Prevent cache poisoning via a Response Content-Type header

@@ -12,6 +12,7 @@
 namespace Symfony\Component\ErrorHandler\ErrorRenderer;
 
 use Symfony\Component\ErrorHandler\Exception\FlattenException;
+use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\Serializer\Exception\NotEncodableValueException;
 use Symfony\Component\Serializer\SerializerInterface;
@@ -30,6 +31,7 @@ class SerializerErrorRenderer implements ErrorRendererInterface
 
     /**
      * @param string|callable(FlattenException) $format The format as a string or a callable that should return it
+     *                                                  formats not supported by Request::getMimeTypes() should be given as mime types
      * @param bool|callable                     $debug  The debugging mode as a boolean or a callable that should return it
      */
     public function __construct(SerializerInterface $serializer, $format, ErrorRendererInterface $fallbackErrorRenderer = null, $debug = false)
@@ -57,11 +59,16 @@ public function render(\Throwable $exception): FlattenException
 
         try {
             $format = \is_string($this->format) ? $this->format : ($this->format)($flattenException);
+            $headers = [
+                'Content-Type' => Request::getMimeTypes($format)[0] ?? $format,
+                'Vary' => 'Accept',
+            ];
 
             return $flattenException->setAsString($this->serializer->serialize($flattenException, $format, [
                 'exception' => $exception,
                 'debug' => \is_bool($this->debug) ? $this->debug : ($this->debug)($exception),
-            ]));
+            ]))
+            ->setHeaders($flattenException->getHeaders() + $headers);
         } catch (NotEncodableValueException $e) {
             return $this->fallbackErrorRenderer->render($exception);
         }

@@ -1590,7 +1590,9 @@ public function isNoCache()
      * Gets the preferred format for the response by inspecting, in the following order:
      *   * the request format set using setRequestFormat
      *   * the values of the Accept HTTP header
-     *   * the content type of the body of the request.
+     *
+     * Note that if you use this method, you should send the "Vary: Accept" header
+     * in the response to prevent any issues with intermediary HTTP caches.
      */
     public function getPreferredFormat(?string $default = 'html'): ?string
     {

@@ -275,7 +275,7 @@ public function prepare(Request $request)
         } else {
             // Content-type based on the Request
             if (!$headers->has('Content-Type')) {
-                $format = $request->getPreferredFormat(null);
+                $format = $request->getRequestFormat(null);
                 if (null !== $format && $mimeType = $request->getMimeType($format)) {
                     $headers->set('Content-Type', $mimeType);
                 }

@@ -497,12 +497,25 @@ public function testPrepareDoesNothingIfRequestFormatIsNotDefined()
         $this->assertEquals('text/html; charset=UTF-8', $response->headers->get('content-type'));
     }
 
+    /**
+     * Same URL cannot produce different Content-Type based on the value of the Accept header,
+     * unless explicitly stated in the response object.
+     */
+    public function testPrepareDoesNotSetContentTypeBasedOnRequestAcceptHeader()
+    {
+        $response = new Response('foo');
+        $request = Request::create('/');
+        $request->headers->set('Accept', 'application/json');
+        $response->prepare($request);
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('content-type'));
+    }
+
     public function testPrepareSetContentType()
     {
         $response = new Response('foo');
         $request = Request::create('/');
         $request->setRequestFormat('json');
-        $request->headers->remove('accept');
 
         $response->prepare($request);