diff --git a/internal/server/hygiene.go b/internal/server/hygiene.go
index 77901ae2..649f392f 100644
--- a/internal/server/hygiene.go
+++ b/internal/server/hygiene.go
@@ -113,3 +113,42 @@ func requireJSONForCookieWrites(next http.Handler) http.Handler {
 		next.ServeHTTP(w, r)
 	})
 }
+
+// securityHeaders sets the response headers every response should carry.
+// Each is set only when the handler hasn't, so a route with a reason to
+// differ keeps its own value. The document's Content-Security-Policy is set
+// by the SPA handler, which knows the inline-script hashes.
+//
+// HSTS goes out only when the request reached us over HTTPS as the trusted
+// proxy reports it (rule 94): sending it over plain HTTP is ignored by
+// browsers at best, and the app never forces HTTPS.
+func securityHeaders(hops func() int) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			h := w.Header()
+			setDefault(h, "X-Content-Type-Options", "nosniff")
+			setDefault(h, "Referrer-Policy", "strict-origin-when-cross-origin")
+			setDefault(h, "Permissions-Policy", "camera=(), microphone=(), geolocation=(), interest-cohort=()")
+			// frame-ancestors in the document's CSP covers modern browsers;
+			// this covers the rest, and every non-HTML response.
+			setDefault(h, "X-Frame-Options", "DENY")
+			if auth.IsHTTPS(r, hopsOrZero(hops)) {
+				setDefault(h, "Strict-Transport-Security", "max-age=31536000")
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+func setDefault(h http.Header, key, value string) {
+	if h.Get(key) == "" {
+		h.Set(key, value)
+	}
+}
+
+func hopsOrZero(hops func() int) int {
+	if hops == nil {
+		return 0
+	}
+	return hops()
+}
diff --git a/internal/server/hygiene_test.go b/internal/server/hygiene_test.go
index 3e432b96..ae9a1c4f 100644
--- a/internal/server/hygiene_test.go
+++ b/internal/server/hygiene_test.go
@@ -88,3 +88,41 @@ func TestRequireJSONForCookieWrites(t *testing.T) {
 		})
 	}
 }
+
+func TestSecurityHeaders(t *testing.T) {
+	serve := func(hops int, proto string) http.Header {
+		h := securityHeaders(func() int { return hops })(okHandler())
+		req := httptest.NewRequest(http.MethodGet, "/api/me", nil)
+		if proto != "" {
+			req.Header.Set("X-Forwarded-Proto", proto)
+		}
+		w := httptest.NewRecorder()
+		h.ServeHTTP(w, req)
+		return w.Header()
+	}
+
+	base := serve(0, "")
+	for key, want := range map[string]string{
+		"X-Content-Type-Options": "nosniff",
+		"Referrer-Policy":        "strict-origin-when-cross-origin",
+		"X-Frame-Options":        "DENY",
+	} {
+		if got := base.Get(key); got != want {
+			t.Errorf("%s = %q, want %q", key, got, want)
+		}
+	}
+	if !strings.Contains(base.Get("Permissions-Policy"), "microphone=()") {
+		t.Errorf("Permissions-Policy = %q", base.Get("Permissions-Policy"))
+	}
+
+	// Rule 94's three cases for HSTS.
+	if got := serve(0, "https").Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("hops 0 + forwarded https: HSTS = %q, want none", got)
+	}
+	if got := serve(1, "https").Get("Strict-Transport-Security"); got == "" {
+		t.Error("hops 1 + forwarded https: want HSTS")
+	}
+	if got := serve(1, "").Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("plain request: HSTS = %q, want none", got)
+	}
+}
diff --git a/internal/server/server.go b/internal/server/server.go
index 510278ed..efae1029 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -139,6 +139,7 @@ func (s *Server) Router() http.Handler {
 	r.Use(middleware.RequestID)
 	r.Use(requestLog(s.Logger, netSettings.Hops))
 	r.Use(middleware.Recoverer)
+	r.Use(securityHeaders(netSettings.Hops))
 	r.Use(limitRequestBody)
 	r.Use(requireJSONForCookieWrites)
 
diff --git a/web/csp.go b/web/csp.go
new file mode 100644
index 00000000..b255998c
--- /dev/null
+++ b/web/csp.go
@@ -0,0 +1,57 @@
+package web
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"regexp"
+	"strings"
+)
+
+// inlineScriptRe matches each <script> element and captures its attributes
+// and body. index.html is our own build output, so a regexp is enough: there
+// is no hostile markup to out-parse.
+var inlineScriptRe = regexp.MustCompile(`(?is)<script\b([^>]*)>(.*?)</script>`)
+
+var srcAttrRe = regexp.MustCompile(`(?i)\bsrc\s*=`)
+
+// contentSecurityPolicy builds the policy served with index.html.
+//
+// Scripts are allowed from our own origin plus the exact inline scripts the
+// page carries: the theme bootstrap in app.html, the branding global the Vite
+// plugin injects, and SvelteKit's start-up block. Their hashes are taken from
+// the page AFTER the branding template has run, so they match the bytes the
+// browser actually receives, whatever the operator's app name. Any script
+// that differs, including one injected through an XSS, is refused.
+//
+// The rest:
+//   - style-src allows inline styles: Svelte transitions and style:
+//     directives write them, and inline style is not a script vector.
+//   - img-src admits https:/http: because Lidarr suggestion art is a remote
+//     poster URL. Images cannot run code.
+//   - media-src/connect-src stay on our own origin: streams, the API and the
+//     SSE stream are all same-origin. blob: covers Web Audio and object URLs.
+func contentSecurityPolicy(indexHTML []byte) string {
+	scriptSrc := []string{"'self'"}
+	for _, m := range inlineScriptRe.FindAllSubmatch(indexHTML, -1) {
+		if srcAttrRe.Match(m[1]) {
+			continue
+		}
+		sum := sha256.Sum256(m[2])
+		scriptSrc = append(scriptSrc, "'sha256-"+base64.StdEncoding.EncodeToString(sum[:])+"'")
+	}
+	return strings.Join([]string{
+		"default-src 'self'",
+		"base-uri 'self'",
+		"object-src 'none'",
+		"frame-ancestors 'none'",
+		"form-action 'self'",
+		"script-src " + strings.Join(scriptSrc, " "),
+		"style-src 'self' 'unsafe-inline'",
+		"img-src 'self' data: blob: https: http:",
+		"font-src 'self' data:",
+		"media-src 'self' blob:",
+		"connect-src 'self'",
+		"worker-src 'self' blob:",
+		"manifest-src 'self'",
+	}, "; ")
+}
diff --git a/web/csp_test.go b/web/csp_test.go
new file mode 100644
index 00000000..b603597a
--- /dev/null
+++ b/web/csp_test.go
@@ -0,0 +1,60 @@
+package web
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"strings"
+	"testing"
+)
+
+func hashOf(body string) string {
+	sum := sha256.Sum256([]byte(body))
+	return "'sha256-" + base64.StdEncoding.EncodeToString(sum[:]) + "'"
+}
+
+func TestContentSecurityPolicy_HashesInlineScriptsOnly(t *testing.T) {
+	theme := "(function () { document.documentElement.dataset.theme = 'dark'; })();"
+	brand := `window.__MINSTREL__ = { appName: "Minstrel" };`
+	html := "<html><head><script>" + theme + "</script>" +
+		`<script type="module" src="/_app/start.js"></script>` +
+		"<script>" + brand + "</script></head></html>"
+
+	csp := contentSecurityPolicy([]byte(html))
+
+	var scriptSrc string
+	for _, d := range strings.Split(csp, "; ") {
+		if strings.HasPrefix(d, "script-src ") {
+			scriptSrc = d
+		}
+	}
+	if scriptSrc == "" {
+		t.Fatalf("no script-src in %q", csp)
+	}
+	for _, want := range []string{"'self'", hashOf(theme), hashOf(brand)} {
+		if !strings.Contains(scriptSrc, want) {
+			t.Errorf("script-src %q missing %s", scriptSrc, want)
+		}
+	}
+	if strings.Count(scriptSrc, "'sha256-") != 2 {
+		t.Errorf("script-src %q: want exactly the two inline scripts hashed, not the src= one", scriptSrc)
+	}
+	if strings.Contains(scriptSrc, "unsafe-inline") || strings.Contains(scriptSrc, "unsafe-eval") {
+		t.Errorf("script-src must not fall back to unsafe-*: %q", scriptSrc)
+	}
+	for _, want := range []string{"frame-ancestors 'none'", "object-src 'none'", "connect-src 'self'"} {
+		if !strings.Contains(csp, want) {
+			t.Errorf("csp missing %q", want)
+		}
+	}
+}
+
+// The hash must be of the page as served, after the branding template has
+// substituted the operator's app name, or a renamed instance would refuse
+// its own bootstrap script.
+func TestContentSecurityPolicy_TracksTemplatedContent(t *testing.T) {
+	a := contentSecurityPolicy([]byte(`<script>window.__MINSTREL__ = { appName: "A" };</script>`))
+	b := contentSecurityPolicy([]byte(`<script>window.__MINSTREL__ = { appName: "B" };</script>`))
+	if a == b {
+		t.Error("different script bodies produced the same policy")
+	}
+}
diff --git a/web/embed.go b/web/embed.go
index 2c63b3c9..690c41c7 100644
--- a/web/embed.go
+++ b/web/embed.go
@@ -47,12 +47,14 @@ func Handler(branding config.BrandingConfig) http.Handler {
 		panic("web: branding template failed: " + err.Error())
 	}
 
+	csp := contentSecurityPolicy(index)
+
 	fileServer := http.FileServer(http.FS(sub))
 
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		clean := path.Clean(r.URL.Path)
 		if clean == "/" || clean == "." {
-			serveIndex(w, index)
+			serveIndex(w, index, csp)
 			return
 		}
 		name := strings.TrimPrefix(clean, "/")
@@ -60,12 +62,16 @@ func Handler(branding config.BrandingConfig) http.Handler {
 			fileServer.ServeHTTP(w, r)
 			return
 		}
-		serveIndex(w, index)
+		serveIndex(w, index, csp)
 	})
 }
 
-func serveIndex(w http.ResponseWriter, index []byte) {
+func serveIndex(w http.ResponseWriter, index []byte, csp string) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	// The policy only means anything on the document; JSON, audio and image
+	// responses can't run script, so it rides here rather than on every
+	// response.
+	w.Header().Set("Content-Security-Policy", csp)
 	w.Header().Set("Cache-Control", "no-cache")
 	_, _ = w.Write(index)
 }