docs(adr-0021): phase-2 security baseline (helmet, CORS, CSRF, rate-limit, error envelope) (#124)
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## Summary

Documents the security middleware stack shipped across the five most recent BFF PRs (#115, #117, #120, #122, #123) as a single MADR ADR. Today the rationale for each choice lives in code comments and PR descriptions; the next contributor reaching for `csurf`, a cookie-only CSRF, or a hardcoded localhost CORS fallback won't have a single place to read why those are wrong here.

ADR-0021 covers:

- **Response envelope** — `{ error: { code, message, traceId } }`, single contract shared between Nest's `StructuredErrorFilter` and raw Express middlewares (CSRF, rate-limit) via the exported `errorResponse()` helper. Status code → code mapping documented. 500s never leak the underlying exception.
- **CSRF — session-bound double-submit**, not pure cookie-vs-header. Rationale: a subdomain-takeover cookie injection can't bypass the comparison because the source of truth is the server-side session token, not the cookie. Cookie is the SPA's read-only mirror.
- **CORS allowlist** — `CORS_ALLOWED_ORIGINS` mandatory at boot, no fallback. "Works in dev, breaks in prod" is the exact trap the validator catches.
- **Rate limiting** — buckets keyed by session id (auth) or IP (anonymous), 10/min on `/auth/login` + `/auth/callback`, 120/min general, `/api/health` skipped. In-memory v1 store; Redis-backed migration is one constructor arg.
- **Helmet config** — defaults plus three overrides (HSTS prod-only, `crossOriginResourcePolicy: cross-origin`, CSP prod-only). Each override has a code-anchored justification.

## Scope

This is the **implementation-level** security ADR. The **strategic** security baseline ADR — OWASP ASVS reference level, HDS / GDPR / NIS 2 framing, RSSI sign-off — remains paused per the note in [CLAUDE.md](CLAUDE.md). When that ADR lands it'll either confirm 0021 or supersede pieces of it; 0021 is explicit about which choices are tactical and revisitable.

## Notable structural choices in the ADR

- **"Considered Options" mirrors the actual debate.** For CSRF: session-bound vs pure double-submit vs `csurf` vs synchronizer. For rate-limit bucket key: sessionID-then-IP vs IP-only vs Entra `oid`. Each rejected option has its "Bad, because …" so the reader sees why we didn't go that way.
- **Each "Decision Outcome" line points at the file / function that enforces it.** Cross-references are absolute paths so they survive folder reorgs.
- **The "Consequences" section is brutally honest about trade-offs.** In-memory rate-limit doesn't scale horizontally. The CSRF cookie is XSS-readable. No `details` field in the envelope for field-level validation errors yet. These aren't hidden in the prose.

## Other doc touches

- [docs/decisions/README.md](docs/decisions/README.md) index entry added.
- [notes/handoff.md](notes/handoff.md) refreshed (gitignored; not part of the commit but useful for the next session).

## Noted for a separate PR (out of scope here)

[CLAUDE.md](CLAUDE.md) §"Repository status" still says "The Nx workspace is **not yet bootstrapped**" and refs ADRs up to 0020. The whole paragraph is stale (the project is fully scaffolded, 22 ADRs in place). Worth a small dedicated docs PR.

## Test plan

- [x] `pnpm exec prettier --check docs/decisions/` → clean.
- [x] ADR follows the MADR 4.0.0 template (frontmatter with `status`, `date`, `decision-makers`, `tags`; sections in the canonical order).
- [x] Tags drawn from the vocabulary in [docs/decisions/README.md](docs/decisions/README.md#tag-vocabulary): `security`, `backend`.
- [x] Index in `docs/decisions/README.md` updated in the same change.
- [x] Cross-references to ADRs 0009, 0010, 0012, 0015 verified.
- [ ] Renders in Gitea / IDE markdown preview without parser warnings.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #124
This commit was merged in pull request #124.
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2026-05-13 23:54:45 +02:00
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ADRs are listed in numerical order. To slice by topic, filter on the `Tags` column.
| # | Title | Status | Tags | Date |
| ------------------------------------------------------------ | --------------------------------------------------------------------------------------------------------------- | -------- | -------------------------------------------------------------- | ---------- |
| [0001](0001-use-adrs-to-record-architectural-decisions.md) | Use ADRs to record architectural decisions | accepted | `process` | 2026-04-29 |
| [0002](0002-adopt-nx-monorepo-apps-preset.md) | Adopt Nx monorepo with the `apps` preset | accepted | `infrastructure`, `frontend`, `backend` | 2026-04-29 |
| [0003](0003-workspace-and-app-naming-convention.md) | Workspace and app naming convention | accepted | `process` | 2026-04-29 |
| [0004](0004-frontend-stack-angular-csr-zoneless-signals.md) | Frontend stack — Angular (latest LTS), standalone, zoneless, Signals, CSR-only, Vitest | accepted | `frontend` | 2026-04-29 |
| [0005](0005-backend-stack-nestjs.md) | Backend stack — NestJS over Express, Fastify, Hono | accepted | `backend` | 2026-04-29 |
| [0006](0006-persistence-postgresql-prisma.md) | Persistence — PostgreSQL with Prisma | accepted | `data`, `backend` | 2026-04-29 |
| [0007](0007-pre-commit-hooks-and-conventional-commits.md) | Pre-commit hooks and Conventional Commits | accepted | `process` | 2026-04-29 |
| [0008](0008-identity-model-entra-workforce-dual-audience.md) | Identity model — multi-tenant Entra ID for workforce, dual-audience design for future External ID | accepted | `security`, `data` | 2026-04-29 |
| [0009](0009-auth-flow-oidc-pkce-msal-node.md) | Authentication flow — OIDC Authorization Code + PKCE via MSAL Node, BFF session pattern | accepted | `security`, `backend` | 2026-04-29 |
| [0010](0010-session-management-redis.md) | Session management — opaque session IDs in cookies, payload in self-hosted Redis with AES-GCM at rest | accepted | `security`, `backend`, `infrastructure` | 2026-04-29 |
| [0011](0011-mfa-enforcement-entra-conditional-access.md) | MFA enforcement — Entra ID Conditional Access baseline, BFF claim sanity-check, step-up hooks designed-in | accepted | `security` | 2026-04-29 |
| [0012](0012-observability-pino-opentelemetry.md) | Observability — Pino structured logs + OpenTelemetry tracing, W3C Trace Context propagation, stdout + collector | accepted | `observability`, `backend`, `frontend` | 2026-04-29 |
| [0013](0013-audit-trail-separated-postgres-append-only.md) | Audit trail — separated append-only Postgres schema, decoupled from app logs | accepted | `security`, `observability`, `data` | 2026-04-29 |
| [0014](0014-downstream-api-access-obo-pattern.md) | Downstream API access — On-Behalf-Of pattern, unified `DownstreamApiClient`, audience-aware authorization | accepted | `security`, `backend` | 2026-04-29 |
| [0015](0015-cicd-gitea-actions.md) | CI/CD pipeline — Gitea Actions, trunk-based + squash-merge, thin YAML over portable scripts | accepted | `infrastructure`, `process` | 2026-04-30 |
| [0016](0016-accessibility-baseline-wcag-aa-targeted-aaa.md) | Accessibility baseline — WCAG 2.2 AA + targeted AAA, Angular CDK + spartan-ng + Tailwind, APF panel testing | accepted | `accessibility`, `frontend`, `process` | 2026-04-30 |
| [0017](0017-performance-budgets-lighthouse-ci.md) | Performance budgets — Core Web Vitals + Lighthouse CI gates, bundle budgets, BFF p95/p99 SLOs | accepted | `performance`, `frontend`, `backend`, `process` | 2026-04-30 |
| [0018](0018-environment-configuration-strategy.md) | Environment configuration — Angular `environment.ts`, BFF env vars, audit pool split | accepted | `frontend`, `backend`, `infrastructure`, `process` | 2026-05-10 |
| [0019](0019-internationalisation-angular-localize.md) | Internationalisation — `@angular/localize`, build-time per-locale bundles, `/fr` + `/en` path-based routing | accepted | `frontend`, `accessibility`, `performance`, `process` | 2026-05-11 |
| [0020](0020-portal-admin-app.md) | `portal-admin` — dedicated SPA for portal administration, sharing the existing BFF | accepted | `frontend`, `backend`, `security`, `infrastructure`, `process` | 2026-05-11 |
| # | Title | Status | Tags | Date |
| ------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------- | -------- | -------------------------------------------------------------- | ---------- |
| [0001](0001-use-adrs-to-record-architectural-decisions.md) | Use ADRs to record architectural decisions | accepted | `process` | 2026-04-29 |
| [0002](0002-adopt-nx-monorepo-apps-preset.md) | Adopt Nx monorepo with the `apps` preset | accepted | `infrastructure`, `frontend`, `backend` | 2026-04-29 |
| [0003](0003-workspace-and-app-naming-convention.md) | Workspace and app naming convention | accepted | `process` | 2026-04-29 |
| [0004](0004-frontend-stack-angular-csr-zoneless-signals.md) | Frontend stack — Angular (latest LTS), standalone, zoneless, Signals, CSR-only, Vitest | accepted | `frontend` | 2026-04-29 |
| [0005](0005-backend-stack-nestjs.md) | Backend stack — NestJS over Express, Fastify, Hono | accepted | `backend` | 2026-04-29 |
| [0006](0006-persistence-postgresql-prisma.md) | Persistence — PostgreSQL with Prisma | accepted | `data`, `backend` | 2026-04-29 |
| [0007](0007-pre-commit-hooks-and-conventional-commits.md) | Pre-commit hooks and Conventional Commits | accepted | `process` | 2026-04-29 |
| [0008](0008-identity-model-entra-workforce-dual-audience.md) | Identity model — multi-tenant Entra ID for workforce, dual-audience design for future External ID | accepted | `security`, `data` | 2026-04-29 |
| [0009](0009-auth-flow-oidc-pkce-msal-node.md) | Authentication flow — OIDC Authorization Code + PKCE via MSAL Node, BFF session pattern | accepted | `security`, `backend` | 2026-04-29 |
| [0010](0010-session-management-redis.md) | Session management — opaque session IDs in cookies, payload in self-hosted Redis with AES-GCM at rest | accepted | `security`, `backend`, `infrastructure` | 2026-04-29 |
| [0011](0011-mfa-enforcement-entra-conditional-access.md) | MFA enforcement — Entra ID Conditional Access baseline, BFF claim sanity-check, step-up hooks designed-in | accepted | `security` | 2026-04-29 |
| [0012](0012-observability-pino-opentelemetry.md) | Observability — Pino structured logs + OpenTelemetry tracing, W3C Trace Context propagation, stdout + collector | accepted | `observability`, `backend`, `frontend` | 2026-04-29 |
| [0013](0013-audit-trail-separated-postgres-append-only.md) | Audit trail — separated append-only Postgres schema, decoupled from app logs | accepted | `security`, `observability`, `data` | 2026-04-29 |
| [0014](0014-downstream-api-access-obo-pattern.md) | Downstream API access — On-Behalf-Of pattern, unified `DownstreamApiClient`, audience-aware authorization | accepted | `security`, `backend` | 2026-04-29 |
| [0015](0015-cicd-gitea-actions.md) | CI/CD pipeline — Gitea Actions, trunk-based + squash-merge, thin YAML over portable scripts | accepted | `infrastructure`, `process` | 2026-04-30 |
| [0016](0016-accessibility-baseline-wcag-aa-targeted-aaa.md) | Accessibility baseline — WCAG 2.2 AA + targeted AAA, Angular CDK + spartan-ng + Tailwind, APF panel testing | accepted | `accessibility`, `frontend`, `process` | 2026-04-30 |
| [0017](0017-performance-budgets-lighthouse-ci.md) | Performance budgets — Core Web Vitals + Lighthouse CI gates, bundle budgets, BFF p95/p99 SLOs | accepted | `performance`, `frontend`, `backend`, `process` | 2026-04-30 |
| [0018](0018-environment-configuration-strategy.md) | Environment configuration — Angular `environment.ts`, BFF env vars, audit pool split | accepted | `frontend`, `backend`, `infrastructure`, `process` | 2026-05-10 |
| [0019](0019-internationalisation-angular-localize.md) | Internationalisation — `@angular/localize`, build-time per-locale bundles, `/fr` + `/en` path-based routing | accepted | `frontend`, `accessibility`, `performance`, `process` | 2026-05-11 |
| [0020](0020-portal-admin-app.md) | `portal-admin` — dedicated SPA for portal administration, sharing the existing BFF | accepted | `frontend`, `backend`, `security`, `infrastructure`, `process` | 2026-05-11 |
| [0021](0021-phase-2-security-baseline.md) | Phase-2 security baseline — helmet, CORS allowlist, double-submit CSRF, rate limiting, structured error envelope | accepted | `security`, `backend` | 2026-05-13 |