Craft CMS 4.x before 4.18.8 and 5.x before 5.11.0 contains a remote code execution vulnerability in the Twig group filter, which invokes non-string arrow arguments as callables without the checkArrow guard used by other arrow-accepting filters. Authenticated control panel users can pass an array-callable such as FileHelper::writeToFile via element-select condition rules to write a webshell to the webroot and execute code as the web user.