diff --git a/src/it/php/web/unittest/IntegrationTest.class.php b/src/it/php/web/unittest/IntegrationTest.class.php
index 3096af75..945d0e6e 100755
--- a/src/it/php/web/unittest/IntegrationTest.class.php
+++ b/src/it/php/web/unittest/IntegrationTest.class.php
@@ -170,7 +170,7 @@ public function with_large_cookie($length) {
   public function websocket_message($input, $output) {
     try {
       $ws= new WebSocket($this->server->connection, '/ws');
-      $ws->connect();
+      $ws->connect(['Origin' => 'http://localhost', 'Host' => 'localhost:80']);
       $ws->send($input);
       $result= $ws->receive();
     } finally {
@@ -183,7 +183,7 @@ public function websocket_message($input, $output) {
   public function invalid_utf8_passed_to_websocket_text_message() {
     try {
       $ws= new WebSocket($this->server->connection, '/ws');
-      $ws->connect();
+      $ws->connect(['Origin' => 'http://localhost', 'Host' => 'localhost:80']);
       $ws->send("\xfc");
       $ws->receive();
     } finally {
diff --git a/src/main/php/web/handler/WebSocket.class.php b/src/main/php/web/handler/WebSocket.class.php
index 8d3cdb1a..43be3610 100755
--- a/src/main/php/web/handler/WebSocket.class.php
+++ b/src/main/php/web/handler/WebSocket.class.php
@@ -1,5 +1,6 @@
 <?php namespace web\handler;
 
+use util\URI;
 use web\Handler;
 use web\io\EventSink;
 use websocket\Listeners;
@@ -14,10 +15,55 @@ class WebSocket implements Handler {
   const GUID= '258EAFA5-E914-47DA-95CA-C5AB0DC85B11';
 
   private $listener;
+  private $allowed= [];
 
-  /** @param function(websocket.protocol.Connection, string|util.Bytes): var|websocket.Listener $listener */
-  public function __construct($listener) {
+  /**
+   * Creates a new websocket handler
+   *
+   * @param function(websocket.protocol.Connection, string|util.Bytes): var|websocket.Listener $listener
+   * @param string[] $origins
+   */
+  public function __construct($listener, array $origins= []) {
     $this->listener= Listeners::cast($listener);
+    foreach ($origins as $allowed) {
+      $this->allowed[]= '#^'.strtr(preg_quote($allowed, '#'), ['\\*' => '.+']).'$#i';
+    }
+  }
+
+  /**
+   * Returns canonicalized base URI
+   *
+   * @param  util.URI $uri
+   * @return string
+   */
+  private function base($uri) {
+    static $ports= ['http' => 80, 'https' => 443];
+
+    return $uri->scheme().'://'.$uri->host().':'.($uri->port() ?? $ports[$uri->scheme()] ?? 0);
+  }
+
+  /**
+   * Verifies request `Origin` header matches the allowed origins. This
+   * header cannot be set by client-side JavaScript in browsers!
+   *
+   * @param   web.Request $request
+   * @param   web.Response $response
+   * @return  bool
+   */
+  public function verify($request, $response) {
+    if ($origin= $request->header('Origin')) {
+      $base= $this->base(new URI($origin));
+      foreach ($this->allowed as $pattern) {
+        if (preg_match($pattern, $base)) return true;
+      }
+
+      // Same-origin policy
+      if (0 === strcasecmp($this->base($request->uri()), $base)) return true;
+    }
+
+    $response->answer(403);
+    $response->send('Origin not allowed', 'text/plain');
+    return false;
   }
 
   /**
@@ -30,15 +76,26 @@ public function __construct($listener) {
   public function handle($request, $response) {
     switch ($version= (int)$request->header('Sec-WebSocket-Version')) {
       case 13: // RFC 6455
+        if (!$this->verify($request, $response)) return;
+
         $key= $request->header('Sec-WebSocket-Key');
         $response->answer(101);
         $response->header('Sec-WebSocket-Accept', base64_encode(sha1($key.self::GUID, true)));
         foreach ($this->listener->protocols ?? [] as $protocol) {
           $response->header('Sec-WebSocket-Protocol', $protocol, true);
         }
-        break;
+
+        // Signal server implementation to switch protocols
+        yield 'connection' => ['websocket', [
+          'path'     => $request->uri()->resource(),
+          'headers'  => $request->headers(),
+          'listener' => $this->listener,
+        ]];
+        return;
 
       case 9: // Reserved version, use for WS <-> SSE translation
+        if (!$this->verify($request, $response)) return;
+
         $response->answer(200);
         $response->header('Content-Type', 'text/event-stream');
         $response->header('Transfer-Encoding', 'chunked');
@@ -60,11 +117,5 @@ public function handle($request, $response) {
         $response->send('This service does not support WebSocket version '.$version, 'text/plain');
         return;
     }
-
-    yield 'connection' => ['websocket', [
-      'path'     => $request->uri()->resource(),
-      'headers'  => $request->headers(),
-      'listener' => $this->listener,
-    ]];
   }
 }
\ No newline at end of file
diff --git a/src/test/php/web/unittest/handler/WebSocketTest.class.php b/src/test/php/web/unittest/handler/WebSocketTest.class.php
index 392ec832..8b2ca030 100755
--- a/src/test/php/web/unittest/handler/WebSocketTest.class.php
+++ b/src/test/php/web/unittest/handler/WebSocketTest.class.php
@@ -1,6 +1,6 @@
 <?php namespace web\unittest\handler;
 
-use test\{Assert, Test};
+use test\{Assert, Test, Values};
 use util\Bytes;
 use web\handler\WebSocket;
 use web\io\{TestInput, TestOutput};
@@ -9,7 +9,7 @@
 class WebSocketTest {
 
   /** Handles a request and returns the response generated from the handler */
-  private function handle(Request $request): Response {
+  private function handle(Request $request, $origins= ['*']): Response {
     $response= new Response(new TestOutput());
     $echo= function($conn, $payload) {
       if ($payload instanceof Bytes) {
@@ -18,10 +18,47 @@ private function handle(Request $request): Response {
         $conn->send('Re: '.$payload);
       }
     };
-    (new WebSocket($echo))->handle($request, $response)->next();
+    (new WebSocket($echo, $origins))->handle($request, $response)->next();
     return $response;
   }
 
+  /** @return iterable */
+  private function same() {
+
+    // By default, enforces same-origin policy
+    yield ['http://localhost:80', 101];
+    yield ['http://localhost', 101];
+    yield ['http://Localhost', 101];
+
+    // Not allowed: Differing hosts, ports or scheme
+    yield ['http://localhost:81', 403];
+    yield ['http://example.localhost', 403];
+    yield ['http://localhost.example.com', 403];
+    yield ['https://localhost', 403];
+    yield ['http://localhost:81@evil.example.com', 403];
+    yield ['http://evil.example.com/#http://localhost', 403];
+    yield ['http://evil.example.com/?page=http://localhost', 403];
+  }
+
+  /** @return iterable */
+  private function origins() {
+
+    // We allow all ports and schemes on localhost
+    yield ['http://localhost', 101];
+    yield ['https://localhost', 101];
+    yield ['http://localhost:8080', 101];
+    yield ['https://localhost:8443', 101];
+    yield ['http://Localhost', 101];
+    yield ['http://api.localhost', 101];
+
+    // Not allowed: Other localhost subdomains and unrelated domains
+    yield ['http://example.localhost', 403];
+    yield ['http://localhost.example.com', 403];
+    yield ['http://localhost:81@evil.example.com', 403];
+    yield ['http://evil.example.com/#http://localhost', 403];
+    yield ['http://evil.example.com/?page=http://localhost', 403];
+  }
+
   #[Test]
   public function can_create() {
     new WebSocket(function($conn, $payload) { });
@@ -30,6 +67,7 @@ public function can_create() {
   #[Test]
   public function switching_protocols() {
     $response= $this->handle(new Request(new TestInput('GET', '/ws', [
+      'Origin'                => 'http://localhost:8080',
       'Sec-WebSocket-Version' => 13,
       'Sec-WebSocket-Key'     => 'test',
     ])));
@@ -37,9 +75,44 @@ public function switching_protocols() {
     Assert::equals('tNpbgC8ZQDOcSkHAWopKzQjJ1hI=', $response->headers()['Sec-WebSocket-Accept']);
   }
 
+  #[Test]
+  public function missing_origin() {
+    $request= new Request(new TestInput('GET', '/ws', [
+      'Sec-WebSocket-Version' => 13,
+      'Sec-WebSocket-Key'     => 'test',
+    ]));
+
+    Assert::equals(403, $this->handle($request)->status());
+  }
+
+  #[Test, Values(from: 'same')]
+  public function verify_same_origin($origin, $expected) {
+    $request= new Request(new TestInput('GET', '/ws', [
+      'Origin'                => $origin,
+      'Host'                  => 'localhost:80',
+      'Sec-WebSocket-Version' => 13,
+      'Sec-WebSocket-Key'     => 'test',
+    ]));
+
+    Assert::equals($expected, $this->handle($request, [])->status());
+  }
+
+  #[Test, Values(from: 'origins')]
+  public function verify_localhost_origin($origin, $expected) {
+    $request= new Request(new TestInput('GET', '/ws', [
+      'Origin'                => $origin,
+      'Host'                  => 'localhost:8080',
+      'Sec-WebSocket-Version' => 13,
+      'Sec-WebSocket-Key'     => 'test',
+    ]));
+
+    Assert::equals($expected, $this->handle($request, ['*://localhost:*', '*://api.localhost:*'])->status());
+  }
+
   #[Test]
   public function translate_text_message() {
     $response= $this->handle(new Request(new TestInput('POST', '/ws', [
+      'Origin'                => 'http://localhost:8080',
       'Sec-WebSocket-Version' => 9,
       'Sec-WebSocket-Id'      => 123,
       'Content-Type'          => 'text/plain',
@@ -53,6 +126,7 @@ public function translate_text_message() {
   #[Test]
   public function translate_binary_message() {
     $response= $this->handle(new Request(new TestInput('POST', '/ws', [
+      'Origin'                => 'http://localhost:8080',
       'Sec-WebSocket-Version' => 9,
       'Sec-WebSocket-Id'      => 123,
       'Content-Type'          => 'application/octet-stream',