CVE-2021-21391: Ckeditor CKEDITOR5-Engine

Medium severity, CVSS 6.5. EPSS: 1.7% chance of exploitation in the next 30 days.

CKEditor 5 provides a WYSIWYG editing solution. This CVE affects the following npm packages: ckeditor5-engine, ckeditor5-font, ckeditor5-image, ckeditor5-list, ckeditor5-markdown-gfm, ckeditor5-media-embed, ckeditor5-paste-from-office, and ckeditor5-widget. Following an internal audit, a regular expression denial of service (ReDoS) vulnerability has been discovered in multiple CKEditor 5 packages. The vulnerability allowed to abuse particular regular expressions, which could cause a significant performance drop resulting in a browser tab freeze. It affects all users using the CKEditor 5 packages listed above at version <= 26.0.0. The problem has been recognized and patched. The fix will be available in version 27.0.0.

Affected products

• Ckeditor CKEDITOR5-Engine: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Font: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Image: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-List: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Markdown-Gfm: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Media-Embed: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Paste-From-Office: before 27.0.0 (fixed in 27.0.0)
• Ckeditor CKEDITOR5-Widget: before 27.0.0 (fixed in 27.0.0)

Published 2021-04-29. Last modified 2026-06-17.