Regular expression Denial of Service in multiple packages

Exploitation

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

CVSS

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

EPSS

• CVE-2021-21391: 1.67% estimated exploitation activity in the next 30 days; percentile 76.78 (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 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. Patches The problem has been recognized and patched. The fix will be available in version 27.0.0. For more information Email us at security@cksource.com if you have any questions or comments about this advisory. Acknowledgements The CKEditor 5 team would like to thank Yeting Li for recognizing and reporting these vulnerabilities.

Published by the source

6 Apr 2021, 17:28 UTC

Last modified by the source

8 Jul 2026, 06:27 UTC

Source last checked

26 Oct 2026, 12:20 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3rh3-wfr4-76mj

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-3rh3-wfr4-76mj

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# Patch Next advisory brief: CVE-2021-21391

Advisory: Regular expression Denial of Service in multiple packages
IDs: CVE-2021-21391, GHSA-3rh3-wfr4-76mj
Record: https://patchnext.com/advisories/cve-2021-21391 (revision 2, source checked 26 Oct 2026, 12:20 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-font 27.0.0; @ckeditor/ckeditor5-markdown-gfm 27.0.0; @ckeditor/ckeditor5-list 27.0.0; @ckeditor/ckeditor5-paste-from-office 27.0.0; @ckeditor/ckeditor5-widget 27.0.0; @ckeditor/ckeditor5-media-embed 27.0.0; @ckeditor/ckeditor5-image 27.0.0; @ckeditor/ckeditor5-engine 27.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @ckeditor/ckeditor5-font: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-markdown-gfm: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-list: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-paste-from-office: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-widget: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-media-embed: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-image: affected <= 26.0.0; fixed in 27.0.0
- npm @ckeditor/ckeditor5-engine: affected <= 26.0.0; fixed in 27.0.0

## Suggested inspection (read-only)
- `npm ls @ckeditor/ckeditor5-font --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-markdown-gfm --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-list --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-paste-from-office --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-widget --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-media-embed --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-image --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @ckeditor/ckeditor5-engine --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-font, @ckeditor/ckeditor5-markdown-gfm, @ckeditor/ckeditor5-list, @ckeditor/ckeditor5-paste-from-office, @ckeditor/ckeditor5-widget, @ckeditor/ckeditor5-media-embed, @ckeditor/ckeditor5-image, @ckeditor/ckeditor5-engine.
- 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-3rh3-wfr4-76mj (OSV record GHSA-3rh3-wfr4-76mj lists affected ranges: npm @ckeditor/ckeditor5-engine >=0 =0 =0 =0 =0 =0 =0 =0 <27.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-3rh3-wfr4-76mj (GitHub-reviewed advisory GHSA-3rh3-wfr4-76mj lists affected ranges: npm @ckeditor/ckeditor5-engine >=0 =0 =0 =0 =0 =0 =0 =0 <=26.0.0)

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