safeweb: add opt-in inline style CSP toggle (#11551)

Allow the use of inline styles with safeweb via an opt-in configuration
item. This will append `style-src "self" "unsafe-inline"` to the default
CSP. The `style-src` directive will be used in lieu of the fallback
`default-src "self"` directive.

Updates tailscale/corp#8027

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

safeweb/http.go

@@ -26,6 +26,10 @@
 //   - X-Content-Type-Options header on responses set to "nosniff" to prevent MIME type sniffing attacks.
 //   - Referer-Policy header set to "same-origin" to prevent leaking referrer information to third parties.
 //
+// By default the Content-Security-Policy header will disallow inline styles.
+// This can be overridden by setting the CSPAllowInlineStyles field to true in
+// the safeweb.Config struct.
+//
 // # API routes
 //
 // safeweb inspects the Content-Type header of incoming requests to the API mux
@@ -118,6 +122,11 @@ type Config struct {
 	// If this is not provided, the Server will generate a random CSRF secret on
 	// startup.
 	CSRFSecret []byte
+
+	// CSPAllowInlineStyles specifies whether to include `style-src:
+	// unsafe-inline` in the Content-Security-Policy header to permit the use of
+	// inline CSS.
+	CSPAllowInlineStyles bool
 }
 
 func (c *Config) setDefaults() error {
@@ -144,6 +153,15 @@ func (c Config) newHandler() http.Handler {
 	// as otherwise the browser will reject the cookie
 	csrfProtect := csrf.Protect(c.CSRFSecret, csrf.Secure(c.SecureContext))
 
+	var csp string
+	if c.CSPAllowInlineStyles {
+		csp = defaultCSP + `; style-src 'self' 'unsafe-inline'`
+	} else {
+		// if no style-src is provided the browser will fallback to the
+		// default-src directive which disallows inline styles.
+		csp = defaultCSP
+	}
+
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if _, p := c.BrowserMux.Handler(r); p == "" {
 			// disallow x-www-form-urlencoded requests to the API
@@ -161,8 +179,7 @@ func (c Config) newHandler() http.Handler {
 			return
 		}
 
-		// TODO(@patrickod) consider templating additions to the CSP header.
-		w.Header().Set("Content-Security-Policy", defaultCSP)
+		w.Header().Set("Content-Security-Policy", csp)
 		w.Header().Set("X-Content-Type-Options", "nosniff")
 		w.Header().Set("Referer-Policy", "same-origin")
 		csrfProtect(c.BrowserMux).ServeHTTP(w, r)

safeweb/http_test.go

@@ -6,6 +6,8 @@ package safeweb
 import (
 	"net/http"
 	"net/http/httptest"
+	"strconv"
+	"strings"
 	"testing"
 
 	"github.com/gorilla/csrf"
@@ -364,3 +366,29 @@ func TestRefererPolicy(t *testing.T) {
 		})
 	}
 }
+
+func TestCSPAllowInlineStyles(t *testing.T) {
+	for _, allow := range []bool{false, true} {
+		t.Run(strconv.FormatBool(allow), func(t *testing.T) {
+			h := &http.ServeMux{}
+			h.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Write([]byte("ok"))
+			}))
+			s, err := NewServer(Config{BrowserMux: h, CSPAllowInlineStyles: allow})
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			req := httptest.NewRequest("GET", "/", nil)
+			w := httptest.NewRecorder()
+			s.h.Handler.ServeHTTP(w, req)
+			resp := w.Result()
+
+			csp := resp.Header.Get("Content-Security-Policy")
+			allowsStyles := strings.Contains(csp, "style-src 'self' 'unsafe-inline'")
+			if allowsStyles != allow {
+				t.Fatalf("CSP inline styles want: %v; got: %v", allow, allowsStyles)
+			}
+		})
+	}
+}