diff --git a/chrome/browser/ui/views/permissions/permission_prompt_bubble.cc b/chrome/browser/ui/views/permissions/permission_prompt_bubble.cc
--- a/chrome/browser/ui/views/permissions/permission_prompt_bubble.cc
+++ b/chrome/browser/ui/views/permissions/permission_prompt_bubble.cc
@@ -40,19 +40,16 @@ PermissionPromptBubble::~PermissionPromptBubble() {
 }

 void PermissionPromptBubble::ShowBubble() {
-  FullscreenController* fullscreen_controller = browser()
-                                                    ->GetFeatures()
-                                                    .exclusive_access_manager()
-                                                    ->fullscreen_controller();
-  CHECK(fullscreen_controller);
-  if (fullscreen_controller->IsTabFullscreen()) {
-    auto blocker =
-        web_contents()->ForSecurityDropFullscreen(display::kInvalidDisplayId);
-    if (!blocker) {
-      return;
-    }
-    fullscreen_blocker_ = std::move(*blocker);
+  // Unconditionally hold a fullscreen blocker for the lifetime of the bubble.
+  // ForSecurityDropFullscreen() both drops any current tab-fullscreen *and*
+  // increments fullscreen_blocker_count_ so the tab cannot enter fullscreen
+  // while the bubble is up (CWE-1021 mitigation).
+  auto blocker =
+      web_contents()->ForSecurityDropFullscreen(display::kInvalidDisplayId);
+  if (!blocker) {
+    return;
   }
+  fullscreen_blocker_ = std::move(*blocker);

   raw_ptr<PermissionPromptBubbleBaseView> prompt_bubble =
       CreatePermissionPromptBubbleView(browser(), delegate()->GetWeakPtr(),