CKEditor 5 Markdown plugin Regular expression Denial of Service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2021-21254: 1.81% estimated exploitation activity in the next 30 days; percentile 78.69 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact A regular expression denial of service (ReDoS) vulnerability has been discovered in the CKEditor 5 Markdown plugin code. The vulnerability allowed to abuse a link recognition regular expression, which could cause a significant performance drop resulting in a browser tab freeze. It affects all users using the CKEditor 5 Markdown plugin at version <= 24.0.0. Patches The problem has been recognized and patched. The fix will be available in version 25.0.0. Workarounds The user can work around the issue by: - Upgrading CKEditor 5 to version 25.0.0. - Disabling the Markdown plugin. More information If you have any questions or comments about this advisory: Email us at security@cksource.com Acknowledgements The CKEditor 5 team would like to thank Erik Krogh Kristensen from the GitHub team for recognizing this vulnerability and Alvaro Muñoz from GitHub for reporting it.

Published by the source

29 Jan 2021, 21:51 UTC

Last modified by the source

8 Jul 2026, 06:29 UTC

Source last checked

26 Oct 2026, 12:17 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-hgmg-hhc8-g5wr

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-hgmg-hhc8-g5wr

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# Patch Next advisory brief: CVE-2021-21254

Advisory: CKEditor 5 Markdown plugin Regular expression Denial of Service
IDs: CVE-2021-21254, GHSA-hgmg-hhc8-g5wr
Record: https://patchnext.com/advisories/cve-2021-21254 (revision 2, source checked 26 Oct 2026, 12:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @ckeditor/ckeditor5-markdown-gfm 25.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @ckeditor/ckeditor5-markdown-gfm: affected <= 24.0.0; fixed in 25.0.0

## Suggested inspection (read-only)
- `npm ls @ckeditor/ckeditor5-markdown-gfm --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @ckeditor/ckeditor5-markdown-gfm.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-hgmg-hhc8-g5wr (OSV record GHSA-hgmg-hhc8-g5wr lists affected ranges: npm @ckeditor/ckeditor5-markdown-gfm >=0 <25.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-hgmg-hhc8-g5wr (GitHub-reviewed advisory GHSA-hgmg-hhc8-g5wr lists affected ranges: npm @ckeditor/ckeditor5-markdown-gfm >=0 <=24.0.0)

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