diff --git a/src/StackFormation/PreProcessor/Stage/Tree/ExpandCslPort.php b/src/StackFormation/PreProcessor/Stage/Tree/ExpandCslPort.php
new file mode 100644
index 0000000..ca091c6
--- /dev/null
+++ b/src/StackFormation/PreProcessor/Stage/Tree/ExpandCslPort.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace StackFormation\PreProcessor\Stage\Tree;
+
+use StackFormation\PreProcessor\Stage\AbstractTreePreProcessorStage;
+use StackFormation\PreProcessor\Rootline;
+
+class ExpandCslPort extends AbstractTreePreProcessorStage
+{
+    /**
+     * This one converts "Port" into "FromPort" and "ToPort" and will resolve comma-separated lists
+     *
+     * @param string $path
+     * @param string $value
+     * @param Rootline $rootLineReferences
+     * @return bool
+     */
+    function invoke($path, $value, Rootline $rootLineReferences) {
+        if (!preg_match('+/SecurityGroupIngress/.*/Port$+', $path)) {
+            return false;  // indicate that nothing has been touched
+        }
+
+        $parentRootlineItem = $rootLineReferences->parent(1); /* @var $parentRootlineItem RootlineItem */
+        $parent = $parentRootlineItem->getValue(); /* @var $parent RecursiveArrayObject */
+
+        $grandParentRootlineItem = $rootLineReferences->parent(2); /* @var $grandParentRootlineItem RootlineItem */
+        $grandParent = $grandParentRootlineItem->getValue(); /* @var $grandParent RecursiveArrayObject */
+
+        // remove original item
+        $grandParent->offsetUnset($parentRootlineItem->getKey());
+
+        // add a new line for every csl item
+        foreach (explode(',', $value) as $port) {
+            $newItem = clone $parent;
+            $newItem->FromPort = $port;
+            $newItem->ToPort = $port;
+            $newItem->offsetUnset('Port');
+            $grandParent->append($newItem);
+        }
+
+        return true; // indicate that something has changed
+    }
+}