Create the host socket with mode 0600 from the start

skills/chrome-control/native-host/host.js

@@ -301,20 +301,26 @@ function lstatIfExists(file) {
 	}
 }
 function listenUnix() {
-	server.listen(socketPath, () => {
-		try {
-			const bound = node_fs.default.lstatSync(socketPath);
-			boundSocket = {
-				dev: bound.dev,
-				ino: bound.ino
-			};
-			ownsSocket = true;
-			node_fs.default.chmodSync(socketPath, 384);
-			releaseStartupLock();
-		} catch {
-			refuseEndpoint();
-		}
-	});
+	const previousUmask = node_process.default.umask(127);
+	try {
+		server.listen(socketPath, onUnixListening);
+	} finally {
+		node_process.default.umask(previousUmask);
+	}
+}
+function onUnixListening() {
+	try {
+		const bound = node_fs.default.lstatSync(socketPath);
+		boundSocket = {
+			dev: bound.dev,
+			ino: bound.ino
+		};
+		ownsSocket = true;
+		node_fs.default.chmodSync(socketPath, 384);
+		releaseStartupLock();
+	} catch {
+		refuseEndpoint();
+	}
 }
 function removeOwnSocket() {
 	try {

src/native-host/host.ts

@@ -243,17 +243,23 @@ function lstatIfExists(file: string) {
 // A bind never replaces an existing file, so a racing host fails with
 // EADDRINUSE instead of taking over this endpoint.
 function listenUnix() {
-  server.listen(socketPath, () => {
-    try {
-      const bound = fs.lstatSync(socketPath);
-      boundSocket = { dev: bound.dev, ino: bound.ino };
-      ownsSocket = true;
-      fs.chmodSync(socketPath, 0o600);
-      releaseStartupLock();
-    } catch {
-      refuseEndpoint();
-    }
-  });
+  // The bind inside listen is synchronous, so this umask makes the socket
+  // 0600 from the moment it exists instead of after the chmod below.
+  const previousUmask = process.umask(0o177);
+  try { server.listen(socketPath, onUnixListening); }
+  finally { process.umask(previousUmask); }
+}
+
+function onUnixListening() {
+  try {
+    const bound = fs.lstatSync(socketPath);
+    boundSocket = { dev: bound.dev, ino: bound.ino };
+    ownsSocket = true;
+    fs.chmodSync(socketPath, 0o600);
+    releaseStartupLock();
+  } catch {
+    refuseEndpoint();
+  }
 }
 
 // Another host may have replaced this endpoint after a stale-socket recovery,

tests/security/host.test.ts

@@ -14,6 +14,7 @@ async function host(env: NodeJS.ProcessEnv = {}, protocolVersion = 2) {
   const child = spawn(process.execPath, ["dist/native-host/host.js"], {
     env: { ...process.env, OPZERO_CHROME_HOST_SOCKET: endpoint, ...env }, stdio: ["pipe", "pipe", "pipe"]
   });
+  child.stdin.on("error", () => undefined);
   cleanup.push(() => { child.kill(); fs.rmSync(directory, { recursive: true, force: true }); });
   const native: any[] = [];
   let buffer = Buffer.alloc(0);
@@ -185,6 +186,7 @@ function spawnHost(endpoint: string) {
   const child = spawn(process.execPath, ["dist/native-host/host.js"], {
     env: { ...process.env, OPZERO_CHROME_HOST_SOCKET: endpoint }, stdio: ["pipe", "ignore", "pipe"]
   });
+  child.stdin.on("error", () => undefined);
   cleanup.push(() => child.kill());
   const exited = new Promise<number | null>(resolve => child.once("exit", resolve));
   return { child, exited };