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feat(portal-bff): rate limiting + structured error filter (#123)
## Summary

Closes the phase-2 hardening list that `main.ts` has been advertising since the security PR (#122). Two new middlewares + one alignment pass on the response shape so every BFF error follows a single contract.

### Structured error filter

A global `ExceptionFilter` (registered via `app.useGlobalFilters(...)` at the top of `bootstrap()`) normalises every 4xx/5xx response to a single envelope :

```json
{
  "error": {
    "code": "csrf",
    "message": "CSRF token missing or invalid",
    "traceId": "abc123…"
  }
}
```

- `code` — stable token the SPA can `switch` on. Either explicit on the `HttpException`'s response object (`new UnauthorizedException({ code: 'unauthenticated', message: '...' })`) or derived from the status (`STATUS_CODE_MAP` for the common cases, `'http_error'` fallback). 500s always use `'internal'`.
- `message` — safe human-readable text. **500s never leak the underlying exception** (the full message + stack go to the Pino `error` log line as `err: exception` — Pino's stack-serialiser does the rest).
- `traceId` — current OTel trace id (or `null` when no span is active). Makes cross-correlation with the audit log + Pino lines trivial.

An exported `errorResponse(code, message)` helper produces the same envelope for code paths that write the response directly (raw Express middlewares like the CSRF one, the rate-limit handler) — single contract everywhere.

### Rate limiting

`express-rate-limit` mounted after the session middleware:

- **Dynamic max per request**: 10/min on `/api/auth/login` + `/api/auth/callback` (`RATE_LIMIT_AUTH_PER_MINUTE` env), 120/min everywhere else (`RATE_LIMIT_PER_MINUTE`).
- **Bucket key** = session id when the request carries an active session, remote IP otherwise. A single attacker can't dodge the limit by rotating sessions; an authenticated user gets per-account fairness regardless of source IP.
- **`/api/health` is skipped** so orchestrator polls don't burn the user quota.
- 429 response uses the same envelope as everything else (`{ error: { code: 'rate_limited', … } }`) via the shared `errorResponse()` helper.
- In-memory store (single-instance v1 per ADR-0015). Redis-backed store is a one-line config change when we scale out.

### Alignment pass

- **CSRF middleware** previously returned `{ error: 'csrf' }`. Now returns the full envelope via `errorResponse('csrf', 'CSRF token missing or invalid')`.
- **`/auth/me` 401** previously wrote `{ error: 'unauthenticated' }` directly. Now throws `UnauthorizedException({ code: 'unauthenticated', message: 'Unauthenticated' })` so the filter formats it. Identical response shape on the wire as the CSRF path.

Both spec assertions updated to the new shape.

### Type-resolution fix (transitive)

`@types/express@4.17.25` was being pulled in transitively by `http-proxy-middleware` (Nx's webpack-dev-server). `express-rate-limit`'s `.d.ts` files import `'express'` and the type resolver was matching the v4 copy, causing `Request` type mismatches with our v5-based code. Added `"@types/express": "^5.0.6"` to `pnpm.overrides` so the workspace pins a single version everywhere.

## Notable choices

**`StructuredErrorFilter` is the source of truth, but raw middlewares are still allowed to write responses directly** (rate-limit, CSRF). The reason: Nest's filter chain only handles exceptions thrown from controllers/guards/interceptors. Express middleware short-circuits before that. Both paths now use the same envelope shape through the `errorResponse()` helper.

**No `traceId` in non-5xx responses?** It IS included. The filter writes it on every status — useful for any client-server debugging conversation ("send me your traceId from the 403 you got").

**500s strip the exception message.** Even if a developer accidentally surfaces a sensitive detail via `throw new Error('connection to postgres://user:secret@host failed')`, the response body just says "Internal server error". The full message goes to the log — visible to ops, never to clients. This is the standard secure-by-default for unhandled errors.

**Dynamic `max` per request, not two separate `rateLimit()` instances.** Two instances would each maintain a separate store, so the `/auth/login` bucket would be independent of the general one for the same IP. A single instance with a path-conditional max gives consistent bucket accounting.

## Out of scope

- Redis-backed rate-limit store. v1 ships in-memory; the BFF runs as a single instance. The migration is `new RedisStore({ ... })` when we scale out (ADR-0015 mentions this).
- Per-user override of `RATE_LIMIT_PER_MINUTE` (e.g. admins / service accounts with higher quotas). No code path for this in v1.
- CSP fine-tuning for portal-shell + portal-admin once Caddy serves them.

## Test plan

- [x] `pnpm nx test portal-bff` (clean env) → **199/199 pass** (+25 specs: StructuredErrorFilter, rate-limit middleware, CSRF + /me alignments).
- [x] `pnpm nx test feature-auth` (clean env) → **28/28 pass**.
- [x] `pnpm nx test portal-shell` (clean env) → **34/34 pass**.
- [x] `pnpm nx run-many -t lint build --projects=portal-bff,feature-auth,portal-shell` → clean.
- [x] Prettier-clean.
- [x] CI clean-env repro: every env var unset (including new `RATE_LIMIT_*`) → 261/261 pass.
- [ ] Manual smoke against running BFF:
  - [ ] Throw any error from a controller → response is `{ error: { code, message, traceId } }`. Pino log has the full exception under `err`.
  - [ ] Curl `/api/auth/me` without a session cookie → 401 + same envelope, `code: 'unauthenticated'`.
  - [ ] Hit `/api/auth/login` 11 times in a minute → 11th returns 429 + `code: 'rate_limited'`. `/api/health` hit 100 times → all 200.
  - [ ] POST without `X-CSRF-Token` → 403 + `code: 'csrf'`.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #123
2026-05-13 21:34:33 +02:00

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# BFF environment template
# Copy to .env (which is gitignored) and fill in actual values for local development.
# Production values are managed by the platform's secret manager (see future infrastructure ADR).
# Postgres connection (per ADR-0006)
# Local dev default: dockerised Postgres on port 5432, schema 'public'.
# Username / password / db must match infra/local/.env (POSTGRES_USER /
# POSTGRES_PASSWORD / POSTGRES_DB) — those are the source of truth,
# this is the BFF view of the same connection.
#
# IMPORTANT — URL encoding. The password is part of the URL userinfo
# segment, so any of these characters must be URL-encoded:
# @ → %40 # → %23 : → %3A / → %2F ? → %3F
# % → %25 & → %26 = → %3D + → %2B ; → %3B
# i.e. if your POSTGRES_PASSWORD is "p@ss#1", DATABASE_URL must read
# "postgresql://portal:p%40ss%231@localhost:5432/portal_dev?schema=public"
# The BFF aborts at boot with a clear error if it detects an unencoded
# special character (see apps/portal-bff/src/config/check-database-url.ts).
DATABASE_URL="postgresql://portal:portal_dev_change_me@localhost:5432/portal_dev?schema=public"
# Observability (per ADR-0012)
# All OTEL_* keys are honoured by the OpenTelemetry SDK directly — see
# apps/portal-bff/src/observability/tracing.ts for the bootstrap.
# Pino log level: 'info' in prod, 'debug' in dev (default if unset).
LOG_LEVEL=debug
OTEL_SERVICE_NAME=portal-bff
OTEL_SERVICE_VERSION=dev
# Default endpoint targets the Collector provisioned in
# infra/local/dev.compose.yml. The /v1/traces suffix is required by
# the HTTP/Protobuf transport.
OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318/v1/traces
OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf
# v1 samples 100 % at the app; tail sampling is delegated to the
# Collector (per ADR-0012). Override only for spike investigations.
OTEL_TRACES_SAMPLER=always_on
# Identity / Entra ID app registration (per ADR-0008 / ADR-0009)
# Values come from the project's Entra application registration in the
# Azure Admin Center → App registrations → APF Portal. The four
# *_INSTANCE_URL / *_TENANT_ID / *_CLIENT_ID / *_CLIENT_SECRET keys
# are mandatory; the BFF refuses to boot without them (see
# apps/portal-bff/src/config/check-entra-config.ts).
#
# ENTRA_INSTANCE_URL is the Microsoft login endpoint — usually
# https://login.microsoftonline.com/. The authority used by MSAL is
# `${ENTRA_INSTANCE_URL}${ENTRA_TENANT_ID}` for single-tenant flows,
# or `${ENTRA_INSTANCE_URL}organizations` / `common` for multi-tenant
# (per ADR-0008's dual-audience design). v1 uses the tenant-scoped
# authority; the multi-tenant switch lands when External ID activation
# is needed.
#
# ENTRA_CLIENT_SECRET is the high-value secret of this set. Never
# commit a real value. Production manages it via the deploy platform's
# secret manager (future infrastructure ADR).
ENTRA_INSTANCE_URL=https://login.microsoftonline.com/
ENTRA_TENANT_ID=00000000-0000-0000-0000-000000000000
ENTRA_CLIENT_ID=00000000-0000-0000-0000-000000000000
ENTRA_CLIENT_SECRET=replace_with_real_value
# Redirect URIs registered in Entra alongside the same client id. Both
# `/auth/callback` and `/auth/logout` paths are mounted by the BFF
# once the OIDC routes land in a subsequent PR.
ENTRA_REDIRECT_URI=http://localhost:3000/api/auth/callback
ENTRA_POST_LOGOUT_REDIRECT_URI=http://localhost:4200/
# Cookie signing secret (per ADR-0009 §"Cookies"). Used to sign the
# transient pre-auth cookie that carries the OIDC `state` + PKCE
# verifier between the /auth/login redirect and the /auth/callback
# round-trip, and (once ADR-0010 ships) the session cookie's
# integrity layer. Mandatory at boot — the BFF aborts if missing or
# obviously weak (less than 32 base64-decoded bytes ≈ 256 bits of
# entropy). Generate a fresh value per environment:
#
# node -e "console.log(require('crypto').randomBytes(32).toString('base64url'))"
SESSION_SECRET=replace_with_32_random_bytes_base64url
# Redis connection (per ADR-0010). The BFF uses `ioredis` for session
# storage (today: just the connection; the express-session +
# connect-redis middleware lands in the next PR).
#
# REDIS_URL — full URL form including auth. Must match `infra/local/.env`
# (REDIS_PASSWORD + REDIS_PORT) when running against the local Compose
# stack. Production wiring uses Sentinel (REDIS_SENTINEL_HOSTS +
# REDIS_SENTINEL_NAME — future-vars block below) and TLS; the current
# variable supports the dev single-instance shape only.
REDIS_URL=redis://default:redis_dev_change_me@localhost:6379/0
# Session payload encryption (per ADR-0010 §"At-rest encryption").
# AES-256-GCM key for encrypting the session JSON that connect-redis
# writes to Redis, so a Redis dump never carries raw user identities
# / future tokens / claims in plaintext. **Distinct** from
# SESSION_SECRET, which only signs the cookie's session-id — never
# reuse one for the other. Mandatory at boot.
#
# node -e "console.log(require('crypto').randomBytes(32).toString('base64url'))"
SESSION_ENCRYPTION_KEY=replace_with_32_random_bytes_base64url
# Session timeouts (per ADR-0010). Both optional with sensible
# defaults; override only when staging / prod policy diverges.
# SESSION_IDLE_TIMEOUT_SECONDS — sliding window. Each request
# extends the cookie's `expires` by this many seconds.
# Default 1800 (30 min).
# SESSION_ABSOLUTE_TIMEOUT_SECONDS — hard ceiling. Session is
# destroyed regardless of activity at this age.
# Default 43200 (12 h).
# SESSION_IDLE_TIMEOUT_SECONDS=1800
# SESSION_ABSOLUTE_TIMEOUT_SECONDS=43200
# Per-environment salt used to pseudonymise the user id before it
# lands in audit rows (per ADR-0013 §"Schema") and in Pino app log
# lines (per ADR-0012 §"User id hashing"). Same value must be used
# on both sides so audit and app logs join on `actor_id_hash`.
#
# Rotation invalidates the join key — old rows / log lines can no
# longer be correlated with the new hash. Treat as long-lived per
# environment. Mandatory at boot.
#
# node -e "console.log(require('crypto').randomBytes(32).toString('base64url'))"
LOG_USER_ID_SALT=replace_with_32_random_bytes_base64url
# CORS allowlist (per ADR-0009 §"CORS"). Comma-separated list of
# origins (scheme://host[:port]) allowed to call the BFF with
# credentials. The BFF refuses to start without this — silently
# defaulting to localhost is the classic "works in dev, breaks in
# prod" trap.
#
# Local dev: the portal-shell dev server on :4200. Add :4201 once
# portal-admin grows its own dev server.
CORS_ALLOWED_ORIGINS=http://localhost:4200
# Rate limiting (per ADR-0015 §"DoS mitigation"). Both optional
# with conservative defaults; override per environment when traffic
# patterns demand it. The BFF keys buckets by session id when the
# request is authenticated, by remote IP otherwise — rotating
# sessions doesn't dodge the limit.
# RATE_LIMIT_PER_MINUTE — general bucket. Default 120 (~ 2 r/s).
# RATE_LIMIT_AUTH_PER_MINUTE — stricter bucket on /auth/login and
# /auth/callback. Default 10/min, to
# slow brute-force / replay loops
# without inconveniencing legit users.
# RATE_LIMIT_PER_MINUTE=120
# RATE_LIMIT_AUTH_PER_MINUTE=10
# Future env vars introduced by upcoming phases / ADRs:
#
# Auth flow (ADR-0009) — additional keys wired as the routes land:
# ENTRA_CLIENT_CERT_PATH (alternative to ENTRA_CLIENT_SECRET)
# ENTRA_ACCEPTED_TENANT_IDS (CSV; restricts which tenants can sign in
# in the multi-tenant phase — empty means
# "only ENTRA_TENANT_ID is accepted")
#
# Sessions (ADR-0010) — additional keys wired as the layers land:
# REDIS_SENTINEL_HOSTS (CSV `host:port,host:port,…`; prod HA)
# REDIS_SENTINEL_NAME (master name in Sentinel; prod HA)
# REDIS_TLS ('true' in prod)
#
# MFA (ADR-0011):
# MFA_FRESHNESS_SECONDS (default 600)
#
# Audit trail (ADR-0013):
# AUDIT_DATABASE_URL (separate creds, role 'audit_writer')
# AUDIT_ARCHIVER_DATABASE_URL (role 'audit_archiver', for the retention purge job)
# AUDIT_RETENTION_DAYS (default 365)
#
# Downstream API access (ADR-0014):
# OBO_CACHE_ENCRYPTION_KEY (32-byte base64, distinct from SESSION_ENCRYPTION_KEY)
# BFF_JWKS_PRIVATE_KEY_PATH
# BFF_JWKS_KID
# <SERVICE>_API_BASE_URL (per integrated downstream)
# <SERVICE>_TIMEOUT_MS (optional, defaults to 5000)