Use CSP header to restrict resource loading

This helps mitigate XSS exploits.
Users will have to save the settings again to make the custom CSS
work.

model/session.go

@@ -27,6 +27,7 @@ type Settings struct {
 	AntiDopamineMode      bool   `json:"adm,omitempty"`
 	HideUnsupportedNotifs bool   `json:"hun,omitempty"`
 	CSS                   string `json:"css,omitempty"`
+	CSSHash               string `json:"cssh,omitempty"`
 }
 
 func NewSettings() *Settings {
@@ -43,5 +44,6 @@ func NewSettings() *Settings {
 		AntiDopamineMode:      false,
 		HideUnsupportedNotifs: false,
 		CSS:                   "",
+		CSSHash:               "",
 	}
 }

service/service.go

@@ -1,6 +1,8 @@
 package service
 
 import (
+	"crypto/sha256"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"mime/multipart"
@@ -1014,8 +1016,18 @@ func (s *service) SaveSettings(c *client, settings *model.Settings) (err error)
 	default:
 		return errInvalidArgument
 	}
-	if len(settings.CSS) > 1<<20 {
+	if len(settings.CSS) > 0 {
-		return errInvalidArgument
+		if len(settings.CSS) > 1<<20 {
+			return errInvalidArgument
+		}
+		// For some reason, browsers convert CRLF to LF before calculating
+		// the hash of the inline resources.
+		settings.CSS = strings.ReplaceAll(settings.CSS, "\x0d\x0a", "\x0a")
+
+		h := sha256.Sum256([]byte(settings.CSS))
+		settings.CSSHash = base64.StdEncoding.EncodeToString(h[:])
+	} else {
+		settings.CSSHash = ""
 	}
 	c.s.Settings = *settings
 	return c.setSession(c.s)

service/transport.go

@@ -26,6 +26,16 @@ const (
 	CSRF
 )
 
+const csp = "default-src 'none';" +
+	" img-src *;" +
+	" media-src *;" +
+	" font-src *;" +
+	" child-src *;" +
+	" connect-src 'self';" +
+	" form-action 'self';" +
+	" script-src 'self';" +
+	" style-src 'self'"
+
 func NewHandler(s *service, verbose bool, staticDir string) http.Handler {
 	r := mux.NewRouter()
 
@@ -58,14 +68,14 @@ func NewHandler(s *service, verbose bool, staticDir string) http.Handler {
 				}(time.Now())
 			}
 
-			var ct string
+			h := c.w.Header()
 			switch rt {
 			case HTML:
-				ct = "text/html; charset=utf-8"
+				h.Set("Content-Type", "text/html; charset=utf-8")
+				h.Set("Content-Security-Policy", csp)
 			case JSON:
-				ct = "application/json"
+				h.Set("Content-Type", "application/json")
 			}
-			c.w.Header().Add("Content-Type", ct)
 
 			err = c.authenticate(at, s.instance)
 			if err != nil {
@@ -73,6 +83,13 @@ func NewHandler(s *service, verbose bool, staticDir string) http.Handler {
 				return
 			}
 
+			// Override the CSP header to allow custom CSS
+			if rt == HTML && len(c.s.Settings.CSS) > 0 &&
+				len(c.s.Settings.CSSHash) > 0 {
+				v := fmt.Sprintf("%s 'sha256-%s'", csp, c.s.Settings.CSSHash)
+				h.Set("Content-Security-Policy", v)
+			}
+
 			err = f(c)
 			if err != nil {
 				writeError(c, err, rt, req.Method == http.MethodGet)