diff --git a/components/guest_view/browser/guest_view_base.cc b/components/guest_view/browser/guest_view_base.cc
--- a/components/guest_view/browser/guest_view_base.cc
+++ b/components/guest_view/browser/guest_view_base.cc
@@ -89,10 +89,21 @@ class GuestViewBase::OwnerContentsObserver : public WebContentsObserver {

   void RenderFrameHostChanged(content::RenderFrameHost* old_host,
                               content::RenderFrameHost* new_host) override {
     if (old_host && guest_->owner_rfh_id_ == old_host->GetGlobalId() &&
         new_host && guest_->element_instance_id_ == kInstanceIDNone) {
+      // SECURITY: Do not transfer ownership across processes. A cross-process
+      // navigation of the embedder must not silently grant the new (possibly
+      // cross-origin) renderer the ability to attach to this guest, since
+      // CanEmbedderAccessInstanceID derives the allowed process from
+      // owner_rfh() and owner_host_ is not refreshed here.
+      if (old_host->GetProcess()->GetID() !=
+          new_host->GetProcess()->GetID()) {
+        guest_->owner_rfh_id_ = content::GlobalRenderFrameHostId();
+        DestroyGuestIfUnattached(&*guest_);
+        return;
+      }
       // TODO(crbug.com/40202416): Do something similar for MPArch.
       guest_->owner_rfh_id_ = new_host->GetGlobalId();
     }
   }