diff --git a/extensions/renderer/guest_view/mime_handler_view/post_message_support.cc b/extensions/renderer/guest_view/mime_handler_view/post_message_support.cc
index d14bb357965ae..c2b2317a23891 100644
--- a/extensions/renderer/guest_view/mime_handler_view/post_message_support.cc
+++ b/extensions/renderer/guest_view/mime_handler_view/post_message_support.cc
@@ -181,12 +181,19 @@ void PostMessageSupport::SetActive() {
   v8::Isolate* isolate = source->GetAgentGroupScheduler()->Isolate();
   v8::HandleScope handle_scope(isolate);
   v8::Context::Scope context_scope(source->MainWorldScriptContext());
-  for (const auto& pending_message : pending_messages_) {
+  // PostJavaScriptMessage() runs script (structured clone of attacker-supplied
+  // values) and may re-enter delegate_->GetTargetFrame(), either of which can
+  // synchronously delete |this| and free |pending_messages_|'s backing buffer.
+  // Move the queue onto the stack and bail out if |this| goes away.
+  auto weak_this = weak_factory_.GetWeakPtr();
+  std::vector<v8::Global<v8::Value>> messages = std::move(pending_messages_);
+  for (const auto& pending_message : messages) {
     PostJavaScriptMessage(isolate,
                           v8::Local<v8::Value>::New(isolate, pending_message));
+    if (!weak_this) {
+      return;
+    }
   }
-
-  pending_messages_.clear();
 }
 
 }  // namespace extensions