Squidex is an open source headless CMS and content management hub. Affected versions are missing origin verification in a postMessage handler which introduces a Cross-Site Scripting (XSS) vulnerability. The editor-sdk.js file defines three different class-like functions, which employ a global message event listener: SquidexSidebar, SquidexWidget, and SquidexFormField. The registered event listener takes some action based on the type of the received message. For example, when the SquidexFormField receives a message with the type valueChanged, the value property is updated. The SquidexFormField class is for example used in the editor-editorjs.html file, which can be accessed via the public wwwroot folder. It uses the onValueChanged method to register a callback function, which passes the value provided from the message event to the editor.render. Passing an attacker-controlled value to this function introduces a Cross-Site Scripting (XSS) vulnerability.