CVE-2021-21391: Regular expression Denial of Service in multiple packages

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-04-29 · updated 2026-06-17
Weakness: CWE-400

Description

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

Ckeditor5-Engine: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Font: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Image: before 27.0.0 — fixed in 27.0.0
Ckeditor5-List: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Markdown-Gfm: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Media-Embed: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Paste-From-Office: before 27.0.0 — fixed in 27.0.0
Ckeditor5-Widget: before 27.0.0 — fixed in 27.0.0

How to fix

Upgrade to a fixed version: Ckeditor5-Engine 27.0.0; Ckeditor5-Font 27.0.0; Ckeditor5-Image 27.0.0; Ckeditor5-List 27.0.0; Ckeditor5-Markdown-Gfm 27.0.0; Ckeditor5-Media-Embed 27.0.0; Ckeditor5-Paste-From-Office 27.0.0; Ckeditor5-Widget 27.0.0. Check the vendor advisory for mitigations.

References

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