CKEditor 5 is a JavaScript rich text editor. A cross-site scripting vulnerability has been discovered affecting three optional CKEditor 5's packages in versions prior to 35.0.1. The vulnerability allowed to trigger a JavaScript code after fulfilling special conditions. The affected packages are `@ckeditor/ckeditor5-markdown-gfm`, `@ckeditor/ckeditor5-html-support`, and `@ckeditor/ckeditor5-html-embed`. The specific conditions are 1) Using one of the affected packages. In case of `ckeditor5-html-support` and `ckeditor5-html-embed`, additionally, it was required to use a configuration that allows unsafe markup inside the editor. 2) Destroying the editor instance and 3) Initializing the editor on an element and using an element other than `<textarea>` as a base. The root cause of the issue was a mechanism responsible for updating the source element with the markup coming from the CKEditor 5 data pipeline after destroying the editor. This vulnerability might affect a small percent of inte…