From 5bbe2304ff2e55562635d67bf9e786c852447ce4 Mon Sep 17 00:00:00 2001
From: Julien Gautier <jgautier.webdev@gmail.com>
Date: Wed, 13 May 2026 20:50:44 +0200
Subject: [PATCH] feat(portal-bff): helmet + env-driven CORS allowlist +
 double-submit CSRF (#122)
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## Summary

Phase-2 security baseline that the `main.ts` placeholder note has been advertising since the auth/session work began. Three independent middlewares + their SPA counterparts, all mounted in a single PR because they only become meaningful together.

### Helmet on the BFF

`helmet()` with three overrides matching our specific shape:

- **HSTS only in production** — dev runs on plain HTTP, HSTS is just noise.
- **`crossOriginResourcePolicy: 'cross-origin'`** — the SPA on its own origin reads JSON from the BFF; the default `same-origin` would block it.
- **CSP disabled in non-production** — the BFF doesn't render HTML, so CSP on JSON responses is mostly inert, but Helmet's default CSP triggers noisy `connect-src` violations in browser devtools that we don't need.

Everything else is Helmet defaults: `X-Frame-Options=SAMEORIGIN`, `X-Content-Type-Options=nosniff`, `Referrer-Policy=no-referrer`, `X-Powered-By` removed, etc.

### CORS allowlist, env-driven

`CORS_ALLOWED_ORIGINS` env (comma-separated) is now **mandatory** at boot. The BFF refuses to start without it via `readCorsAllowlist()` — same boot-time validator family as `assertSessionSecret` etc. The previous hardcoded `http://localhost:4200` fallback is gone; getting CORS wrong silently is the kind of "works in dev, breaks in prod" trap the validator is specifically designed to catch. `X-CSRF-Token` is now in the allowed headers.

### Double-submit CSRF

- BFF mints a 256-bit `csrfToken` at session creation (`/auth/callback`), stored on `req.session.csrfToken` and mirrored to a JS-readable cookie (`__Host-portal_csrf` prod / `portal_csrf` dev). The cookie is the SPA's read-only view; the server-side session is the source of truth.
- `createCsrfMiddleware` (mounted after the session middleware in `main.ts`) compares the `X-CSRF-Token` header with `req.session.csrfToken` using `crypto.timingSafeEqual`. Skips:
  - safe methods (`GET / HEAD / OPTIONS`),
  - anonymous requests (no `req.session.user`),
  - `/api/auth/login` and `/api/auth/callback` (those mint the token themselves).
- Mismatch → `403 {"error":"csrf"}` with a structured Pino warn.
- SPA's `csrfInterceptor` reads the cookie via `document.cookie` and copies its value into `X-CSRF-Token` on every mutating BFF request. The header is omitted on `GET / HEAD / OPTIONS` (BFF skips them anyway) and on non-BFF origins.
- Logout and the absolute-timeout middleware both clear the CSRF cookie alongside the session cookie.

## Notable choices

**Session-bound double-submit, not pure cookie-vs-header.** A naive "compare cookie with header" check is defeated when an attacker can plant a cookie (subdomain takeover, etc.). Comparing the header to the server-side session-stored token instead means the attacker would also need to be the authenticated user — which is what CSRF defense is supposed to prevent in the first place.

**No CSRF for anonymous mutating routes (v1).** None exist today; we don't have an unauthenticated POST endpoint anywhere. Generating a CSRF token for anonymous sessions would conflict with `saveUninitialized: false` on express-session and add complexity we don't need yet. Anonymous public-form CSRF defenses (site-key, captcha) land if and when those routes ship.

**`SameSite=Lax`, not `Strict`, on the CSRF cookie.** Matches the session cookie's policy so the two travel together on the SPA→BFF cross-origin same-site fetch (different ports = different origin, same registrable domain). The double-submit pattern is what gives the protection; `SameSite=Lax` is a belt-and-braces layer.

**`csrfInterceptor` runs after `bffCredentialsInterceptor` and before `bffUnauthorizedInterceptor` in the chain.** Order: credentials first (set `withCredentials`), then CSRF (set the header), then unauthorized handling (catch 401s). Forward order, no surprises.

**`CORS_ALLOWED_ORIGINS` has no localhost fallback.** I considered keeping the fallback for ergonomics but it makes the BFF silently misconfigured if someone forgets the env. The error message points straight at the file to edit.

## Out of scope (next PRs)

- Rate limiting + structured error filter (still in the phase-2 to-do).
- CSP fine-tuning when we have actual HTML pages (portal-shell + portal-admin static serving).
- CSRF token rotation on idle-extension (today the token lives the session's lifetime; refreshing on each request would invalidate in-flight mutations).

## Test plan

- [x] `pnpm nx run-many -t test --projects=portal-bff,feature-auth,portal-shell` clean env → **177 + 28 + 34 = 239/239 pass** (was 144 + 19 + 34 = 197 before; +42 specs across CSRF middleware, CSRF cookie helpers, CORS allowlist parser, csrfInterceptor, and extended auth.controller / absolute-timeout coverage).
- [x] `pnpm nx run-many -t lint build --projects=portal-bff,feature-auth,portal-shell` → clean.
- [x] **CI clean-env repro** (lesson from prior PRs): every env var unset (including new `CORS_ALLOWED_ORIGINS`) → tests still pass. The BFF refuses to boot without `CORS_ALLOWED_ORIGINS`, which is the intended behaviour.
- [x] Prettier-clean.
- [ ] Manual smoke against running BFF:
  - [ ] Sign in → `__Host-portal_csrf` (prod) / `portal_csrf` (dev) cookie set, value matches `audit.events.payload->>actorIdHash`-style traceability via `req.session.csrfToken` in Redis.
  - [ ] Hit a future POST route from the SPA → request carries `X-CSRF-Token`, BFF accepts.
  - [ ] Forge a POST without the header (curl) → 403 `{"error":"csrf"}`.
  - [ ] Sign out → both cookies cleared.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: https://git.unespace.com/julien/apf_portal/pulls/122
---
 apps/portal-bff/.env.example                  |  10 +
 apps/portal-bff/src/app/app.module.ts         |   2 +
 .../src/auth/auth.controller.spec.ts          |  30 +++
 apps/portal-bff/src/auth/auth.controller.ts   |  16 +-
 .../src/config/check-cors-allowlist.spec.ts   |  75 ++++++++
 .../src/config/check-cors-allowlist.ts        |  78 ++++++++
 apps/portal-bff/src/main.ts                   |  67 +++++--
 .../src/security/csrf-cookie.spec.ts          |  49 +++++
 apps/portal-bff/src/security/csrf-cookie.ts   |  53 ++++++
 .../src/security/csrf.middleware.spec.ts      | 177 ++++++++++++++++++
 .../src/security/csrf.middleware.ts           | 102 ++++++++++
 .../src/security/security.module.ts           |  25 +++
 .../portal-bff/src/security/security.token.ts |  11 ++
 .../absolute-timeout.middleware.spec.ts       |   1 +
 .../session/absolute-timeout.middleware.ts    |   2 +
 apps/portal-bff/src/session/session.types.ts  |  10 +
 apps/portal-shell/src/app/app.config.ts       |  15 +-
 .../src/environments/environment.ts           |   9 +
 libs/feature/auth/src/index.ts                |   3 +-
 libs/feature/auth/src/lib/auth.config.ts      |   9 +
 .../auth/src/lib/csrf.interceptor.spec.ts     | 115 ++++++++++++
 libs/feature/auth/src/lib/csrf.interceptor.ts |  56 ++++++
 package.json                                  |   1 +
 pnpm-lock.yaml                                |   9 +
 24 files changed, 902 insertions(+), 23 deletions(-)
 create mode 100644 apps/portal-bff/src/config/check-cors-allowlist.spec.ts
 create mode 100644 apps/portal-bff/src/config/check-cors-allowlist.ts
 create mode 100644 apps/portal-bff/src/security/csrf-cookie.spec.ts
 create mode 100644 apps/portal-bff/src/security/csrf-cookie.ts
 create mode 100644 apps/portal-bff/src/security/csrf.middleware.spec.ts
 create mode 100644 apps/portal-bff/src/security/csrf.middleware.ts
 create mode 100644 apps/portal-bff/src/security/security.module.ts
 create mode 100644 apps/portal-bff/src/security/security.token.ts
 create mode 100644 libs/feature/auth/src/lib/csrf.interceptor.spec.ts
 create mode 100644 libs/feature/auth/src/lib/csrf.interceptor.ts

diff --git a/apps/portal-bff/.env.example b/apps/portal-bff/.env.example
index 943ac57..d4b9c1c 100644
--- a/apps/portal-bff/.env.example
+++ b/apps/portal-bff/.env.example
@@ -117,6 +117,16 @@ SESSION_ENCRYPTION_KEY=replace_with_32_random_bytes_base64url
 #   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
+
 # Future env vars introduced by upcoming phases / ADRs:
 #
 # Auth flow (ADR-0009) — additional keys wired as the routes land:
diff --git a/apps/portal-bff/src/app/app.module.ts b/apps/portal-bff/src/app/app.module.ts
index 06d4a0d..626a5a3 100644
--- a/apps/portal-bff/src/app/app.module.ts
+++ b/apps/portal-bff/src/app/app.module.ts
@@ -7,6 +7,7 @@ import { HealthModule } from '../health/health.module';
 import { AuditModule } from '../audit/audit.module';
 import { AuthModule } from '../auth/auth.module';
 import { RedisModule } from '../redis/redis.module';
+import { SecurityModule } from '../security/security.module';
 import { SessionModule } from '../session/session.module';
 
 @Module({
@@ -17,6 +18,7 @@ import { SessionModule } from '../session/session.module';
     AuthModule,
     RedisModule,
     SessionModule,
+    SecurityModule,
     HealthModule,
   ],
   controllers: [AppController],
diff --git a/apps/portal-bff/src/auth/auth.controller.spec.ts b/apps/portal-bff/src/auth/auth.controller.spec.ts
index 799f3d5..73c4d87 100644
--- a/apps/portal-bff/src/auth/auth.controller.spec.ts
+++ b/apps/portal-bff/src/auth/auth.controller.spec.ts
@@ -55,6 +55,7 @@ interface SessionStub {
   user?: AuthenticatedUser;
   createdAt?: number;
   absoluteExpiresAt?: number;
+  csrfToken?: string;
   save: jest.Mock;
   destroy: jest.Mock;
 }
@@ -261,6 +262,34 @@ describe('AuthController.callback', () => {
     }
   });
 
+  it('mints a CSRF token, writes it to the session, and mirrors it to the JS-readable cookie', async () => {
+    const { controller } = makeController();
+    const res = makeResStub();
+    const session = makeSessionStub();
+    const req = makeReqStub({
+      signedCookies: { [PRE_AUTH_COOKIE_NAME]: JSON.stringify(PRE_AUTH) },
+      session,
+    });
+
+    await controller.callback(req, res, 'auth-code', PRE_AUTH.state);
+
+    // Server-side source of truth lives on the session.
+    expect(typeof session.csrfToken).toBe('string');
+    expect((session.csrfToken as string).length).toBeGreaterThan(20);
+
+    // Cookie mirror — name picked from sessionCookieName's twin
+    // (NODE_ENV-conditional). In the test default (development),
+    // it's the unprefixed variant.
+    const csrfCookieCall = res.cookie.mock.calls.find(
+      (c) => c[0] === 'portal_csrf' || c[0] === '__Host-portal_csrf',
+    );
+    expect(csrfCookieCall).toBeDefined();
+    expect(csrfCookieCall?.[1]).toBe(session.csrfToken);
+    // Must NOT be HttpOnly — the SPA reads it to echo back via the
+    // X-CSRF-Token header.
+    expect(csrfCookieCall?.[2]).toMatchObject({ httpOnly: false });
+  });
+
   it('registers the new session in the user_sessions index after save', async () => {
     const { controller, userSessionIndex } = makeController();
     const res = makeResStub();
@@ -501,6 +530,7 @@ describe('AuthController.logout', () => {
     });
     expect(session.destroy).toHaveBeenCalledTimes(1);
     expect(res.clearCookie).toHaveBeenCalledWith('portal_session', { path: '/' });
+    expect(res.clearCookie).toHaveBeenCalledWith('portal_csrf', { path: '/' });
     expect(logger.log).toHaveBeenCalledWith(
       { event: 'auth.signed_out', wasAuthenticated: true },
       'AuthLogout',
diff --git a/apps/portal-bff/src/auth/auth.controller.ts b/apps/portal-bff/src/auth/auth.controller.ts
index 0579e65..b2d7edb 100644
--- a/apps/portal-bff/src/auth/auth.controller.ts
+++ b/apps/portal-bff/src/auth/auth.controller.ts
@@ -1,7 +1,9 @@
+import { randomBytes } from 'node:crypto';
 import { Controller, Get, Inject, Query, Req, Res } from '@nestjs/common';
 import type { Request, Response } from 'express';
 import { Logger } from 'nestjs-pino';
 import { AuditWriter } from '../audit/audit.service';
+import { csrfCookieName, csrfCookieOptions } from '../security/csrf-cookie';
 import { readSessionTimeouts, sessionCookieName } from '../session/session-cookie';
 import { UserSessionIndexService } from '../session/user-session-index.service';
 import {
@@ -117,19 +119,30 @@ export class AuthController {
     try {
       const user = await this.authService.completeAuthCodeFlow(code, state, preAuth);
       const now = Date.now();
-      const { absoluteSeconds } = readSessionTimeouts();
+      const { idleSeconds, absoluteSeconds } = readSessionTimeouts();
+      const csrfToken = randomBytes(32).toString('base64url');
       req.session.user = user;
       req.session.createdAt = now;
       // Hard ceiling per ADR-0010 §"TTL policy" — checked on every
       // request by the absolute-timeout middleware, independent of
       // idle TTL.
       req.session.absoluteExpiresAt = now + absoluteSeconds * 1000;
+      // CSRF token per ADR-0009 §"Double-submit CSRF". Server-side
+      // source of truth lives on the session; the cookie below is
+      // the SPA's read-only mirror used to echo the value in the
+      // X-CSRF-Token header.
+      req.session.csrfToken = csrfToken;
       // Force the save before the redirect: express-session writes
       // on response end, but the 302 we're about to emit closes the
       // response before the async store-write would otherwise
       // complete. Without this, the browser hits the SPA before
       // Redis carries the new payload.
       await saveSession(req);
+      // Mirror the CSRF token to a JS-readable cookie. maxAge
+      // matches the session's idle TTL so the cookie expires at
+      // the same time as the session (rolling, refreshed on each
+      // request alongside express-session's own cookie).
+      res.cookie(csrfCookieName(), csrfToken, csrfCookieOptions(idleSeconds * 1000));
       // Register the freshly-minted session id in the per-user
       // index so a future admin "logout everywhere" can enumerate
       // and revoke. Best-effort: a Redis hiccup here doesn't fail
@@ -232,6 +245,7 @@ export class AuthController {
     }
 
     res.clearCookie(sessionCookieName(), { path: '/' });
+    res.clearCookie(csrfCookieName(), { path: '/' });
     this.logger.log({ event: 'auth.signed_out', wasAuthenticated }, 'AuthLogout');
     res.redirect(302, logoutUrl);
   }
diff --git a/apps/portal-bff/src/config/check-cors-allowlist.spec.ts b/apps/portal-bff/src/config/check-cors-allowlist.spec.ts
new file mode 100644
index 0000000..c7f1ce9
--- /dev/null
+++ b/apps/portal-bff/src/config/check-cors-allowlist.spec.ts
@@ -0,0 +1,75 @@
+import { readCorsAllowlist } from './check-cors-allowlist';
+
+describe('readCorsAllowlist', () => {
+  const original = process.env['CORS_ALLOWED_ORIGINS'];
+
+  afterEach(() => {
+    if (original === undefined) {
+      delete process.env['CORS_ALLOWED_ORIGINS'];
+    } else {
+      process.env['CORS_ALLOWED_ORIGINS'] = original;
+    }
+  });
+
+  it('parses a single origin', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'http://localhost:4200';
+    expect(readCorsAllowlist()).toEqual(['http://localhost:4200']);
+  });
+
+  it('parses a comma-separated list and trims whitespace', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] =
+      'http://localhost:4200,  https://portal.apf.example  ,https://admin.portal.apf.example';
+    expect(readCorsAllowlist()).toEqual([
+      'http://localhost:4200',
+      'https://portal.apf.example',
+      'https://admin.portal.apf.example',
+    ]);
+  });
+
+  it('filters out empty entries (trailing commas tolerated)', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'http://localhost:4200,';
+    expect(readCorsAllowlist()).toEqual(['http://localhost:4200']);
+  });
+
+  it('throws when the var is unset', () => {
+    delete process.env['CORS_ALLOWED_ORIGINS'];
+    expect(() => readCorsAllowlist()).toThrow(/CORS_ALLOWED_ORIGINS is not set/);
+  });
+
+  it('throws when the var is the empty string', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = '';
+    expect(() => readCorsAllowlist()).toThrow(/CORS_ALLOWED_ORIGINS is not set/);
+  });
+
+  it('throws when the parsed list is empty (only commas / whitespace)', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = ' , , ';
+    expect(() => readCorsAllowlist()).toThrow(/empty after parsing/);
+  });
+
+  it('throws on a non-URL entry', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'not-a-url';
+    expect(() => readCorsAllowlist()).toThrow(/not a valid URL/);
+  });
+
+  it('throws on a non-http(s) scheme', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'ftp://files.apf.example';
+    expect(() => readCorsAllowlist()).toThrow(/must use http: or https:/);
+  });
+
+  it('throws when an entry has a path', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'https://portal.apf.example/foo';
+    expect(() => readCorsAllowlist()).toThrow(/must be a bare origin/);
+  });
+
+  it('accepts a trailing slash by treating it as the root path', () => {
+    // `new URL('https://x.example/').pathname` is "/" — we accept
+    // that as equivalent to a bare origin.
+    process.env['CORS_ALLOWED_ORIGINS'] = 'https://portal.apf.example/';
+    expect(readCorsAllowlist()).toEqual(['https://portal.apf.example/']);
+  });
+
+  it('throws when an entry has a query string', () => {
+    process.env['CORS_ALLOWED_ORIGINS'] = 'https://portal.apf.example?x=1';
+    expect(() => readCorsAllowlist()).toThrow(/no query string or fragment/);
+  });
+});
diff --git a/apps/portal-bff/src/config/check-cors-allowlist.ts b/apps/portal-bff/src/config/check-cors-allowlist.ts
new file mode 100644
index 0000000..470b3cb
--- /dev/null
+++ b/apps/portal-bff/src/config/check-cors-allowlist.ts
@@ -0,0 +1,78 @@
+/**
+ * Parse and validate the `CORS_ALLOWED_ORIGINS` env var into a list
+ * of origin strings ready to be passed to `app.enableCors({ origin
+ * }) `.
+ *
+ * Wired in `main.ts` (no eager boot-time pre-flight check — the
+ * value is consumed at CORS configuration time, after `NestFactory.
+ * create`). The validator runs there to fail fast on a malformed
+ * URL.
+ *
+ * Format: comma-separated list of origins (`<scheme>://<host>[:port]`).
+ * Whitespace around each entry is trimmed. Empty entries are
+ * filtered out. Trailing slashes are not allowed (an "origin" is
+ * scheme+host+port; nothing after).
+ *
+ * No fallback to a dev default. The BFF refuses to start without
+ * an explicit allowlist — getting CORS wrong silently is exactly
+ * the kind of "works in dev, breaks in prod" issue this guard is
+ * supposed to catch.
+ *
+ * Production should set `CORS_ALLOWED_ORIGINS` to the SPA / admin
+ * SPA's deploy origins (e.g.
+ * `https://portal.apf.example,https://admin.portal.apf.example`).
+ * Dev `.env` lists `http://localhost:4200` (and 4201 once
+ * portal-admin grows a dev server).
+ */
+export function readCorsAllowlist(): readonly string[] {
+  const raw = process.env['CORS_ALLOWED_ORIGINS'];
+  if (raw === undefined || raw === '') {
+    throw new Error(
+      `CORS_ALLOWED_ORIGINS is not set. Add the SPA origin(s) to ` +
+        `apps/portal-bff/.env, e.g. CORS_ALLOWED_ORIGINS=http://localhost:4200`,
+    );
+  }
+
+  const origins = raw
+    .split(',')
+    .map((o) => o.trim())
+    .filter((o) => o.length > 0);
+
+  if (origins.length === 0) {
+    throw new Error(`CORS_ALLOWED_ORIGINS is empty after parsing: "${raw}"`);
+  }
+
+  for (const origin of origins) {
+    assertOriginShape(origin);
+  }
+
+  return origins;
+}
+
+function assertOriginShape(origin: string): void {
+  let url: URL;
+  try {
+    url = new URL(origin);
+  } catch {
+    throw new Error(`CORS_ALLOWED_ORIGINS entry "${origin}" is not a valid URL`);
+  }
+
+  if (url.protocol !== 'http:' && url.protocol !== 'https:') {
+    throw new Error(
+      `CORS_ALLOWED_ORIGINS entry "${origin}" must use http: or https:, got "${url.protocol}"`,
+    );
+  }
+
+  if (url.pathname !== '/' && url.pathname !== '') {
+    throw new Error(
+      `CORS_ALLOWED_ORIGINS entry "${origin}" must be a bare origin ` +
+        `(scheme://host[:port]) with no path. Got pathname "${url.pathname}".`,
+    );
+  }
+
+  if (url.search !== '' || url.hash !== '') {
+    throw new Error(
+      `CORS_ALLOWED_ORIGINS entry "${origin}" must be a bare origin — no query string or fragment.`,
+    );
+  }
+}
diff --git a/apps/portal-bff/src/main.ts b/apps/portal-bff/src/main.ts
index 07d8eee..201c78a 100644
--- a/apps/portal-bff/src/main.ts
+++ b/apps/portal-bff/src/main.ts
@@ -6,14 +6,17 @@ import './observability/tracing';
 import { ValidationPipe } from '@nestjs/common';
 import { NestFactory } from '@nestjs/core';
 import cookieParser from 'cookie-parser';
+import helmet from 'helmet';
 import { Logger } from 'nestjs-pino';
 import { AppModule } from './app/app.module';
+import { readCorsAllowlist } from './config/check-cors-allowlist';
 import { assertDatabaseUrl } from './config/check-database-url';
 import { assertEntraConfig } from './config/check-entra-config';
 import { assertRedisConfig } from './config/check-redis-config';
 import { assertLogUserIdSalt } from './config/check-log-user-id-salt';
 import { assertSessionEncryptionKey } from './config/check-session-encryption-key';
 import { assertSessionSecret } from './config/check-session-secret';
+import { CSRF_MIDDLEWARE } from './security/security.token';
 import {
   SESSION_ABSOLUTE_TIMEOUT_MIDDLEWARE,
   SESSION_MIDDLEWARE,
@@ -57,21 +60,43 @@ async function bootstrap() {
   const app = await NestFactory.create(AppModule, { bufferLogs: true });
   app.useLogger(app.get(Logger));
 
-  // CORS — minimal dev-time allowlist. The SPA running on
-  // http://localhost:4200 issues fetches to the BFF and must be
-  // able to send the W3C `traceparent` (and `tracestate`) headers
-  // that `@opentelemetry/instrumentation-fetch` injects, so the BFF
-  // can pick up the parent span id and emit child spans on the same
-  // trace. The full security-grade allowlist (per-environment
-  // origins, credentials policy, helmet stack, etc.) lands with the
-  // phase-2 security ADR — for now this is the minimum needed for
-  // end-to-end tracing.
+  // Security headers (phase-2). Defaults from `helmet()` are good
+  // for an API server returning JSON: X-Frame-Options=SAMEORIGIN,
+  // X-Content-Type-Options=nosniff, Referrer-Policy=no-referrer,
+  // X-Powered-By removed, etc. CSP defaults apply too but the BFF
+  // doesn't render HTML, so they're inert here.
+  //
+  // Three overrides for our specific shape:
+  //   - HSTS only in production (dev runs on plain HTTP).
+  //   - crossOriginResourcePolicy: 'cross-origin' so the SPA on its
+  //     own origin can read JSON from the BFF without being blocked
+  //     by Spectre-class CORP protections.
+  //   - contentSecurityPolicy: false in dev — Helmet's default CSP
+  //     blocks `connect-src` from anything but 'self', which is
+  //     fine for HTML pages but irrelevant for JSON responses and
+  //     noisy in browser devtools.
+  app.use(
+    helmet({
+      hsts: process.env['NODE_ENV'] === 'production',
+      crossOriginResourcePolicy: { policy: 'cross-origin' },
+      contentSecurityPolicy: process.env['NODE_ENV'] === 'production',
+    }),
+  );
+
+  // CORS allowlist — env-driven via `CORS_ALLOWED_ORIGINS`, parsed
+  // and validated at boot. No hardcoded localhost fallback: getting
+  // CORS wrong silently is exactly the kind of "works in dev, breaks
+  // in prod" issue this validator is meant to catch.
   app.enableCors({
-    origin: (process.env['CORS_ALLOWED_ORIGINS'] ?? 'http://localhost:4200')
-      .split(',')
-      .map((o) => o.trim())
-      .filter(Boolean),
-    allowedHeaders: ['Content-Type', 'Accept', 'Authorization', 'traceparent', 'tracestate'],
+    origin: [...readCorsAllowlist()],
+    allowedHeaders: [
+      'Content-Type',
+      'Accept',
+      'Authorization',
+      'X-CSRF-Token',
+      'traceparent',
+      'tracestate',
+    ],
     credentials: true,
   });
 
@@ -104,9 +129,17 @@ async function bootstrap() {
   // needed).
   app.use(app.get<RequestHandler>(SESSION_ABSOLUTE_TIMEOUT_MIDDLEWARE));
 
-  // Phase-2 security ADRs will harden the above: helmet, real CORS
-  // allowlist, CSRF protection, rate limiting, auth guards,
-  // structured error filter.
+  // Double-submit CSRF (ADR-0009 §"CSRF defense"). Mounted after
+  // the session middleware so `req.session.csrfToken` is available
+  // for comparison with the `X-CSRF-Token` request header. Skips
+  // safe methods (GET / HEAD / OPTIONS), anonymous requests, and
+  // the auth entry routes that *mint* the token (`/auth/login`,
+  // `/auth/callback`).
+  app.use(app.get<RequestHandler>(CSRF_MIDDLEWARE));
+
+  // Phase-2 hardening still pending: rate limiting + structured
+  // error filter (helmet, CORS allowlist, CSRF protection landed
+  // with this PR).
 
   const globalPrefix = 'api';
   app.setGlobalPrefix(globalPrefix);
diff --git a/apps/portal-bff/src/security/csrf-cookie.spec.ts b/apps/portal-bff/src/security/csrf-cookie.spec.ts
new file mode 100644
index 0000000..781155e
--- /dev/null
+++ b/apps/portal-bff/src/security/csrf-cookie.spec.ts
@@ -0,0 +1,49 @@
+import { csrfCookieName, csrfCookieOptions } from './csrf-cookie';
+
+describe('csrf-cookie', () => {
+  const originalNodeEnv = process.env['NODE_ENV'];
+  afterEach(() => {
+    if (originalNodeEnv === undefined) {
+      delete process.env['NODE_ENV'];
+    } else {
+      process.env['NODE_ENV'] = originalNodeEnv;
+    }
+  });
+
+  describe('csrfCookieName', () => {
+    it('uses __Host-portal_csrf in production', () => {
+      process.env['NODE_ENV'] = 'production';
+      expect(csrfCookieName()).toBe('__Host-portal_csrf');
+    });
+
+    it('uses portal_csrf in development (HTTP, no Secure)', () => {
+      process.env['NODE_ENV'] = 'development';
+      expect(csrfCookieName()).toBe('portal_csrf');
+    });
+  });
+
+  describe('csrfCookieOptions', () => {
+    it('is NOT HttpOnly (SPA must read the value to echo it back)', () => {
+      expect(csrfCookieOptions(1800_000).httpOnly).toBe(false);
+    });
+
+    it('uses SameSite=Lax to travel with the session cookie', () => {
+      expect(csrfCookieOptions(1800_000).sameSite).toBe('lax');
+    });
+
+    it('sets Secure only in production', () => {
+      process.env['NODE_ENV'] = 'production';
+      expect(csrfCookieOptions(1800_000).secure).toBe(true);
+      process.env['NODE_ENV'] = 'development';
+      expect(csrfCookieOptions(1800_000).secure).toBe(false);
+    });
+
+    it('pins path=/', () => {
+      expect(csrfCookieOptions(1800_000).path).toBe('/');
+    });
+
+    it('passes maxAge through unchanged', () => {
+      expect(csrfCookieOptions(123456).maxAge).toBe(123456);
+    });
+  });
+});
diff --git a/apps/portal-bff/src/security/csrf-cookie.ts b/apps/portal-bff/src/security/csrf-cookie.ts
new file mode 100644
index 0000000..915d3b6
--- /dev/null
+++ b/apps/portal-bff/src/security/csrf-cookie.ts
@@ -0,0 +1,53 @@
+import type { CookieOptions } from 'express';
+
+/**
+ * Name of the CSRF cookie that pairs with the `__Host-portal_session`
+ * cookie. Per ADR-0009 §"Double-submit CSRF": the BFF generates a
+ * random token at session creation and writes it both to the session
+ * payload (server-side) and to this cookie (readable by the SPA).
+ * Every mutating request must echo the value back in an
+ * `X-CSRF-Token` header; the middleware compares the header against
+ * the *session-stored* token, not the cookie — the cookie is just
+ * the SPA's UX way of reading the value it's expected to send back.
+ *
+ * Same env-conditional naming as the session cookie:
+ *   - production: `__Host-portal_csrf` (prefix mandates Secure +
+ *     Path=/ + no Domain — defeats subdomain cookie injection)
+ *   - development: `portal_csrf` (no Secure available on HTTP)
+ */
+const PRODUCTION_NAME = '__Host-portal_csrf';
+const DEVELOPMENT_NAME = 'portal_csrf';
+
+export function csrfCookieName(): string {
+  return process.env['NODE_ENV'] === 'production' ? PRODUCTION_NAME : DEVELOPMENT_NAME;
+}
+
+/**
+ * Cookie options for the CSRF cookie.
+ *
+ * Crucially **NOT** `HttpOnly`: the SPA needs JavaScript access to
+ * read the token and echo it back in the `X-CSRF-Token` header. The
+ * tradeoff is acceptable because the cookie is _not_ the auth
+ * credential — the session id cookie (`__Host-portal_session`, which
+ * IS HttpOnly) is. An XSS that reads this CSRF cookie still cannot
+ * impersonate the user without also stealing the session cookie.
+ *
+ * `SameSite=Lax` (not Strict): matches the session cookie so the
+ * two travel together on the SPA→BFF cross-origin same-site fetch
+ * (different ports = different origin but same registrable domain).
+ * Strict would drop on top-level POST navigations originating from
+ * a different origin, which is correct for raw CSRF defense but
+ * loses parity with the session cookie's policy. The double-submit
+ * pattern itself is what gives the protection here; SameSite=Lax is
+ * a belt-and-braces layer.
+ */
+export function csrfCookieOptions(maxAgeMs: number): CookieOptions {
+  const isProduction = process.env['NODE_ENV'] === 'production';
+  return {
+    httpOnly: false,
+    sameSite: 'lax',
+    secure: isProduction,
+    path: '/',
+    maxAge: maxAgeMs,
+  };
+}
diff --git a/apps/portal-bff/src/security/csrf.middleware.spec.ts b/apps/portal-bff/src/security/csrf.middleware.spec.ts
new file mode 100644
index 0000000..887fbc1
--- /dev/null
+++ b/apps/portal-bff/src/security/csrf.middleware.spec.ts
@@ -0,0 +1,177 @@
+import type { NextFunction, Request, Response } from 'express';
+import type { Logger } from 'nestjs-pino';
+import { createCsrfMiddleware } from './csrf.middleware';
+
+interface SessionStub {
+  user?: { oid: string };
+  csrfToken?: string;
+}
+
+function makeReq(opts: {
+  method: string;
+  path: string;
+  headers?: Record<string, string>;
+  session?: SessionStub;
+}): Request {
+  const headers = opts.headers ?? {};
+  return {
+    method: opts.method,
+    path: opts.path,
+    session: opts.session,
+    get: (name: string) => headers[name],
+  } as unknown as Request;
+}
+
+function makeRes() {
+  const res = {
+    status: jest.fn(),
+    json: jest.fn(),
+  };
+  res.status.mockReturnValue(res);
+  res.json.mockReturnValue(res);
+  return res as unknown as Response & { status: jest.Mock; json: jest.Mock };
+}
+
+function makeLogger() {
+  return {
+    log: jest.fn(),
+    warn: jest.fn(),
+    error: jest.fn(),
+    debug: jest.fn(),
+  } as unknown as Logger & { warn: jest.Mock };
+}
+
+describe('csrf middleware', () => {
+  const next: NextFunction = jest.fn();
+  beforeEach(() => (next as jest.Mock).mockReset());
+
+  describe('skip cases', () => {
+    it.each(['GET', 'HEAD', 'OPTIONS'])('passes through safe method %s', (method) => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({ method, path: '/api/anything' });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+      expect(res.status).not.toHaveBeenCalled();
+    });
+
+    it('passes through anonymous mutating requests (no req.session.user)', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({ method: 'POST', path: '/api/public', session: {} });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+    });
+
+    it('passes through /api/auth/login (CSRF-exempt: session is born here)', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/auth/login',
+        session: { user: { oid: 'u' }, csrfToken: 'irrelevant' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+    });
+
+    it('passes through /api/auth/callback (CSRF-exempt)', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/auth/callback',
+        session: { user: { oid: 'u' }, csrfToken: 'irrelevant' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  describe('enforcement', () => {
+    it('rejects with 403 when an authenticated mutation lacks X-CSRF-Token', () => {
+      const logger = makeLogger();
+      const csrf = createCsrfMiddleware(logger);
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/profile',
+        session: { user: { oid: 'u' }, csrfToken: 'expected-token' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(res.status).toHaveBeenCalledWith(403);
+      expect(res.json).toHaveBeenCalledWith({ error: 'csrf' });
+      expect(next).not.toHaveBeenCalled();
+      expect(logger.warn).toHaveBeenCalledWith(
+        expect.objectContaining({ event: 'csrf.reject', hasHeaderToken: false }),
+        'Csrf',
+      );
+    });
+
+    it('rejects with 403 when the header value mismatches the session token', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/profile',
+        headers: { 'X-CSRF-Token': 'wrong-token' },
+        session: { user: { oid: 'u' }, csrfToken: 'expected-token' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(res.status).toHaveBeenCalledWith(403);
+    });
+
+    it('rejects with 403 when the session has no csrfToken (auth without CSRF — should never happen)', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/profile',
+        headers: { 'X-CSRF-Token': 'anything' },
+        session: { user: { oid: 'u' } },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(res.status).toHaveBeenCalledWith(403);
+    });
+
+    it('passes when header matches the session token exactly', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/profile',
+        headers: { 'X-CSRF-Token': 'expected-token' },
+        session: { user: { oid: 'u' }, csrfToken: 'expected-token' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+      expect(res.status).not.toHaveBeenCalled();
+    });
+
+    it.each(['PUT', 'PATCH', 'DELETE'])('enforces on mutation method %s like POST', (method) => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method,
+        path: '/api/profile',
+        headers: { 'X-CSRF-Token': 'tok' },
+        session: { user: { oid: 'u' }, csrfToken: 'tok' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(next).toHaveBeenCalledTimes(1);
+    });
+
+    it('uses constant-time comparison (different lengths short-circuit as unequal)', () => {
+      const csrf = createCsrfMiddleware(makeLogger());
+      const req = makeReq({
+        method: 'POST',
+        path: '/api/profile',
+        headers: { 'X-CSRF-Token': 'short' },
+        session: { user: { oid: 'u' }, csrfToken: 'much-longer-token-value' },
+      });
+      const res = makeRes();
+      csrf(req, res, next);
+      expect(res.status).toHaveBeenCalledWith(403);
+    });
+  });
+});
diff --git a/apps/portal-bff/src/security/csrf.middleware.ts b/apps/portal-bff/src/security/csrf.middleware.ts
new file mode 100644
index 0000000..71d3199
--- /dev/null
+++ b/apps/portal-bff/src/security/csrf.middleware.ts
@@ -0,0 +1,102 @@
+import { timingSafeEqual } from 'node:crypto';
+import type { NextFunction, Request, RequestHandler, Response } from 'express';
+import { Logger } from 'nestjs-pino';
+
+/**
+ * Methods that, per RFC 7231 / 9110, MUST NOT alter server state.
+ * The CSRF check skips them — there is nothing to forge for routes
+ * that don't change anything.
+ */
+const SAFE_METHODS = new Set(['GET', 'HEAD', 'OPTIONS']);
+
+/**
+ * Paths the CSRF middleware lets through unconditionally. These are
+ * the OIDC entry points that *create* the session (and the token
+ * along with it) — there is no token to verify yet.
+ *
+ * `GET /auth/logout` is a safe method so it's already covered by
+ * `SAFE_METHODS`; if a future PR moves logout to POST, the bypass
+ * here would need extending.
+ */
+const CSRF_EXEMPT_PREFIXES = ['/api/auth/login', '/api/auth/callback'];
+
+/**
+ * Double-submit CSRF middleware (per ADR-0009 §"CSRF defense").
+ *
+ * Contract:
+ *   - GET / HEAD / OPTIONS pass through.
+ *   - Unauthenticated requests pass through. CSRF protects an
+ *     authenticated identity from being abused by another origin;
+ *     there is no identity to abuse on anonymous routes. (Anonymous
+ *     POST endpoints, if they ever exist, will need a different
+ *     defense — site-key, captcha, etc.)
+ *   - `/api/auth/login` and `/api/auth/callback` are exempt: they
+ *     are the routes that *establish* the session and thereby
+ *     mint the CSRF token.
+ *   - Any other request must carry `X-CSRF-Token` whose value
+ *     equals `req.session.csrfToken`. Comparison is
+ *     `crypto.timingSafeEqual` to avoid leaking the token via
+ *     response-time differences.
+ *   - Mismatch / missing → 403 `{ error: 'csrf' }`. No further
+ *     processing.
+ *
+ * The middleware reads the token from the **session**, not the
+ * cookie. The cookie is the SPA's read-only mirror; an attacker
+ * who can plant a cookie (subdomain takeover, etc.) cannot also
+ * plant a matching session-stored token without already being the
+ * authenticated user. Tying the check to the server-side session
+ * is what distinguishes "session-bound double-submit" from the
+ * weaker pure cookie-vs-header comparison.
+ */
+export function createCsrfMiddleware(logger: Logger): RequestHandler {
+  return function csrf(req: Request, res: Response, next: NextFunction): void {
+    if (SAFE_METHODS.has(req.method.toUpperCase())) {
+      next();
+      return;
+    }
+
+    const session = req.session;
+    if (!session?.user) {
+      // Anonymous mutating request — out of scope for CSRF v1.
+      next();
+      return;
+    }
+
+    if (CSRF_EXEMPT_PREFIXES.some((p) => req.path.startsWith(p))) {
+      next();
+      return;
+    }
+
+    const expected = session.csrfToken;
+    const headerToken = req.get('X-CSRF-Token');
+
+    if (!expected || !headerToken || !equalsTimingSafe(headerToken, expected)) {
+      logger.warn(
+        {
+          event: 'csrf.reject',
+          path: req.path,
+          method: req.method,
+          hasHeaderToken: Boolean(headerToken),
+          hasSessionToken: Boolean(expected),
+        },
+        'Csrf',
+      );
+      res.status(403).json({ error: 'csrf' });
+      return;
+    }
+
+    next();
+  };
+}
+
+function equalsTimingSafe(a: string, b: string): boolean {
+  // `timingSafeEqual` requires equal-length buffers. Different
+  // lengths short-circuit as unequal — we accept the early return
+  // here because the length itself is not a secret.
+  const ba = Buffer.from(a, 'utf8');
+  const bb = Buffer.from(b, 'utf8');
+  if (ba.length !== bb.length) {
+    return false;
+  }
+  return timingSafeEqual(ba, bb);
+}
diff --git a/apps/portal-bff/src/security/security.module.ts b/apps/portal-bff/src/security/security.module.ts
new file mode 100644
index 0000000..8b69837
--- /dev/null
+++ b/apps/portal-bff/src/security/security.module.ts
@@ -0,0 +1,25 @@
+import { Module } from '@nestjs/common';
+import { Logger } from 'nestjs-pino';
+import { createCsrfMiddleware } from './csrf.middleware';
+import { CSRF_MIDDLEWARE, type RequestHandler } from './security.token';
+
+/**
+ * Phase-2 security primitives (per the `main.ts` placeholder note
+ * left in earlier PRs). For now: the CSRF middleware. Helmet and
+ * the CORS allowlist parser are wired directly in `main.ts` because
+ * they are pure stateless configuration — they don't need DI.
+ *
+ * Future phase-2 work that will fit here: rate limiting, structured
+ * error filter, request-size limits.
+ */
+@Module({
+  providers: [
+    {
+      provide: CSRF_MIDDLEWARE,
+      inject: [Logger],
+      useFactory: (logger: Logger): RequestHandler => createCsrfMiddleware(logger),
+    },
+  ],
+  exports: [CSRF_MIDDLEWARE],
+})
+export class SecurityModule {}
diff --git a/apps/portal-bff/src/security/security.token.ts b/apps/portal-bff/src/security/security.token.ts
new file mode 100644
index 0000000..b67affc
--- /dev/null
+++ b/apps/portal-bff/src/security/security.token.ts
@@ -0,0 +1,11 @@
+import type { RequestHandler } from 'express';
+
+/**
+ * DI token for the double-submit CSRF middleware (per ADR-0009).
+ * Resolved at bootstrap in `main.ts` and mounted with `app.use(...)`
+ * after the session middleware so `req.session.csrfToken` is
+ * available for comparison.
+ */
+export const CSRF_MIDDLEWARE = 'CSRF_MIDDLEWARE';
+
+export type { RequestHandler };
diff --git a/apps/portal-bff/src/session/absolute-timeout.middleware.spec.ts b/apps/portal-bff/src/session/absolute-timeout.middleware.spec.ts
index 5334b47..2aa27d4 100644
--- a/apps/portal-bff/src/session/absolute-timeout.middleware.spec.ts
+++ b/apps/portal-bff/src/session/absolute-timeout.middleware.spec.ts
@@ -162,6 +162,7 @@ describe('absoluteTimeout middleware', () => {
     expect(session.destroy).toHaveBeenCalledTimes(1);
     expect(index.remove).toHaveBeenCalledWith(USER.oid, 'session-abc');
     expect(res.clearCookie).toHaveBeenCalledWith('portal_session', { path: '/' });
+    expect(res.clearCookie).toHaveBeenCalledWith('portal_csrf', { path: '/' });
     expect(audit.sessionExpired).toHaveBeenCalledWith({
       actor: { oid: USER.oid },
       sessionId: 'session-abc',
diff --git a/apps/portal-bff/src/session/absolute-timeout.middleware.ts b/apps/portal-bff/src/session/absolute-timeout.middleware.ts
index 78dfb70..a16a8fd 100644
--- a/apps/portal-bff/src/session/absolute-timeout.middleware.ts
+++ b/apps/portal-bff/src/session/absolute-timeout.middleware.ts
@@ -1,6 +1,7 @@
 import type { NextFunction, Request, RequestHandler, Response } from 'express';
 import { Logger } from 'nestjs-pino';
 import { AuditWriter } from '../audit/audit.service';
+import { csrfCookieName } from '../security/csrf-cookie';
 import { sessionCookieName } from './session-cookie';
 import { UserSessionIndexService } from './user-session-index.service';
 
@@ -80,6 +81,7 @@ export function createAbsoluteTimeoutMiddleware(
 
     await index.remove(userId, sessionId);
     res.clearCookie(sessionCookieName(), { path: '/' });
+    res.clearCookie(csrfCookieName(), { path: '/' });
 
     // Audit the expiry — blocking per ADR-0013: if the audit row
     // can't be written, the request fails (the user sees a 5xx,
diff --git a/apps/portal-bff/src/session/session.types.ts b/apps/portal-bff/src/session/session.types.ts
index 81d1c16..7faf560 100644
--- a/apps/portal-bff/src/session/session.types.ts
+++ b/apps/portal-bff/src/session/session.types.ts
@@ -33,6 +33,16 @@ declare module 'express-session' {
      * the session.
      */
     absoluteExpiresAt?: number;
+    /**
+     * Double-submit CSRF token (per ADR-0009). 32 random bytes
+     * base64url-encoded; minted at session creation alongside
+     * `user` and lasts the session lifetime. The same value is
+     * mirrored to the `__Host-portal_csrf` cookie (HttpOnly: false
+     * so the SPA can read it) and compared with the `X-CSRF-Token`
+     * request header by the CSRF middleware. The cookie is the
+     * SPA's read-only mirror; the source of truth lives here.
+     */
+    csrfToken?: string;
   }
 }
 
diff --git a/apps/portal-shell/src/app/app.config.ts b/apps/portal-shell/src/app/app.config.ts
index 56fc460..7581b85 100644
--- a/apps/portal-shell/src/app/app.config.ts
+++ b/apps/portal-shell/src/app/app.config.ts
@@ -7,8 +7,10 @@ import { provideHttpClient, withFetch, withInterceptors } from '@angular/common/
 import { provideRouter } from '@angular/router';
 import {
   AUTH_BFF_BASE_URL,
+  AUTH_CSRF_COOKIE_NAME,
   bffCredentialsInterceptor,
   bffUnauthorizedInterceptor,
+  csrfInterceptor,
 } from 'feature-auth';
 import { environment } from '../environments/environment';
 import { appRoutes } from './app.routes';
@@ -33,13 +35,18 @@ export const appConfig: ApplicationConfig = {
     //     when a BFF route (other than `/auth/me` itself) answers
     //     401, keeping the SPA's auth state in sync after server-
     //     side session destruction (absolute-timeout, manual revoke).
+    //   - `csrfInterceptor` reads the BFF's CSRF cookie and copies
+    //     the value into the `X-CSRF-Token` header on mutating BFF
+    //     requests (POST / PUT / PATCH / DELETE), per the
+    //     double-submit pattern from ADR-0009.
     provideHttpClient(
       withFetch(),
-      withInterceptors([bffCredentialsInterceptor, bffUnauthorizedInterceptor]),
+      withInterceptors([bffCredentialsInterceptor, csrfInterceptor, bffUnauthorizedInterceptor]),
     ),
-    // `feature-auth` is environment-agnostic and reads the BFF base
-    // URL from this token — provided once per app from `environment.ts`
-    // (per ADR-0018).
+    // `feature-auth` is environment-agnostic and reads its BFF
+    // config from these tokens — provided once per app from
+    // `environment.ts` (per ADR-0018).
     { provide: AUTH_BFF_BASE_URL, useValue: environment.bffApiBaseUrl },
+    { provide: AUTH_CSRF_COOKIE_NAME, useValue: environment.bffCsrfCookieName },
   ],
 };
diff --git a/apps/portal-shell/src/environments/environment.ts b/apps/portal-shell/src/environments/environment.ts
index cb13b92..121f3dd 100644
--- a/apps/portal-shell/src/environments/environment.ts
+++ b/apps/portal-shell/src/environments/environment.ts
@@ -25,6 +25,15 @@ export const environment = {
    */
   bffApiBaseUrl: 'http://localhost:3000/api',
 
+  /**
+   * Name of the BFF's CSRF cookie. Mirrors the BFF's
+   * `csrfCookieName()` convention: `__Host-portal_csrf` in
+   * production (Secure-required, hence excluded from HTTP dev),
+   * `portal_csrf` otherwise. Read by the `csrfInterceptor` to
+   * echo the value back in the `X-CSRF-Token` request header.
+   */
+  bffCsrfCookieName: 'portal_csrf',
+
   /**
    * OTLP/HTTP traces endpoint. Targets the Collector in
    * `infra/local/dev.compose.yml`. CORS is enabled there — see
diff --git a/libs/feature/auth/src/index.ts b/libs/feature/auth/src/index.ts
index ce9cbb6..4d52ea0 100644
--- a/libs/feature/auth/src/index.ts
+++ b/libs/feature/auth/src/index.ts
@@ -1,6 +1,7 @@
-export { AUTH_BFF_BASE_URL, AUTH_NAVIGATOR } from './lib/auth.config';
+export { AUTH_BFF_BASE_URL, AUTH_CSRF_COOKIE_NAME, AUTH_NAVIGATOR } from './lib/auth.config';
 export { authGuard } from './lib/auth.guard';
 export { AuthService } from './lib/auth.service';
 export type { AuthState, CurrentUser } from './lib/auth.types';
 export { bffCredentialsInterceptor } from './lib/bff-credentials.interceptor';
 export { bffUnauthorizedInterceptor } from './lib/bff-unauthorized.interceptor';
+export { csrfInterceptor } from './lib/csrf.interceptor';
diff --git a/libs/feature/auth/src/lib/auth.config.ts b/libs/feature/auth/src/lib/auth.config.ts
index a8507e0..16c3920 100644
--- a/libs/feature/auth/src/lib/auth.config.ts
+++ b/libs/feature/auth/src/lib/auth.config.ts
@@ -26,3 +26,12 @@ export const AUTH_NAVIGATOR = new InjectionToken<(url: string) => void>('AUTH_NA
   providedIn: 'root',
   factory: () => (url: string) => window.location.assign(url),
 });
+
+/**
+ * Name of the BFF's CSRF cookie that the `csrfInterceptor` reads
+ * to echo back in the `X-CSRF-Token` header. Provided by the host
+ * from its per-environment config (`environment.bffCsrfCookieName`)
+ * so the lib doesn't have to know the env-conditional naming
+ * (`portal_csrf` dev / `__Host-portal_csrf` prod) the BFF picks.
+ */
+export const AUTH_CSRF_COOKIE_NAME = new InjectionToken<string>('AUTH_CSRF_COOKIE_NAME');
diff --git a/libs/feature/auth/src/lib/csrf.interceptor.spec.ts b/libs/feature/auth/src/lib/csrf.interceptor.spec.ts
new file mode 100644
index 0000000..e095376
--- /dev/null
+++ b/libs/feature/auth/src/lib/csrf.interceptor.spec.ts
@@ -0,0 +1,115 @@
+import { HttpClient, provideHttpClient, withInterceptors } from '@angular/common/http';
+import { HttpTestingController, provideHttpClientTesting } from '@angular/common/http/testing';
+import { TestBed } from '@angular/core/testing';
+import { AUTH_BFF_BASE_URL, AUTH_CSRF_COOKIE_NAME } from './auth.config';
+import { csrfInterceptor } from './csrf.interceptor';
+
+const BFF_BASE = 'http://bff.test/api';
+const CSRF_COOKIE = 'portal_csrf';
+
+function setup() {
+  TestBed.configureTestingModule({
+    providers: [
+      provideHttpClient(withInterceptors([csrfInterceptor])),
+      provideHttpClientTesting(),
+      { provide: AUTH_BFF_BASE_URL, useValue: BFF_BASE },
+      { provide: AUTH_CSRF_COOKIE_NAME, useValue: CSRF_COOKIE },
+    ],
+  });
+  return {
+    http: TestBed.inject(HttpClient),
+    httpCtrl: TestBed.inject(HttpTestingController),
+  };
+}
+
+/**
+ * jsdom keeps `document.cookie` as a single mutable string. Wipe it
+ * around each test so prior writes don't leak.
+ */
+function setCookie(name: string, value: string): void {
+  document.cookie = `${name}=${value}; path=/`;
+}
+function clearAllCookies(): void {
+  const all = document.cookie.split(';');
+  for (const c of all) {
+    const eq = c.indexOf('=');
+    const name = (eq > -1 ? c.slice(0, eq) : c).trim();
+    if (name) document.cookie = `${name}=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/`;
+  }
+}
+
+describe('csrfInterceptor', () => {
+  beforeEach(() => clearAllCookies());
+  afterEach(() => {
+    clearAllCookies();
+    TestBed.resetTestingModule();
+  });
+
+  it('echoes the cookie value into the X-CSRF-Token header on POST', () => {
+    setCookie(CSRF_COOKIE, 'tok-abc');
+    const { http, httpCtrl } = setup();
+    http.post(`${BFF_BASE}/profile`, { name: 'Jane' }).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/profile`);
+    expect(req.request.headers.get('X-CSRF-Token')).toBe('tok-abc');
+    req.flush({});
+  });
+
+  it.each(['PUT', 'PATCH', 'DELETE'])('sets the header on %s too', (method) => {
+    setCookie(CSRF_COOKIE, 'tok-abc');
+    const { http, httpCtrl } = setup();
+    http.request(method, `${BFF_BASE}/profile`, { body: {} }).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/profile`);
+    expect(req.request.headers.get('X-CSRF-Token')).toBe('tok-abc');
+    req.flush({});
+  });
+
+  it('does NOT set the header on GET (BFF skips CSRF on safe methods)', () => {
+    setCookie(CSRF_COOKIE, 'tok-abc');
+    const { http, httpCtrl } = setup();
+    http.get(`${BFF_BASE}/me`).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/me`);
+    expect(req.request.headers.has('X-CSRF-Token')).toBe(false);
+    req.flush({});
+  });
+
+  it('does NOT touch requests to other origins', () => {
+    setCookie(CSRF_COOKIE, 'tok-abc');
+    const { http, httpCtrl } = setup();
+    http.post('http://otel.example/v1/traces', {}).subscribe();
+    const req = httpCtrl.expectOne('http://otel.example/v1/traces');
+    expect(req.request.headers.has('X-CSRF-Token')).toBe(false);
+    req.flush({});
+  });
+
+  it('omits the header when the cookie is missing (lets the BFF respond with its 403)', () => {
+    // No setCookie call — cookie isn't set.
+    const { http, httpCtrl } = setup();
+    http.post(`${BFF_BASE}/profile`, {}).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/profile`);
+    expect(req.request.headers.has('X-CSRF-Token')).toBe(false);
+    req.flush({});
+  });
+
+  it('URL-decodes the cookie value (cookies are URI-encoded in transit)', () => {
+    // Cookies preserve their stored form; if anything URL-encodes
+    // the token (current BFF doesn't, base64url is safe), we still
+    // round-trip cleanly.
+    setCookie(CSRF_COOKIE, encodeURIComponent('A B+/='));
+    const { http, httpCtrl } = setup();
+    http.post(`${BFF_BASE}/profile`, {}).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/profile`);
+    expect(req.request.headers.get('X-CSRF-Token')).toBe('A B+/=');
+    req.flush({});
+  });
+
+  it('finds the right cookie when others are also set', () => {
+    setCookie('other_cookie', 'noise');
+    setCookie(CSRF_COOKIE, 'tok-abc');
+    setCookie('yet_another', 'noise');
+    const { http, httpCtrl } = setup();
+    http.post(`${BFF_BASE}/profile`, {}).subscribe();
+    const req = httpCtrl.expectOne(`${BFF_BASE}/profile`);
+    expect(req.request.headers.get('X-CSRF-Token')).toBe('tok-abc');
+    req.flush({});
+  });
+});
diff --git a/libs/feature/auth/src/lib/csrf.interceptor.ts b/libs/feature/auth/src/lib/csrf.interceptor.ts
new file mode 100644
index 0000000..65075d6
--- /dev/null
+++ b/libs/feature/auth/src/lib/csrf.interceptor.ts
@@ -0,0 +1,56 @@
+import type { HttpHandlerFn, HttpInterceptorFn, HttpRequest } from '@angular/common/http';
+import { inject } from '@angular/core';
+import { AUTH_BFF_BASE_URL, AUTH_CSRF_COOKIE_NAME } from './auth.config';
+
+/**
+ * Double-submit CSRF helper interceptor (per ADR-0009). Reads the
+ * BFF-issued `__Host-portal_csrf` cookie (or its dev twin
+ * `portal_csrf`) and copies the value into an `X-CSRF-Token` header
+ * on every BFF request whose method mutates state.
+ *
+ * GET / HEAD / OPTIONS pass through without a header — the BFF
+ * skips CSRF on safe methods anyway, and adding the header would
+ * just be noise.
+ *
+ * Non-BFF origins pass through untouched. Same posture as the other
+ * BFF-only interceptors in this lib.
+ *
+ * If the cookie is missing (anonymous, freshly-cleared session,
+ * etc.), the request goes through without the header — the BFF
+ * skips CSRF on anonymous requests, and on the authenticated path
+ * the BFF will answer 403 which the SPA can surface as "please
+ * sign in again".
+ */
+const SAFE_METHODS = new Set(['GET', 'HEAD', 'OPTIONS']);
+
+export const csrfInterceptor: HttpInterceptorFn = (
+  req: HttpRequest<unknown>,
+  next: HttpHandlerFn,
+) => {
+  const bffBaseUrl = inject(AUTH_BFF_BASE_URL);
+  const cookieName = inject(AUTH_CSRF_COOKIE_NAME);
+
+  if (!req.url.startsWith(bffBaseUrl)) return next(req);
+  if (SAFE_METHODS.has(req.method.toUpperCase())) return next(req);
+
+  const token = readCookie(cookieName);
+  if (!token) return next(req);
+
+  return next(req.clone({ setHeaders: { 'X-CSRF-Token': token } }));
+};
+
+function readCookie(name: string): string | null {
+  // `document.cookie` is the only browser API to read non-HttpOnly
+  // cookies. The CSRF cookie is deliberately *not* HttpOnly so we
+  // can do exactly this; the session cookie remains HttpOnly.
+  const cookies = typeof document !== 'undefined' ? document.cookie : '';
+  for (const raw of cookies.split(';')) {
+    const eq = raw.indexOf('=');
+    if (eq < 0) continue;
+    const key = raw.slice(0, eq).trim();
+    if (key === name) {
+      return decodeURIComponent(raw.slice(eq + 1));
+    }
+  }
+  return null;
+}
diff --git a/package.json b/package.json
index 45a45bf..d662321 100644
--- a/package.json
+++ b/package.json
@@ -151,6 +151,7 @@
     "connect-redis": "^9.0.0",
     "cookie-parser": "^1.4.7",
     "express-session": "^1.19.0",
+    "helmet": "^8.1.0",
     "ioredis": "^5.10.1",
     "lucide-angular": "^1.0.0",
     "nestjs-cls": "^6.2.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index f957793..0d7c37c 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -126,6 +126,9 @@ importers:
       express-session:
         specifier: ^1.19.0
         version: 1.19.0
+      helmet:
+        specifier: ^8.1.0
+        version: 8.1.0
       ioredis:
         specifier: ^5.10.1
         version: 5.10.1
@@ -6510,6 +6513,10 @@ packages:
     resolution: {integrity: sha512-F/1DnUGPopORZi0ni+CvrCgHQ5FyEAHRLSApuYWMmrbSwoN2Mn/7k+Gl38gJnR7yyDZk6WLXwiGod1JOWNDKGw==}
     hasBin: true
 
+  helmet@8.1.0:
+    resolution: {integrity: sha512-jOiHyAZsmnr8LqoPGmCjYAaiuWwjAPLgY8ZX2XrmHawt99/u1y6RgrZMTeoPfpUbV96HOalYgz1qzkRbw54Pmg==}
+    engines: {node: '>=18.0.0'}
+
   help-me@5.0.0:
     resolution: {integrity: sha512-7xgomUX6ADmcYzFik0HzAxh/73YlKR9bmFzf51CZwR+b6YtzU2m0u49hQCqV6SvlqIqsaxovfwdvbnsw3b/zpg==}
 
@@ -17339,6 +17346,8 @@ snapshots:
 
   he@1.2.0: {}
 
+  helmet@8.1.0: {}
+
   help-me@5.0.0: {}
 
   homedir-polyfill@1.0.3: