fix: use time-constant comparison for CSRF tokens (#9875)

This hardens the framework against a theoretical timing attack based on
comparing how quickly a request with an invalid CSRF token is rejected.

(cherry picked from commit 292b3a0)

flow-server/src/main/java/com/vaadin/flow/server/VaadinService.java

@@ -29,6 +29,8 @@
 import java.io.Serializable;
 import java.lang.reflect.Constructor;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -1941,7 +1943,9 @@ public static boolean isCsrfTokenValid(UI ui, String requestToken) {
                 .isXsrfProtectionEnabled()) {
             String uiToken = ui.getCsrfToken();
 
-            if (uiToken == null || !uiToken.equals(requestToken)) {
+            if (uiToken == null || !MessageDigest.isEqual(
+                    uiToken.getBytes(StandardCharsets.UTF_8),
+                    requestToken.getBytes(StandardCharsets.UTF_8))) {
                 return false;
             }
         }