diff --git a/internal/httpapi/csp_header_test.go b/internal/httpapi/csp_header_test.go
index d0b37fe..3275dea 100644
--- a/internal/httpapi/csp_header_test.go
+++ b/internal/httpapi/csp_header_test.go
@@ -3,6 +3,8 @@ package httpapi
 import (
 	"strings"
 	"testing"
+
+	"github.com/kristianwind/verdande/internal/config"
 )
 
 // Politikken skal stå i svaret, og den skal stadig være stram.
@@ -50,6 +52,35 @@ func TestEveryResponseCarriesAStrictCSP(t *testing.T) {
 	}
 }
 
+// HSTS følger med, når instansen kører over HTTPS — og kun der.
+//
+// Sessionen bor i en __Host-cookie, der kun findes over TLS, så hele modellen
+// forudsætter allerede HTTPS. HSTS er det, der holder den forudsætning mod en
+// netværksangriber; men bedt om over ren HTTP ville den bede en browser om at
+// kræve et certifikat, en dev-instans ikke har. Begge halvdele skal stå prøven.
+func TestHSTSTracksHTTPS(t *testing.T) {
+	overHTTP := newTestServer(t) // BaseURL er http://localhost
+	resp, _ := overHTTP.do(t, "GET", "/api/v1/ping", nil)
+	if got := resp.Header.Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("HSTS sat over ren HTTP: %q", got)
+	}
+
+	overHTTPS := newTestServerWith(t, func(c *config.Config) {
+		c.BaseURL = "https://todo.example.dk"
+	})
+	resp, _ = overHTTPS.do(t, "GET", "/api/v1/ping", nil)
+	if got := resp.Header.Get("Strict-Transport-Security"); got == "" {
+		t.Error("ingen HSTS over HTTPS")
+	} else if !strings.Contains(got, "includeSubDomains") {
+		t.Errorf("HSTS uden includeSubDomains: %q", got)
+	}
+	// Permissions-Policy står på hvert svar, uanset skema: den lukker for kamera,
+	// mikrofon og placering, som appen aldrig beder om.
+	if got := resp.Header.Get("Permissions-Policy"); !strings.Contains(got, "camera=()") {
+		t.Errorf("Permissions-Policy lukker ikke for kameraet: %q", got)
+	}
+}
+
 // Et bilag må ikke kunne blive læst som et dokument.
 func TestAttachmentsAreNotServedAsDocuments(t *testing.T) {
 	for _, mime := range []string{"image/svg+xml", "text/html", "application/xhtml+xml", "text/xml"} {
diff --git a/internal/httpapi/server.go b/internal/httpapi/server.go
index 8418b67..9ee7e37 100644
--- a/internal/httpapi/server.go
+++ b/internal/httpapi/server.go
@@ -665,6 +665,26 @@ func (s *Server) securityHeaders(next http.Handler) http.Handler {
 		h.Set("X-Frame-Options", "DENY")
 		h.Set("Referrer-Policy", "same-origin")
 		h.Set("Content-Security-Policy", s.csp)
+		// The session lives in a __Host- cookie that only exists over HTTPS, so the
+		// whole security model already assumes TLS. HSTS is what makes that
+		// assumption hold against a network attacker: without it, the very first
+		// navigation can be stripped to http:// and the cookie handed over in the
+		// clear. Gated on the same condition as the cookie's Secure flag, so a
+		// plain-HTTP dev instance is never told to demand a certificate it cannot
+		// present. Two years, subdomains included; no preload directive, because
+		// asking to be hard-coded into browsers is a decision the operator makes
+		// for their domain, not one a self-hosted app makes for them.
+		if s.secureCookies() {
+			h.Set("Strict-Transport-Security", "max-age=63072000; includeSubDomains")
+		}
+		// verdande uses none of the powerful browser features, so the safest policy
+		// is to turn them all off: an injected element cannot ask for the camera,
+		// the microphone or the location if the page itself has already declined
+		// them. Named explicitly rather than with a wildcard because the wildcard
+		// form of this header is not universally understood, and a directive a
+		// browser ignores is not a control.
+		h.Set("Permissions-Policy", "camera=(), microphone=(), geolocation=(), "+
+			"payment=(), usb=(), interest-cohort=()")
 		next.ServeHTTP(w, r)
 	})
 }