diff --git a/internal/api/middleware/security_headers.go b/internal/api/middleware/security_headers.go
index c2f6401b5..1580bfe1c 100644
--- a/internal/api/middleware/security_headers.go
+++ b/internal/api/middleware/security_headers.go
@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"net"
 	"strings"
 
 	"github.com/gin-gonic/gin"
@@ -48,13 +49,14 @@ func SecurityHeaders() gin.HandlerFunc {
 
 // PageHeaders is the part of SecurityHeaders a service serving framable pages
 // can carry (the forms service): no content sniffing, a referrer policy, and
-// HSTS over TLS. Each page sets its own CSP.
-func PageHeaders() gin.HandlerFunc {
+// HSTS over TLS on ownHost only, never on a customer's domain. Each page sets its own CSP.
+func PageHeaders(ownHost string) gin.HandlerFunc {
+	ownHost = strings.ToLower(ownHost)
 	return func(c *gin.Context) {
 		h := c.Writer.Header()
 		h.Set("X-Content-Type-Options", "nosniff")
 		h.Set("Referrer-Policy", "strict-origin-when-cross-origin")
-		if requestIsHTTPS(c) {
+		if ownHost != "" && requestIsHTTPS(c) && strings.EqualFold(requestHostname(c.Request.Host), ownHost) {
 			h.Set("Strict-Transport-Security", "max-age=31536000; includeSubDomains")
 		}
 		c.Next()
@@ -79,3 +81,11 @@ func requestIsHTTPS(c *gin.Context) bool {
 	}
 	return false
 }
+
+// requestHostname is the request's Host without a port.
+func requestHostname(host string) string {
+	if h, _, err := net.SplitHostPort(host); err == nil {
+		return h
+	}
+	return host
+}
diff --git a/internal/formserver/server.go b/internal/formserver/server.go
index d432e3b98..fdda618c9 100644
--- a/internal/formserver/server.go
+++ b/internal/formserver/server.go
@@ -16,6 +16,7 @@ import (
 	"fmt"
 	"html"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strings"
@@ -23,6 +24,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/warmbly/warmbly/internal/api/middleware"
+	"github.com/warmbly/warmbly/internal/config"
 	"github.com/warmbly/warmbly/internal/formwire"
 )
 
@@ -138,7 +140,7 @@ func (s *Server) Router(trustedProxies []string) (*gin.Engine, error) {
 		gin.SetMode(gin.ReleaseMode)
 	}
 	r := gin.New()
-	r.Use(middleware.RequestLogger(), gin.Recovery(), middleware.PageHeaders())
+	r.Use(middleware.RequestLogger(), gin.Recovery(), middleware.PageHeaders(formsHost()))
 	// Same posture as the backend: trust no proxy unless the operator names
 	// it, so a forged X-Forwarded-For cannot dodge the submit limiter.
 	if len(trustedProxies) > 0 {
@@ -433,3 +435,12 @@ func (s *Server) ServeFormsEmbedJS(c *gin.Context) {
 	c.Header("Cache-Control", "public, max-age=3600")
 	c.Data(http.StatusOK, "application/javascript; charset=utf-8", formsEmbedJS)
 }
+
+// formsHost is the hostname of FORMS_DOMAIN, the one host the forms service sends HSTS for.
+func formsHost() string {
+	u, err := url.Parse(config.FormsBaseURL())
+	if err != nil {
+		return ""
+	}
+	return u.Hostname()
+}
diff --git a/site/public/install.sh b/site/public/install.sh
index e72578192..55156909d 100644
--- a/site/public/install.sh
+++ b/site/public/install.sh
@@ -1604,6 +1604,7 @@ render_caddyfile() {
 	header {
 		X-Content-Type-Options "nosniff"
 		Referrer-Policy "strict-origin-when-cross-origin"
+		-Strict-Transport-Security
 		-Server
 	}
 }
diff --git a/site/public/install.sh.sha256 b/site/public/install.sh.sha256
index 02ecf7c18..ef0b66788 100644
--- a/site/public/install.sh.sha256
+++ b/site/public/install.sh.sha256
@@ -1 +1 @@
-098221eed28b874d84250518a001bbf38e3e74520c00bc48d3cc1328d2107f56  install.sh
+d68bebcc66b502d4f78736a1325517aea199e7f3fdc8c517f9d4165ff7985116  install.sh