diff --git a/Dockerfile b/Dockerfile
index 61da87a..a110d40 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -9,6 +9,8 @@ RUN npm run build
 # Serve the static files using a lightweight web server
 FROM nginx:alpine
 COPY --from=builder /app/dist /usr/share/nginx/html
-COPY nginx.conf /etc/nginx/conf.d/default.conf
+# nginx.conf is a template. An env var that is not set stays in it as ${...}, and nginx then fails to start.
+ENV AUTH_HOST="" API_HOST=""
+COPY nginx.conf /etc/nginx/templates/default.conf.template
 EXPOSE 80
 CMD ["nginx", "-g", "daemon off;"]
\ No newline at end of file
diff --git a/README.md b/README.md
index 1a080ee..9dfe24b 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,8 @@ To Be Played is split into three repositories:
 - Tailwind CSS 4, Radix UI primitives (shadcn/ui style components), Lucide icons
 - react-hook-form + zod for forms, i18next for translations (`public/i18n`)
 - Vitest + React Testing Library + jsdom
-- Production image: nginx (Alpine), SPA fallback and long-term caching for hashed assets
+- Production image: nginx (Alpine), SPA fallback, long-term caching for hashed assets, Content Security Policy
+  (report-only for now) and security headers
 - CI: Forgejo Actions (`npm ci`, `npm test`, `npm run build`, Docker image)
 
 ## Project structure
@@ -90,7 +91,7 @@ npm run build
 ## Roadmap
 
 - Mood filter for "What to play next".
-- A Content Security Policy in `nginx.conf`.
+- Enforce the Content Security Policy (it runs in report-only mode now).
 
 ## License
 
diff --git a/nginx.conf b/nginx.conf
index 022f911..a70a83d 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -2,6 +2,15 @@ server {
     listen 80;
     root /usr/share/nginx/html;
 
+    # An nginx template: AUTH_HOST and API_HOST come from the container env at start (see Dockerfile).
+    # Report-Only until production shows no violation in the browser console; then rename the header.
+    add_header Content-Security-Policy-Report-Only "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https://*.steamstatic.com https://steamcdn-a.akamaihd.net https://media.steampowered.com https://images.igdb.com; font-src 'self' data:; connect-src 'self' https://${AUTH_HOST} https://${API_HOST}; manifest-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Permissions-Policy "camera=(), microphone=(), geolocation=()" always;
+    # Keeps the headers above in the locations that add their own Cache-Control (nginx 1.29.3 and later).
+    add_header_inherit merge;
+
     # Vite puts a content hash in these file names, so they never change.
     location /assets/ {
         add_header Cache-Control "public, max-age=31536000, immutable";
diff --git a/src/csp.test.js b/src/csp.test.js
new file mode 100644
index 0000000..c2b6802
--- /dev/null
+++ b/src/csp.test.js
@@ -0,0 +1,41 @@
+import {readdirSync, readFileSync} from 'node:fs';
+import path from 'node:path';
+import {describe, expect, it} from 'vitest';
+
+const root = path.resolve(import.meta.dirname, '..');
+const nginx = readFileSync(path.join(root, 'nginx.conf'), 'utf8');
+const policy = nginx.match(/Content-Security-Policy(?:-Report-Only)? "([^"]+)"/)[1];
+const directive = (name) => policy.split(';').map((part) => part.trim()).find((part) => part.startsWith(`${name} `));
+
+const LINK_ONLY_HOSTS = ['store.steampowered.com'];
+
+const allows = (sources, host) => sources.split(' ').some((source) =>
+    source === `https://${host}` || (source.startsWith('https://*.') && host.endsWith(source.slice('https://*'.length))));
+
+describe('Content Security Policy in nginx.conf', () => {
+    it('allows every image host the code builds URLs for', () => {
+        const code = readdirSync(path.join(root, 'src'), {recursive: true})
+            .filter((file) => /\.jsx?$/.test(file) && !/\.test\.jsx?$/.test(file) && !file.startsWith('test'))
+            .map((file) => readFileSync(path.join(root, 'src', file), 'utf8'))
+            .join('\n');
+        const hosts = new Set([...code.matchAll(/`https:\/\/([a-z0-9.-]+)\//g)].map((match) => match[1]));
+        // Only links: a navigation needs no img-src.
+        LINK_ONLY_HOSTS.forEach((host) => hosts.delete(host));
+        // Hosts the backends pass on: Steam avatars and achievement icons, IGDB pictures.
+        ['avatars.steamstatic.com', 'steamcdn-a.akamaihd.net', 'media.steampowered.com', 'images.igdb.com']
+            .forEach((host) => hosts.add(host));
+
+        for (const host of hosts) {
+            expect(allows(directive('img-src'), host), host).toBe(true);
+        }
+    });
+
+    it('lets the app reach Auth and Core, whose hosts come from the container env', () => {
+        expect(directive('connect-src')).toBe("connect-src 'self' https://${AUTH_HOST} https://${API_HOST}");
+    });
+
+    it('allows no inline or foreign script', () => {
+        expect(directive('script-src')).toBe("script-src 'self'");
+        expect(directive('frame-ancestors')).toBe("frame-ancestors 'none'");
+    });
+});