level redirect; `Lax` allows the cookie on top-level navigation, which is exactly what we need. The threat model trade-off is acceptable.

In **local development** (HTTP `localhost`), browsers reject the `__Host-` prefix because it requires `Secure`. The dev environment runs over **HTTPS via mkcert**-issued local certificates so cookie names and attributes match production exactly. There is no fallback to non-prefixed cookies in dev — keeping a single behaviour avoids "works in dev, breaks in prod" surprises.

**CSRF.** Every non-`GET` request to the BFF must carry an `X-CSRF-Token` header whose value equals the value of the `__Host-portal_csrf` cookie. A NestJS interceptor enforces this on every state-changing endpoint and rejects with 403 otherwise. The Angular SPA ships an `HttpInterceptor` that reads the cookie and injects the header on outgoing requests.

**Routes.** All authentication endpoints live under the `/auth` prefix on the BFF:

| Method | Path             | Purpose                                                                                                                                                                                                      |
| ------ | ---------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `GET`  | `/auth/login`    | starts the flow; accepts an optional `returnTo` query parameter validated against an in-app path allowlist; redirects the browser to Entra's `authorize` endpoint with `state`, `nonce`, and PKCE parameters |
| `GET`  | `/auth/callback` | receives `code` + `state` from Entra; verifies state, exchanges via MSAL Node, validates id_token, creates the session, redirects to `returnTo` (or `/`)                                                     |
| `POST` | `/auth/logout`   | requires `X-CSRF-Token`; invalidates the BFF session; returns 200 with the Entra `end_session_endpoint` URL in JSON; the SPA navigates the browser to that URL                                               |
| `GET`  | `/auth/me`       | returns the current user's id, audience, and a curated subset of claims for the SPA to display (no tokens)                                                                                                   |

**Authorization layer.** A NestJS `AuthGuard` checks for a valid session and rejects with 401 otherwise. A `@CurrentUser()` decorator extracts `{ id, audience, claims }` from the request scope. Every controller (other than `/auth/*` itself, `/health`, etc.) is protected by `AuthGuard` by default — the framework default is "denied", explicit allow-listing is required.

**Configuration.** All Entra-specific values come from environment variables — no tenant ID, client ID, or secret in source. The BFF refuses to start if any required variable is missing.

| Variable                                            | Purpose                                                                             |
| --------------------------------------------------- | ----------------------------------------------------------------------------------- |
| `ENTRA_TENANT_ID`                                   | home tenant id of the BFF (the tenant where the app is registered as the home org)  |
| `ENTRA_CLIENT_ID`                                   | app's client id                                                                     |
| `ENTRA_CLIENT_SECRET` _or_ `ENTRA_CLIENT_CERT_PATH` | confidential-client credential (certificate preferred for production)               |
| `ENTRA_ACCEPTED_TENANT_IDS`                         | comma-separated allowlist for `iss` validation                                      |
| `ENTRA_REDIRECT_URI`                                | absolute URL of the `/auth/callback` endpoint, registered in the app registration   |
| `ENTRA_POST_LOGOUT_REDIRECT_URI`                    | absolute URL the user lands on after Entra logout                                   |
| `SESSION_SECRET`                                    | symmetric secret for cookie signing (rotated procedure covered in a future ops ADR) |

### Consequences

- Good, because the SPA can never leak tokens — the worst an XSS can do is hijack the session cookie, and `HttpOnly` blocks even that.
- Good, because PKCE protects the code-exchange step against authorization-code interception even on the BFF side.
- Good, because the route shape and configuration surface are simple, predictable, and entirely documented.
- Good, because MSAL Node will let us add the On-Behalf-Of flow in the downstream-API ADR with minimal extra work.
- Good, because RP-initiated logout produces a clean cross-app state; users are not silently re-authenticated against their will.
- Good, because the dual-audience claim mapping is enforced at the validation step — workforce vs customer is a server-side decision, not a client-side hint.
- Bad, because the codebase becomes tightly coupled to MSAL Node and Entra. Switching IdP later means more than swapping a library — but the trade-off is accepted given identity is already locked to Entra.
- Bad, because `SameSite=Lax` (instead of `Strict`) is necessary for the callback to work; this is a known and accepted trade-off, mitigated by HTTPS, HSTS, the `__Host-` prefix, and CSRF.
- Bad, because every state-changing call from the SPA must carry `X-CSRF-Token` — small DX overhead, mitigated by a single Angular HTTP interceptor.
- Bad, because mkcert is required in dev to keep cookie behaviour identical to prod — slightly higher onboarding cost; documented in the dev setup guide.
- Bad, because the tenant allowlist (`ENTRA_ACCEPTED_TENANT_IDS`) must be maintained operationally as new B2B partners are onboarded — this is an operational item, not a one-shot.

### Confirmation

- `apps/portal-bff/src/auth/auth.module.ts` provides a single `ConfidentialClientApplication` configured from env, with PKCE enabled.
- `apps/portal-bff/src/auth/auth.controller.ts` exposes the four routes above and no others.
- `AuthGuard` is registered globally; routes that must be public are explicitly marked with a `@Public()` decorator.
- `__Host-portal_session` and `__Host-portal_csrf` are the only cookies set by the BFF (other than infrastructure cookies, none of which carry session data).
- A NestJS interceptor enforces double-submit CSRF on every non-`GET` request.
- The Angular HTTP interceptor in `portal-shell` injects `X-CSRF-Token` on every outgoing request to the BFF.
- `iss` validation rejects any token whose issuer is not in the tenant allowlist; tested with a token from a non-allowlisted tenant.
- Refresh-token rotation is asserted via integration tests (one expired access token + a still-valid refresh token must succeed silently; an expired refresh token must produce 401 and trigger re-authentication).
- RP-initiated logout is asserted: `POST /auth/logout` invalidates the session, returns the `end_session_endpoint` URL, and a subsequent request without re-authentication is denied.

## Pros and Cons of the Options

### Library

#### `@azure/msal-node` (chosen)

- Good, because Microsoft-official, actively maintained, aligned with Entra's quirks and roadmap (B2B, External ID later, OBO).
- Good, because handles refresh-token rotation, multi-tenant authority routing, and JWKS caching out of the box.
- Bad, because Microsoft-specific — couples the BFF to Entra at the protocol-handler layer.

#### `openid-client`

- Good, because a generic, RFC-faithful OIDC client; would work with any IdP.
- Bad, because lacks Microsoft-specific helpers, especially the OBO flow we'll need in the downstream-API ADR.
- Bad, because we'd be writing the Microsoft-specific glue ourselves — bricolage on a security-critical surface.

#### `passport-azure-ad`

- Bad, because deprecated by Microsoft; no new features, only critical fixes. Rejected.

### OAuth flow

#### Authorization Code with PKCE (chosen)

- Good, because IETF BCP for browser-based apps, recommended by Microsoft for confidential clients on the BFF.
- Good, because PKCE adds a binding between the auth-request and the code-exchange that defeats code-interception attacks.

#### Implicit Flow

- Bad, because deprecated; tokens were returned in the URL fragment, exposed to browser history and the SPA. Rejected.

#### Hybrid Flow

- Bad, because legacy; combines Code and Implicit. Rejected.

#### Resource Owner Password Credentials

- Bad, because the BFF would handle the user's credentials directly — forbidden by Microsoft for production. Rejected.

### Cookie shape

#### `__Host-` prefix (chosen)

- Good, because browsers enforce `Secure`, `Path=/`, and absence of `Domain` — no subdomain bleed, no downgrade.
- Bad, because requires HTTPS in dev (mkcert).

#### Without the prefix

- Bad, because subdomain cookies and HTTP-downgrade scenarios become attack surface.

### CSRF

#### Double-submit cookie (chosen)

- Good, because stateless server-side — no token store to maintain.
- Good, because the cookie is `SameSite=Lax`, so it isn't sent on cross-site requests; combined with the header check, an attacker on another origin cannot synthesise a valid request.

#### Synchronizer token

- Good, because slightly stronger isolation (the token is server-only).
- Bad, because requires a server-side store and synchronisation — heavier for marginal gain in our threat model.

#### No protection

- Bad, because cookies are sent on cross-site form submissions; state-changing endpoints would be trivially exploitable. Rejected.

### Logout

#### RP-initiated (chosen)

- Good, because the user's Entra SSO state is cleared too — explicit, no silent re-auth.

#### Local-only

- Bad, because the user remains signed in to Entra; "log out" feels reversible to the attacker who got their hands on the session for a moment, not to the user.

## More Information

- IETF "OAuth 2.0 for Browser-Based Apps" BCP draft: https://datatracker.ietf.org/doc/draft-ietf-oauth-browser-based-apps/
- Microsoft BFF guidance: https://learn.microsoft.com/azure/architecture/patterns/backends-for-frontends
- MSAL Node overview: https://learn.microsoft.com/azure/active-directory/develop/msal-node-overview
- Cookie `__Host-` prefix: https://developer.mozilla.org/docs/Web/HTTP/Cookies#cookie_prefixes
- mkcert: https://github.com/FiloSottile/mkcert
- Related ADRs: [ADR-0005](0005-backend-stack-nestjs.md) (NestJS), [ADR-0008](0008-identity-model-entra-workforce-dual-audience.md) (identity model), and the future ADRs for session storage (Redis), MFA enforcement, downstream On-Behalf-Of flow, and audit trail.