{"id":"GHSA-hgmg-hhc8-g5wr","summary":"CKEditor 5 Markdown plugin Regular expression Denial of Service","details":"### Impact\nA 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 \u003c= 24.0.0. \n\n### Patches\nThe problem has been recognized and patched. The fix will be available in version 25.0.0.\n\n### Workarounds\nThe user can work around the issue by:\n- Upgrading CKEditor 5 to version 25.0.0.\n- Disabling the Markdown plugin.\n\n### More information\nIf you have any questions or comments about this advisory:\n* Email us at [security@cksource.com](mailto:security@cksource.com)\n\n### Acknowledgements\nThe CKEditor 5 team would like to thank Erik Krogh Kristensen from the GitHub team for recognizing this vulnerability and \nAlvaro Muñoz from GitHub for reporting it.","aliases":["CVE-2021-21254"],"modified":"2026-05-07T05:02:13.606149161Z","published":"2021-01-29T21:51:22Z","database_specific":{"nvd_published_at":"2021-01-29T22:15:00Z","cwe_ids":["CWE-400"],"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2021-01-29T21:50:43Z"},"references":[{"type":"WEB","url":"https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21254"},{"type":"PACKAGE","url":"https://github.com/ckeditor/ckeditor5"},{"type":"WEB","url":"https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0"},{"type":"WEB","url":"https://www.npmjs.com/package/@ckeditor/ckeditor5-markdown-gfm"}],"affected":[{"package":{"name":"@ckeditor/ckeditor5-markdown-gfm","ecosystem":"npm","purl":"pkg:npm/%40ckeditor/ckeditor5-markdown-gfm"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"25.0.0"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 24.0.0","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2021/01/GHSA-hgmg-hhc8-g5wr/GHSA-hgmg-hhc8-g5wr.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"}]}