diff --git a/system/extensions/core.php b/system/extensions/core.php
new file mode 100644
index 0000000..312ed50
--- /dev/null
+++ b/system/extensions/core.php
@@ -0,0 +1,91 @@
+<?php
+// Core extension, https://github.com/annaesvensson/yellow-core
+// This file is only needed for backwards compatibility with Datenstrom Yellow 0.8.23
+// Please note that the latest core can be found in file `system/workers/core.php`
+    
+class YellowCore {
+    const VERSION = "0.8.134";
+    const RELEASE = "0.8.23";
+    
+    // Handle initialisation
+    public function load() {
+        ini_set("display_errors", 1);
+        error_reporting(E_ALL);
+    }
+    
+    // Handle request from web browser
+    public function request() {
+        $statusCode = $this->processCoreUpdate();
+        if ($statusCode==303) {
+            header("HTTP/1.0 303 Reload please");
+            header("Location: ".$_SERVER["REQUEST_URI"]);
+            header("Cache-Control: no-cache, no-store");
+        }
+        return $statusCode;
+    }
+    
+    // Handle command from command line
+    public function command($line = "") {
+        $statusCode = $this->processCoreUpdate();
+        if ($statusCode==303) {
+            echo "Flux capacitor has reached 1.21 gigawatts. Please run command again.\n";
+        }
+        return $statusCode<400 ? 0 : 1;
+    }
+    
+    // Update core for new release
+    public function processCoreUpdate() {
+        $statusCode = 303;
+        if (is_file("system/extensions/core.php") && is_file("system/workers/core.php")) {
+            $fileName = "yellow.php";
+            $fileData = $fileDataNew = $this->readFile($fileName);
+            $fileDataNew = str_replace("system/extensions/core.php", "system/workers/core.php", $fileDataNew);
+            if ($fileData!=$fileDataNew && !$this->createFile($fileName, $fileDataNew)) {
+                $statusCode = 500;
+                header("HTTP/1.0 500 Server error");
+                echo "Something went wrong during core update: Can't write file '$fileName'! <br/>\n";
+                echo "Datenstrom Yellow stopped with fatal error. Activate the debug mode for more information. ";
+                echo "See https://datenstrom.se/yellow/help/troubleshooting <br/>\n";
+            }
+            if (function_exists("opcache_reset")) opcache_reset();
+        }
+        return $statusCode;
+    }
+    
+    // Read file, empty string if not found
+    public function readFile($fileName, $sizeMax = 0) {
+        $fileData = "";
+        $fileHandle = @fopen($fileName, "rb");
+        if ($fileHandle) {
+            clearstatcache(true, $fileName);
+            if (flock($fileHandle, LOCK_SH)) {
+                $fileSize = $sizeMax ? $sizeMax : filesize($fileName);
+                if ($fileSize) $fileData = fread($fileHandle, $fileSize);
+                flock($fileHandle, LOCK_UN);
+            }
+            fclose($fileHandle);
+        }
+        return $fileData;
+    }
+    
+    // Create file
+    public function createFile($fileName, $fileData, $mkdir = false) {
+        $ok = false;
+        if ($mkdir) {
+            $path = dirname($fileName);
+            if (!is_string_empty($path) && !is_dir($path)) @mkdir($path, 0777, true);
+        }
+        $fileHandle = @fopen($fileName, "cb");
+        if ($fileHandle) {
+            clearstatcache(true, $fileName);
+            if (flock($fileHandle, LOCK_EX)) {
+                ftruncate($fileHandle, 0);
+                fwrite($fileHandle, $fileData);
+                flock($fileHandle, LOCK_UN);
+            }
+            fclose($fileHandle);
+            $ok = true;
+        }
+        return $ok;
+    }
+}
diff --git a/system/extensions/update-available.ini b/system/extensions/update-available.ini
index f94a78d..1a11bd5 100644
--- a/system/extensions/update-available.ini
+++ b/system/extensions/update-available.ini
@@ -158,7 +158,7 @@ DocumentationLanguage: en, de, sv
 Published: 2024-04-04 14:38:12
 Status: available
 system/workers/core.php: core.php, create, update
-system/extensions/core.php: coreupdate.txt, create, additional
+system/extensions/core.php: coreupdate.txt, update
 system/layouts/default.html: default.html, create, update, careful
 system/layouts/error.html: error.html, create, update, careful
 system/layouts/header.html: header.html, create, update, careful
diff --git a/system/extensions/yellow-extension.ini b/system/extensions/yellow-extension.ini
index a2cc5df..2f23022 100755
--- a/system/extensions/yellow-extension.ini
+++ b/system/extensions/yellow-extension.ini
@@ -11,7 +11,7 @@ DocumentationLanguage: en, de, sv
 Published: 2024-04-04 14:38:12
 Status: available
 system/workers/core.php: core.php, create, update
-system/extensions/core.php: coreupdate.txt, create, additional
+system/extensions/core.php: coreupdate.txt, update
 system/layouts/default.html: default.html, create, update, careful
 system/layouts/error.html: error.html, create, update, careful
 system/layouts/header.html: header.html, create, update, careful