What’s Vulnerable › Vulnerabilities

CVE-2021-21254: Regular expression Denial of Service in Markdown plugin

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-01-29 · updated 2026-06-17
• Weakness: CWE-400

Description

CKEditor 5 is an open source rich text editor framework with a modular architecture. The CKEditor 5 Markdown plugin (@ckeditor/ckeditor5-markdown-gfm) before version 25.0.0 has a regex denial of service (ReDoS) vulnerability. The vulnerability allowed to abuse link recognition regular expression, which could cause a significant performance drop resulting in browser tab freeze. It affects all users using CKEditor 5 Markdown plugin at version <= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.

Affected products

• ckeditor5: before 25.0.0 — fixed in 25.0.0

How to fix

Upgrade to a fixed version: ckeditor5 25.0.0. Check the vendor advisory for mitigations.

References

• https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0
• https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr
• https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm
• https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0
• https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr
• https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm

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