diff --git a/ApiCore-Backend/.env.example b/ApiCore-Backend/.env.example
index 015013e..12cebe9 100644
--- a/ApiCore-Backend/.env.example
+++ b/ApiCore-Backend/.env.example
@@ -14,6 +14,10 @@ SESSION_MAX_AGE="7d"
 
 PORT=3001
 FRONTEND_ORIGIN="http://localhost:3000"
+# Reverse-proxy hops in front of the backend whose X-Forwarded-For is
+# trusted (client IP for rate limits and API-key IP allowlists). "1" for the
+# single K8s ingress; a list such as "loopback, 10.0.0.0/8" also works.
+TRUST_PROXY=1
 
 # Refresh-token cookie requires HTTPS by default. Set to "false" only for
 # local HTTP development - every real deployment must leave this unset (or
diff --git a/ApiCore-Backend/src/app.config.ts b/ApiCore-Backend/src/app.config.ts
index 3f67c90..6a9b67c 100644
--- a/ApiCore-Backend/src/app.config.ts
+++ b/ApiCore-Backend/src/app.config.ts
@@ -4,18 +4,58 @@ import helmet from 'helmet';
 import cookieParser from 'cookie-parser';
 import { AllExceptionsFilter } from './common/filters/all-exceptions.filter';
 import { requestLoggerMiddleware } from './common/middleware/request-logger.middleware';
+import { noStoreMiddleware } from './common/middleware/no-store.middleware';
+
+/**
+ * How many reverse-proxy hops to trust for X-Forwarded-For. It decides the
+ * client IP behind every rate limit and API-key IP allowlist: too few and
+ * all users share the proxy's IP, too many and a client can forge its own.
+ * A hop count ("1", the single K8s ingress) or Express' list syntax
+ * ("loopback, 10.0.0.0/8").
+ */
+export function trustProxySetting(): number | string {
+  const raw = process.env.TRUST_PROXY?.trim();
+  if (!raw) {
+    return 1;
+  }
+  return /^\d+$/.test(raw) ? Number(raw) : raw;
+}
 
 export function configureApp(app: NestExpressApplication): void {
   // Trust the single reverse-proxy hop (K8s ingress) so req.ip reflects the
   // real client IP from X-Forwarded-For — required for per-client throttling
   // to work correctly rather than rate-limiting all traffic together.
-  app.set('trust proxy', 1);
+  app.set('trust proxy', trustProxySetting());
+  // No ETags: they let caches and proxies revalidate (and keep) personal
+  // responses that are meant to be no-store.
+  app.set('etag', false);
   // Default 100kb JSON body limit is too small for the avatar upload, which
   // sends the image as a base64 data: URL in the JSON body (not multipart -
   // see profile/dto/upload-avatar.dto.ts for why) - 5mb comfortably covers
   // the 2MB decoded size cap plus base64 overhead.
   app.useBodyParser('json', { limit: '5mb' });
-  app.use(helmet());
+  app.use(
+    helmet({
+      // This server only answers JSON and files. A strict policy means even
+      // a response that is somehow rendered as a page can load nothing and
+      // cannot be framed. The Swagger UI (opt-in) needs scripts, so it keeps
+      // helmet's default page policy.
+      contentSecurityPolicy:
+        process.env.ENABLE_SWAGGER === 'true'
+          ? undefined
+          : {
+              useDefaults: false,
+              directives: {
+                defaultSrc: ["'none'"],
+                frameAncestors: ["'none'"],
+                baseUri: ["'none'"],
+                formAction: ["'none'"],
+              },
+            },
+      crossOriginResourcePolicy: { policy: 'same-site' },
+    }),
+  );
+  app.use(noStoreMiddleware);
   app.use(cookieParser());
   app.use(requestLoggerMiddleware);
   app.useGlobalFilters(new AllExceptionsFilter());
diff --git a/ApiCore-Backend/src/app.module.ts b/ApiCore-Backend/src/app.module.ts
index 1c76317..38fa3e7 100644
--- a/ApiCore-Backend/src/app.module.ts
+++ b/ApiCore-Backend/src/app.module.ts
@@ -106,9 +106,11 @@ import { IdempotencyCleanupService } from './common/idempotency/idempotency-clea
   ],
   controllers: [],
   providers: [
+    // Throttle first: a request with a wrong token or API key must count
+    // against the limit too, before any database lookup is spent on it.
+    { provide: APP_GUARD, useClass: ThrottlerGuard },
     { provide: APP_GUARD, useClass: JwtAuthGuard },
     { provide: APP_GUARD, useClass: PermissionsGuard },
-    { provide: APP_GUARD, useClass: ThrottlerGuard },
     { provide: APP_INTERCEPTOR, useClass: IdempotencyInterceptor },
     IdempotencyCleanupService,
   ],
diff --git a/ApiCore-Backend/src/common/middleware/no-store.middleware.ts b/ApiCore-Backend/src/common/middleware/no-store.middleware.ts
new file mode 100644
index 0000000..9e7b474
--- /dev/null
+++ b/ApiCore-Backend/src/common/middleware/no-store.middleware.ts
@@ -0,0 +1,22 @@
+import type { NextFunction, Request, Response } from 'express';
+
+// The public origin pages of honey lots are the only answers meant for
+// everyone; they may be cached briefly.
+const CACHEABLE_PREFIX = '/api/v1/public/';
+
+/**
+ * Personal data, tokens and exports must not end up in a browser's
+ * back/forward cache or a shared proxy. Every API answer is no-store unless
+ * it is one of the public, anonymous ones.
+ */
+export function noStoreMiddleware(
+  req: Request,
+  res: Response,
+  next: NextFunction,
+): void {
+  if (!req.path.startsWith(CACHEABLE_PREFIX)) {
+    res.setHeader('Cache-Control', 'no-store');
+    res.setHeader('Pragma', 'no-cache');
+  }
+  next();
+}
diff --git a/ApiCore-Backend/src/common/middleware/request-logger.middleware.ts b/ApiCore-Backend/src/common/middleware/request-logger.middleware.ts
index 75d90f3..8102260 100644
--- a/ApiCore-Backend/src/common/middleware/request-logger.middleware.ts
+++ b/ApiCore-Backend/src/common/middleware/request-logger.middleware.ts
@@ -18,7 +18,9 @@ export function requestLoggerMiddleware(
   const start = Date.now();
   res.on('finish', () => {
     const duration = Date.now() - start;
-    const line = `${req.method} ${req.originalUrl} ${res.statusCode} ${duration}ms`;
+    // Path only: query strings can carry secrets (the SSO callback's
+    // authorization code and state, search terms with personal data).
+    const line = `${req.method} ${req.baseUrl}${req.path} ${res.statusCode} ${duration}ms`;
     if (res.statusCode >= 500) {
       logger.error(line);
     } else if (res.statusCode >= 400) {
diff --git a/ApiCore-Backend/src/main.ts b/ApiCore-Backend/src/main.ts
index 4db0e53..6fdc3a5 100644
--- a/ApiCore-Backend/src/main.ts
+++ b/ApiCore-Backend/src/main.ts
@@ -15,6 +15,21 @@ const REQUIRED_ENV_VARS = ['DATABASE_URL', 'JWT_ACCESS_SECRET'] as const;
 // generate one.
 const MIN_JWT_SECRET_LENGTH = 32;
 
+function assertProductionSettings(): void {
+  if (process.env.NODE_ENV !== 'production') {
+    return;
+  }
+  // Without it CORS and SSO redirects silently fall back to localhost.
+  if (!process.env.FRONTEND_ORIGIN?.startsWith('https://')) {
+    throw new Error(
+      'FRONTEND_ORIGIN must be set to an https:// origin in production',
+    );
+  }
+  if (process.env.COOKIE_SECURE === 'false') {
+    throw new Error('COOKIE_SECURE=false is for local HTTP development only');
+  }
+}
+
 function assertRequiredEnvVars(): void {
   const missing = REQUIRED_ENV_VARS.filter((name) => !process.env[name]);
   if (missing.length > 0) {
@@ -32,6 +47,7 @@ function assertRequiredEnvVars(): void {
 
 async function bootstrap() {
   assertRequiredEnvVars();
+  assertProductionSettings();
 
   const app = await NestFactory.create<NestExpressApplication>(AppModule);
   app.enableShutdownHooks();
diff --git a/ApiCore-Backend/test/security-headers.e2e-spec.ts b/ApiCore-Backend/test/security-headers.e2e-spec.ts
new file mode 100644
index 0000000..4e8acf9
--- /dev/null
+++ b/ApiCore-Backend/test/security-headers.e2e-spec.ts
@@ -0,0 +1,44 @@
+import { INestApplication } from '@nestjs/common';
+import request from 'supertest';
+import { App } from 'supertest/types';
+import { createTestApp } from './utils/create-test-app';
+
+describe('Security headers (e2e)', () => {
+  let app: INestApplication<App>;
+
+  beforeAll(async () => {
+    app = await createTestApp();
+  });
+
+  afterAll(async () => {
+    await app.close();
+  });
+
+  it('marks API answers no-store, unframeable and without an ETag', async () => {
+    const response = await request(app.getHttpServer())
+      .get('/api/v1/auth/sso/status')
+      .expect(200);
+
+    expect(response.headers['cache-control']).toBe('no-store');
+    expect(response.headers['content-security-policy']).toContain(
+      "default-src 'none'",
+    );
+    expect(response.headers['content-security-policy']).toContain(
+      "frame-ancestors 'none'",
+    );
+    expect(response.headers['etag']).toBeUndefined();
+    expect(response.headers['x-powered-by']).toBeUndefined();
+  });
+
+  it('rate-limits requests with a wrong API key like any other', async () => {
+    const server = app.getHttpServer();
+    const statuses: number[] = [];
+    for (let i = 0; i < 105 && !statuses.includes(429); i++) {
+      const response = await request(server)
+        .get('/api/v1/shop-sync')
+        .set('Authorization', `Bearer ack_aaaaaaaa_${'A'.repeat(43)}`);
+      statuses.push(response.status);
+    }
+    expect(statuses).toContain(429);
+  });
+});