feat(server): security headers and a hash-based CSP for the web app (M462 #4980)

There were no security headers at all. Now:
- every response: nosniff, Referrer-Policy strict-origin-when-cross-origin,
  Permissions-Policy (no camera/mic/geolocation), X-Frame-Options DENY;
  each set only if the handler hasn't.
- HSTS only when the trusted proxy reports HTTPS (rule 94); never a redirect.
- index.html carries a Content-Security-Policy whose script-src is 'self'
  plus the sha256 of each inline script in the page as served, computed
  after the branding template runs. No 'unsafe-inline' or 'unsafe-eval'
  for scripts. img-src admits remote https/http because Lidarr suggestion
  art is a remote poster URL.

Hashing in Go rather than via SvelteKit's kit.csp covers the inline scripts
SvelteKit doesn't know about (app.html's theme bootstrap and the branding
global injected at build) and stays correct whatever the app name is.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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()
+}

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)
+	}
+}

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)
 

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'",
+	}, "; ")
+}

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")
+	}
+}

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)
 }