CKEditor5 cross-site scripting vulnerability caused by the editor instance destroying process

Affected packages

@ckeditor/ckeditor5-markdown-gfm
@ckeditor/ckeditor5-html-support
@ckeditor/ckeditor5-html-embed

Impact

A cross-site scripting vulnerability has been discovered affecting three optional CKEditor 5’s packages. The vulnerability allowed to trigger a JavaScript code after fulfilling special conditions:

a) 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,
b) Initializing the editor on an element and using an element other than <textarea> as a base,
c) Destroying the editor instance.

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 integrators that depend on dynamic editor initialization/destroy and use Markdown, General HTML Support or HTML embed features.

Patches

The problem has been recognized and patched. The fix will be available in version 35.0.1.

For more information

Email us at [email protected] if you have any questions or comments about this advisory.