diff --git a/backend/Cargo.lock b/backend/Cargo.lock
index b6679ea..a7f5b86 100644
--- a/backend/Cargo.lock
+++ b/backend/Cargo.lock
@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.128.4"
+version = "0.128.5"
 dependencies = [
  "anyhow",
  "argon2",
diff --git a/backend/Cargo.toml b/backend/Cargo.toml
index 603e10d..d975c4d 100644
--- a/backend/Cargo.toml
+++ b/backend/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.128.4"
+version = "0.128.5"
 edition = "2021"
 default-run = "mangalord"
 
diff --git a/frontend/csp-config.js b/frontend/csp-config.js
new file mode 100644
index 0000000..27b6b56
--- /dev/null
+++ b/frontend/csp-config.js
@@ -0,0 +1,10 @@
+// Shared CSP constants, kept dependency-free so both svelte.config.js (loaded by
+// Node when Vite starts) and the vitest drift-guard test (jsdom env) can import
+// it without pulling in adapter-node / esbuild.
+
+// sha256 of the inline theme <script> in src/app.html, base64-encoded, wrapped
+// for the CSP `script-src` allowlist. SvelteKit's kit.csp hash mode does not
+// cover app.html template scripts, so this is pinned by hand. src/csp-theme-hash.test.ts
+// recomputes it from app.html and fails if it drifts.
+export const THEME_SCRIPT_HASH =
+    "'sha256-qj6Oim9siqow/Su+v47sZHJeJci+M/RQwGXn53eSULQ='";
diff --git a/frontend/e2e/security-headers.spec.ts b/frontend/e2e/security-headers.spec.ts
new file mode 100644
index 0000000..f95b979
--- /dev/null
+++ b/frontend/e2e/security-headers.spec.ts
@@ -0,0 +1,37 @@
+import { test, expect } from './fixtures';
+
+// Defense-in-depth response headers on document navigations (T4 in the security
+// audit). The CSP is emitted by SvelteKit's kit.csp config; the clickjacking /
+// referrer / permissions headers by hooks.server.ts. We assert on the raw
+// response of a document navigation, and separately confirm the inline theme
+// script still executes under the CSP (its sha256 is allowlisted) by checking
+// the data-theme attribute it sets — a CSP block would leave it unset.
+
+test('document responses carry the security headers', async ({ page }) => {
+    const response = await page.goto('/');
+    expect(response, 'navigation returned a response').not.toBeNull();
+    const headers = response!.headers();
+
+    // Clickjacking: both the legacy header and the CSP directive.
+    expect(headers['x-frame-options']).toBe('DENY');
+    expect(headers['content-security-policy']).toContain("frame-ancestors 'none'");
+    // Script-injection surface reduction.
+    expect(headers['content-security-policy']).toContain("object-src 'none'");
+    expect(headers['content-security-policy']).toContain('script-src');
+    // The non-CSP hardening headers.
+    expect(headers['referrer-policy']).toBe('strict-origin-when-cross-origin');
+    expect(headers['x-content-type-options']).toBe('nosniff');
+    expect(headers['permissions-policy']).toContain('geolocation=()');
+});
+
+test('the inline theme script executes under the CSP (hash is allowlisted)', async ({
+    page
+}) => {
+    // If the theme script were CSP-blocked, data-theme would never be set.
+    // Its presence proves the allowlisted sha256 matches the served script.
+    await page.goto('/');
+    const theme = await page.evaluate(() =>
+        document.documentElement.getAttribute('data-theme')
+    );
+    expect(theme === 'light' || theme === 'dark').toBe(true);
+});
diff --git a/frontend/package.json b/frontend/package.json
index 89eb1dd..e04c3e8 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.4",
+  "version": "0.128.5",
   "private": true,
   "type": "module",
   "scripts": {
diff --git a/frontend/src/csp-theme-hash.test.ts b/frontend/src/csp-theme-hash.test.ts
new file mode 100644
index 0000000..5104a4e
--- /dev/null
+++ b/frontend/src/csp-theme-hash.test.ts
@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { createHash } from 'node:crypto';
+import { join } from 'node:path';
+import { THEME_SCRIPT_HASH } from '../csp-config.js';
+
+// The theme-flash-prevention <script> in app.html runs inline, so the CSP
+// script-src must allow it by hash. SvelteKit's kit.csp hash mode does NOT
+// cover app.html template scripts (only what it injects into %sveltekit.head%),
+// so we pin the hash by hand in svelte.config.js. This test recomputes the hash
+// from the actual app.html bytes and fails if THEME_SCRIPT_HASH drifted — i.e.
+// someone edited the theme script without updating the CSP, which would silently
+// CSP-block it (theme flash returns, only a console error to show for it).
+describe('CSP theme-script hash', () => {
+    it('matches the current inline theme script in app.html', () => {
+        // vitest runs with cwd at the frontend package root.
+        const html = readFileSync(join(process.cwd(), 'src/app.html'), 'utf-8');
+        const match = html.match(/<script>(.*?)<\/script>/s);
+        expect(match, 'app.html must contain an inline <script>').not.toBeNull();
+        const digest = createHash('sha256')
+            .update(match![1], 'utf-8')
+            .digest('base64');
+        expect(THEME_SCRIPT_HASH).toBe(`'sha256-${digest}'`);
+    });
+});
diff --git a/frontend/src/hooks.server.test.ts b/frontend/src/hooks.server.test.ts
index e5653a2..efa54cf 100644
--- a/frontend/src/hooks.server.test.ts
+++ b/frontend/src/hooks.server.test.ts
@@ -8,6 +8,7 @@ import {
     type MockInstance
 } from 'vitest';
 import {
+    applySecurityHeaders,
     handle,
     setForwardedFor,
     shouldBypassProxyTimeout,
@@ -353,6 +354,36 @@ describe('stripHopByHopHeaders', () => {
     });
 });
 
+describe('applySecurityHeaders', () => {
+    it('sets the clickjacking and defense-in-depth headers', () => {
+        const h = applySecurityHeaders(new Headers());
+        // Clickjacking: legacy + modern. CSP frame-ancestors is emitted by
+        // SvelteKit's kit.csp config; X-Frame-Options is the belt-and-braces
+        // fallback for older UAs.
+        expect(h.get('x-frame-options')).toBe('DENY');
+        expect(h.get('referrer-policy')).toBe('strict-origin-when-cross-origin');
+        expect(h.get('x-content-type-options')).toBe('nosniff');
+        // Lock down powerful features we never use.
+        const pp = h.get('permissions-policy') ?? '';
+        expect(pp).toContain('camera=()');
+        expect(pp).toContain('microphone=()');
+        expect(pp).toContain('geolocation=()');
+    });
+
+    it('does not clobber a header the response already set', () => {
+        // If a downstream route deliberately set a stricter Referrer-Policy,
+        // the helper must not override it.
+        const h = new Headers({ 'referrer-policy': 'no-referrer' });
+        applySecurityHeaders(h);
+        expect(h.get('referrer-policy')).toBe('no-referrer');
+    });
+
+    it('returns the same Headers instance it was given (mutates in place)', () => {
+        const h = new Headers();
+        expect(applySecurityHeaders(h)).toBe(h);
+    });
+});
+
 describe('setForwardedFor', () => {
     it('overrides any client-supplied x-forwarded-for with the real address', () => {
         const headers = new Headers({ 'x-forwarded-for': '1.2.3.4', 'x-real-ip': '1.2.3.4' });
diff --git a/frontend/src/hooks.server.ts b/frontend/src/hooks.server.ts
index b809092..4ad2fbb 100644
--- a/frontend/src/hooks.server.ts
+++ b/frontend/src/hooks.server.ts
@@ -105,6 +105,38 @@ export function shouldBypassProxyTimeout(headers: Headers): boolean {
     return accept.toLowerCase().includes('text/event-stream');
 }
 
+/**
+ * Defense-in-depth response headers for HTML/document responses.
+ *
+ * The Content-Security-Policy itself is emitted by SvelteKit's `kit.csp`
+ * config (see svelte.config.js) so that `script-src` hashes cover both the
+ * inline theme script in app.html AND SvelteKit's own hydration inline
+ * scripts — a hand-rolled `script-src` here would break hydration on the next
+ * build. What CSP can't (or shouldn't) express, we add here:
+ *
+ * - `X-Frame-Options: DENY` — clickjacking guard for UAs predating CSP
+ *   `frame-ancestors` (which the kit.csp policy also sets).
+ * - `Referrer-Policy` — don't leak full URLs to cross-origin destinations.
+ * - `X-Content-Type-Options: nosniff` — no MIME sniffing on documents.
+ * - `Permissions-Policy` — deny powerful features the app never uses.
+ *
+ * Each header is only set when absent, so a route that deliberately picked a
+ * stricter value keeps it. Mutates and returns the passed `Headers`. Exported
+ * for unit-test coverage.
+ */
+export function applySecurityHeaders(headers: Headers): Headers {
+    const defaults: Record<string, string> = {
+        'x-frame-options': 'DENY',
+        'referrer-policy': 'strict-origin-when-cross-origin',
+        'x-content-type-options': 'nosniff',
+        'permissions-policy': 'camera=(), microphone=(), geolocation=(), interest-cohort=()'
+    };
+    for (const [name, value] of Object.entries(defaults)) {
+        if (!headers.has(name)) headers.set(name, value);
+    }
+    return headers;
+}
+
 export const handle: Handle = async ({ event, resolve }) => {
     if (event.url.pathname.startsWith('/api/')) {
         const target = `${BACKEND_URL}${event.url.pathname}${event.url.search}`;
@@ -199,5 +231,7 @@ export const handle: Handle = async ({ event, resolve }) => {
             headers: stripHopByHopHeaders(upstream.headers)
         });
     }
-    return resolve(event);
+    const response = await resolve(event);
+    applySecurityHeaders(response.headers);
+    return response;
 };
diff --git a/frontend/svelte.config.js b/frontend/svelte.config.js
index 2a214ea..bf77f37 100644
--- a/frontend/svelte.config.js
+++ b/frontend/svelte.config.js
@@ -1,11 +1,34 @@
 import adapter from '@sveltejs/adapter-node';
 import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
+import { THEME_SCRIPT_HASH } from './csp-config.js';
 
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
     preprocess: vitePreprocess(),
     kit: {
-        adapter: adapter({ out: 'build' })
+        adapter: adapter({ out: 'build' }),
+        // Content-Security-Policy. `mode: 'hash'` makes SvelteKit hash the
+        // inline scripts IT injects (the hydration bootstrap) and append those
+        // hashes to `script-src`. It does NOT hash the theme <script> in
+        // app.html — that template script isn't part of `%sveltekit.head%`, so
+        // we pin its sha256 here by hand (THEME_SCRIPT_HASH). If that script is
+        // edited, the hash drifts and the theme-flash guard would be silently
+        // CSP-blocked; `svelte.config.test.js` recomputes the hash from
+        // app.html and fails if it no longer matches this constant.
+        // Styles are left unconstrained (Svelte emits dynamic inline `style=`
+        // attributes); this policy is clickjacking + script-injection defense
+        // in depth, not a full lockdown. The non-CSP defense-in-depth headers
+        // (X-Frame-Options, Referrer-Policy, Permissions-Policy) live in
+        // hooks.server.ts.
+        csp: {
+            mode: 'hash',
+            directives: {
+                'script-src': ['self', THEME_SCRIPT_HASH],
+                'object-src': ['none'],
+                'base-uri': ['self'],
+                'frame-ancestors': ['none']
+            }
+        }
     }
 };