diff --git a/extension/src/popup/popup.ts b/extension/src/popup/popup.ts
index 360cb2e..a808007 100644
--- a/extension/src/popup/popup.ts
+++ b/extension/src/popup/popup.ts
@@ -33,6 +33,12 @@ export interface PopupState {
   activeGroup: string | null;
   error: string | null;
   loading: boolean;
+  // Captured tab snapshot taken at popup-open. Used by fill_credentials
+  // to guard against TOCTOU navigation — the SW re-checks this URL's
+  // hostname against the tab's live URL before forwarding fill_credentials
+  // to the content script. See router/popup-only.ts#handleFillCredentials.
+  capturedTabId: number | null;
+  capturedUrl: string;
 }
 
 let currentState: PopupState = {
@@ -45,6 +51,8 @@ let currentState: PopupState = {
   activeGroup: null,
   error: null,
   loading: false,
+  capturedTabId: null,
+  capturedUrl: '',
 };
 
 export function getState(): PopupState {
@@ -103,6 +111,13 @@ function render(): void {
 // --- Init ---
 
 async function init(): Promise<void> {
+  // Snapshot the active tab at popup-open — the fill path uses this
+  // tabId/url pair so the SW can verify the tab hasn't navigated before
+  // forwarding credentials (audit M5 + TOCTOU close via expectedHost).
+  const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
+  currentState.capturedTabId = tab?.id ?? null;
+  currentState.capturedUrl = tab?.url ?? '';
+
   // Check if extension is configured.
   const setupResp = await sendMessage({ type: 'get_setup_state' });
   if (setupResp.ok) {