CKEditor 5 is a JavaScript rich-text editor. Starting in version 40.0.0 and prior to version 43.1.1, a Cross-Site Scripting (XSS) vulnerability is present in the CKEditor 5 clipboard package. This vulnerability could be triggered by a specific user action, leading to unauthorized JavaScript code execution, if the attacker managed to insert a malicious content into the editor, which might happen with a very specific editor configuration. This vulnerability only affects installations where the Block Toolbar plugin is enabled and either the General HTML Support (with a configuration that permits unsafe markup) or the HTML Embed plugin is also enabled. A fix for the problem is available in version 43.1.1. As a workaround, one may disable the block toolbar plugin.