diff --git a/src/EventListener/MergeHttpHeadersListener.php b/src/EventListener/MergeHttpHeadersListener.php
index d0ab7ce5c3..8858740faf 100644
--- a/src/EventListener/MergeHttpHeadersListener.php
+++ b/src/EventListener/MergeHttpHeadersListener.php
@@ -11,6 +11,8 @@
 namespace Contao\CoreBundle\EventListener;
 
 use Contao\CoreBundle\Framework\ContaoFrameworkInterface;
+use Contao\CoreBundle\HttpKernel\Header\HeaderStorageInterface;
+use Contao\CoreBundle\HttpKernel\Header\NativeHeaderStorage;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
 
@@ -18,6 +20,7 @@
  * Adds HTTP headers sent by Contao to the Symfony response.
  *
  * @author Yanick Witschi <https://github.com/toflar>
+ * @author Andreas Schempp <https://github.com/aschempp>
  */
 class MergeHttpHeadersListener
 {
@@ -27,9 +30,14 @@ class MergeHttpHeadersListener
     private $framework;
 
     /**
-     * @var array|null
+     * @var HeaderStorageInterface
      */
-    private $headers;
+    private $headerStorage;
+
+    /**
+     * @var array
+     */
+    private $headers = [];
 
     /**
      * @var array
@@ -45,13 +53,13 @@ class MergeHttpHeadersListener
     /**
      * Constructor.
      *
-     * @param ContaoFrameworkInterface $framework
-     * @param array|null               $headers   Meant for unit testing only!
+     * @param ContaoFrameworkInterface    $framework
+     * @param HeaderStorageInterface|null $headerStorage
      */
-    public function __construct(ContaoFrameworkInterface $framework, array $headers = null)
+    public function __construct(ContaoFrameworkInterface $framework, HeaderStorageInterface $headerStorage = null)
     {
         $this->framework = $framework;
-        $this->headers = $headers;
+        $this->headerStorage = $headerStorage ?: new NativeHeaderStorage();
     }
 
     /**
@@ -111,45 +119,41 @@ public function onKernelResponse(FilterResponseEvent $event)
             return;
         }
 
-        $event->setResponse($this->mergeHttpHeaders($event->getResponse()));
+        // Fetch remaining headers and add them to the response
+        $this->fetchHttpHeaders();
+        $this->setResponseHeaders($event->getResponse());
     }
 
     /**
-     * Merges the HTTP headers.
+     * Fetches and stores HTTP headers from PHP.
+     */
+    private function fetchHttpHeaders()
+    {
+        $this->headers = array_merge($this->headers, $this->headerStorage->all());
+        $this->headerStorage->clear();
+    }
+
+    /**
+     * Sets the response headers.
      *
      * @param Response $response
-     *
-     * @return Response
      */
-    private function mergeHttpHeaders(Response $response)
+    private function setResponseHeaders(Response $response)
     {
-        foreach ($this->getHeaders() as $header) {
-            list($name, $content) = explode(':', $header, 2);
+        $allowOverrides = [];
 
-            if ('cli' !== PHP_SAPI && !headers_sent()) {
-                header_remove($name);
-            }
+        foreach ($this->headers as $header) {
+            list($name, $content) = explode(':', $header, 2);
 
             $uniqueKey = $this->getUniqueKey($name);
 
             if (in_array($uniqueKey, $this->multiHeaders, true)) {
                 $response->headers->set($uniqueKey, trim($content), false);
-            } elseif (!$response->headers->has($uniqueKey)) {
+            } elseif (isset($allowOverrides[$uniqueKey]) || !$response->headers->has($uniqueKey)) {
+                $allowOverrides[$uniqueKey] = true;
                 $response->headers->set($uniqueKey, trim($content));
             }
         }
-
-        return $response;
-    }
-
-    /**
-     * Returns the headers.
-     *
-     * @return array
-     */
-    private function getHeaders()
-    {
-        return $this->headers ?: headers_list();
     }
 
     /**
diff --git a/src/HttpKernel/Header/HeaderStorageInterface.php b/src/HttpKernel/Header/HeaderStorageInterface.php
new file mode 100644
index 0000000000..a2bc183099
--- /dev/null
+++ b/src/HttpKernel/Header/HeaderStorageInterface.php
@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of Contao.
+ *
+ * Copyright (c) 2005-2017 Leo Feyer
+ *
+ * @license LGPL-3.0+
+ */
+
+namespace Contao\CoreBundle\HttpKernel\Header;
+
+/**
+ * Interface for HTTP header storage.
+ *
+ * @author Andreas Schempp <https://github.com/aschempp>
+ */
+interface HeaderStorageInterface
+{
+    /**
+     * Returns all headers.
+     *
+     * @return array
+     */
+    public function all();
+
+    /**
+     * Adds a header to the storage.
+     *
+     * @param string $header
+     */
+    public function add($header);
+
+    /**
+     * Clears the storage.
+     */
+    public function clear();
+}
diff --git a/src/HttpKernel/Header/MemoryHeaderStorage.php b/src/HttpKernel/Header/MemoryHeaderStorage.php
new file mode 100644
index 0000000000..6a8e8ebef9
--- /dev/null
+++ b/src/HttpKernel/Header/MemoryHeaderStorage.php
@@ -0,0 +1,58 @@
+<?php
+
+/*
+ * This file is part of Contao.
+ *
+ * Copyright (c) 2005-2017 Leo Feyer
+ *
+ * @license LGPL-3.0+
+ */
+
+namespace Contao\CoreBundle\HttpKernel\Header;
+
+/**
+ * Handles HTTP headers in memory (for unit tests).
+ *
+ * @author Andreas Schempp <https://github.com/aschempp>
+ */
+class MemoryHeaderStorage implements HeaderStorageInterface
+{
+    /**
+     * @var array
+     */
+    private $headers = [];
+
+    /**
+     * Constructor.
+     *
+     * @param array $headers
+     */
+    public function __construct(array $headers = [])
+    {
+        $this->headers = $headers;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function all()
+    {
+        return $this->headers;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function add($header)
+    {
+        $this->headers[] = $header;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function clear()
+    {
+        $this->headers = [];
+    }
+}
diff --git a/src/HttpKernel/Header/NativeHeaderStorage.php b/src/HttpKernel/Header/NativeHeaderStorage.php
new file mode 100644
index 0000000000..b7e930e29c
--- /dev/null
+++ b/src/HttpKernel/Header/NativeHeaderStorage.php
@@ -0,0 +1,45 @@
+<?php
+
+/*
+ * This file is part of Contao.
+ *
+ * Copyright (c) 2005-2017 Leo Feyer
+ *
+ * @license LGPL-3.0+
+ */
+
+namespace Contao\CoreBundle\HttpKernel\Header;
+
+/**
+ * Handles HTTP headers in PHP's native methods.
+ *
+ * @author Andreas Schempp <https://github.com/aschempp>
+ */
+class NativeHeaderStorage implements HeaderStorageInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function all()
+    {
+        return headers_list();
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function add($header)
+    {
+        header($header);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function clear()
+    {
+        if ('cli' !== PHP_SAPI && !headers_sent()) {
+            header_remove();
+        }
+    }
+}
diff --git a/tests/EventListener/MergeHttpHeadersListenerTest.php b/tests/EventListener/MergeHttpHeadersListenerTest.php
index 5bfe59c193..645315d06e 100644
--- a/tests/EventListener/MergeHttpHeadersListenerTest.php
+++ b/tests/EventListener/MergeHttpHeadersListenerTest.php
@@ -12,6 +12,7 @@
 
 use Contao\CoreBundle\EventListener\MergeHttpHeadersListener;
 use Contao\CoreBundle\Framework\ContaoFrameworkInterface;
+use Contao\CoreBundle\HttpKernel\Header\MemoryHeaderStorage;
 use Contao\CoreBundle\Tests\TestCase;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
@@ -37,7 +38,7 @@ public function testCanBeInstantiated()
     }
 
     /**
-     * Tests that the headers sent using header() are merged into the response object.
+     * Tests that the headers are merged into the response object.
      */
     public function testMergesTheHeadersSent()
     {
@@ -56,7 +57,7 @@ public function testMergesTheHeadersSent()
             ->willReturn(true)
         ;
 
-        $listener = new MergeHttpHeadersListener($framework, ['Content-Type: text/html']);
+        $listener = new MergeHttpHeadersListener($framework, new MemoryHeaderStorage(['Content-Type: text/html']));
         $listener->onKernelResponse($responseEvent);
 
         $response = $responseEvent->getResponse();
@@ -85,14 +86,14 @@ public function testDoesNotMergeTheHeadersSentIfTheContaoFrameworkIsNotInitializ
             ->willReturn(false)
         ;
 
-        $listener = new MergeHttpHeadersListener($framework, ['Content-Type: text/html']);
+        $listener = new MergeHttpHeadersListener($framework, new MemoryHeaderStorage(['Content-Type: text/html']));
         $listener->onKernelResponse($responseEvent);
 
         $this->assertFalse($responseEvent->getResponse()->headers->has('Content-Type'));
     }
 
     /**
-     * Tests that multi-value headers are not overriden.
+     * Tests that multi-value headers are not overridden.
      */
     public function testDoesNotOverrideMultiValueHeaders()
     {
@@ -114,7 +115,9 @@ public function testDoesNotOverrideMultiValueHeaders()
             ->willReturn(true)
         ;
 
-        $listener = new MergeHttpHeadersListener($framework, ['set-cookie: new-content=foobar']); // lower-case key
+        $headers = new MemoryHeaderStorage(['set-cookie: new-content=foobar']); // lower-case key
+
+        $listener = new MergeHttpHeadersListener($framework, $headers);
         $listener->onKernelResponse($responseEvent);
 
         $response = $responseEvent->getResponse();
@@ -183,4 +186,92 @@ public function testAddsAndRemovesMultiValueHeaders()
             ]
         );
     }
+
+    /**
+     * Tests that headers are inherited from a subrequest.
+     */
+    public function testInheritsHeadersFromSubrequest()
+    {
+        $responseEvent = new FilterResponseEvent(
+            $this->mockKernel(),
+            new Request(),
+            HttpKernelInterface::MASTER_REQUEST,
+            new Response()
+        );
+
+        $framework = $this->createMock(ContaoFrameworkInterface::class);
+
+        $framework
+            ->expects($this->atLeastOnce())
+            ->method('isInitialized')
+            ->willReturn(true)
+        ;
+
+        $headerStorage = new MemoryHeaderStorage(['Content-Type: text/html']);
+
+        $listener = new MergeHttpHeadersListener($framework, $headerStorage);
+        $listener->onKernelResponse($responseEvent);
+
+        $response = $responseEvent->getResponse();
+
+        $this->assertTrue($response->headers->has('Content-Type'));
+        $this->assertSame('text/html', $response->headers->get('Content-Type'));
+
+        $headerStorage->add('Content-Type: application/json');
+
+        $responseEvent->setResponse(new Response());
+        $listener->onKernelResponse($responseEvent);
+
+        $response = $responseEvent->getResponse();
+
+        $this->assertTrue($response->headers->has('Content-Type'));
+        $this->assertSame('application/json', $response->headers->get('Content-Type'));
+    }
+
+    /**
+     * Tests that multi headers are inherited from a subrequest.
+     */
+    public function testInheritsMultiHeadersFromSubrequest()
+    {
+        $responseEvent = new FilterResponseEvent(
+            $this->mockKernel(),
+            new Request(),
+            HttpKernelInterface::MASTER_REQUEST,
+            new Response()
+        );
+
+        $framework = $this->createMock(ContaoFrameworkInterface::class);
+
+        $framework
+            ->expects($this->atLeastOnce())
+            ->method('isInitialized')
+            ->willReturn(true)
+        ;
+
+        $headerStorage = new MemoryHeaderStorage(['Set-Cookie: content=foobar']);
+
+        $listener = new MergeHttpHeadersListener($framework, $headerStorage);
+        $listener->onKernelResponse($responseEvent);
+
+        $response = $responseEvent->getResponse();
+        $allHeaders = $response->headers->get('Set-Cookie', null, false);
+
+        $this->assertTrue($response->headers->has('Set-Cookie'));
+        $this->assertCount(1, $allHeaders);
+        $this->assertSame('content=foobar; path=/', $allHeaders[0]);
+
+        $headerStorage->add('Set-Cookie: new-content=foobar');
+
+        $responseEvent->setResponse(new Response());
+        $listener->onKernelResponse($responseEvent);
+
+        $response = $responseEvent->getResponse();
+
+        $allHeaders = $response->headers->get('Set-Cookie', null, false);
+
+        $this->assertTrue($response->headers->has('Set-Cookie'));
+        $this->assertCount(2, $allHeaders);
+        $this->assertSame('content=foobar; path=/', $allHeaders[0]);
+        $this->assertSame('new-content=foobar; path=/', $allHeaders[1]);
+    }
 }
diff --git a/tests/HttpKernel/Header/MemoryHeaderStorageTest.php b/tests/HttpKernel/Header/MemoryHeaderStorageTest.php
new file mode 100644
index 0000000000..b2634f36e8
--- /dev/null
+++ b/tests/HttpKernel/Header/MemoryHeaderStorageTest.php
@@ -0,0 +1,59 @@
+<?php
+
+/*
+ * This file is part of Contao.
+ *
+ * Copyright (c) 2005-2017 Leo Feyer
+ *
+ * @license LGPL-3.0+
+ */
+
+namespace Contao\CoreBundle\Tests\HttpKernel\Header;
+
+use Contao\CoreBundle\HttpKernel\Header\MemoryHeaderStorage;
+use Contao\CoreBundle\Tests\TestCase;
+
+/**
+ * @author Andreas Schempp <https://github.com/aschempp>
+ */
+class MemoryHeaderStorageTest extends TestCase
+{
+    /**
+     * Tests the object instantiation.
+     */
+    public function testCanBeInstantiated()
+    {
+        $storage = new MemoryHeaderStorage();
+
+        $this->assertInstanceOf('Contao\CoreBundle\HttpKernel\Header\MemoryHeaderStorage', $storage);
+        $this->assertInstanceOf('Contao\CoreBundle\HttpKernel\Header\HeaderStorageInterface', $storage);
+    }
+
+    /**
+     * Tests that all headers are returned.
+     */
+    public function testReturnsAllHeaders()
+    {
+        $storage = new MemoryHeaderStorage(['Foo: Bar']);
+
+        $this->assertSame(['Foo: Bar'], $storage->all());
+
+        $storage->add('Bar: Baz');
+
+        $this->assertSame(['Foo: Bar', 'Bar: Baz'], $storage->all());
+    }
+
+    /**
+     * Tests that existing headers are cleared.
+     */
+    public function testClearsExistingHeaders()
+    {
+        $storage = new MemoryHeaderStorage(['Foo: Bar']);
+
+        $this->assertSame(['Foo: Bar'], $storage->all());
+
+        $storage->clear();
+
+        $this->assertSame([], $storage->all());
+    }
+}
diff --git a/tests/HttpKernel/Header/NativeHeaderStorageTest.php b/tests/HttpKernel/Header/NativeHeaderStorageTest.php
new file mode 100644
index 0000000000..ff58eae6ed
--- /dev/null
+++ b/tests/HttpKernel/Header/NativeHeaderStorageTest.php
@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of Contao.
+ *
+ * Copyright (c) 2005-2017 Leo Feyer
+ *
+ * @license LGPL-3.0+
+ */
+
+namespace Contao\CoreBundle\Tests\HttpKernel\Header;
+
+use Contao\CoreBundle\HttpKernel\Header\NativeHeaderStorage;
+use Contao\CoreBundle\Tests\TestCase;
+
+/**
+ * @author Andreas Schempp <https://github.com/aschempp>
+ */
+class NativeHeaderStorageTest extends TestCase
+{
+    /**
+     * Tests the object instantiation.
+     */
+    public function testCanBeInstantiated()
+    {
+        $storage = new NativeHeaderStorage();
+
+        $this->assertInstanceOf('Contao\CoreBundle\HttpKernel\Header\NativeHeaderStorage', $storage);
+        $this->assertInstanceOf('Contao\CoreBundle\HttpKernel\Header\HeaderStorageInterface', $storage);
+    }
+}