{"id":"CVE-2021-21254","aliases":["GHSA-hgmg-hhc8-g5wr"],"url":"https://o3.security/vulnerability/CVE-2021-21254","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 <= 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.","published":"2021-01-29T22:15:14.797Z","modified":"2026-07-09T14:06:37.686463Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.01792,"percentile":0.76895,"asOf":"2026-09-04"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"@ckeditor/ckeditor5-markdown-gfm","fixedVersion":"25.0.0"}],"fix":null,"references":[{"type":"WEB","url":"https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm"},{"type":"ADVISORY","url":"https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0"},{"type":"ADVISORY","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://www.npmjs.com/package/@ckeditor/ckeditor5-markdown-gfm"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T14:06:37.686463Z"}}