Fix for #4372

framework/filters/HttpCache.php

@@ -129,10 +129,10 @@ class HttpCache extends ActionFilter
      */
     protected function validateCache($lastModified, $etag)
     {
-        if ($lastModified !== null && (!isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) || @strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) < $lastModified)) {
-            return false;
+        if($etag !== null && isset($_SERVER['HTTP_IF_NONE_MATCH']) && $_SERVER['HTTP_IF_NONE_MATCH'] === $etag) {
+            return true;
         } else {
-            return $etag === null || isset($_SERVER['HTTP_IF_NONE_MATCH']) && $_SERVER['HTTP_IF_NONE_MATCH'] === $etag;
+            return $lastModified !== null && isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && @strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $lastModified;
         }
     }