{"id":"CVE-2021-21254","aliases":["EUVD-2021-0505","GHSA-hgmg-hhc8-g5wr"],"status":"published","title":"Regular expression Denial of Service in Markdown plugin","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 \u003c= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.","severity":{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.01814,"percentile":0.78972,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-400"],"published":"2021-01-29T21:51:22Z","modified":"2026-10-20T10:54:27Z","fixes":[{"ecosystem":"npm","package":"@ckeditor/ckeditor5-markdown-gfm","fixed_in":["25.0.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"@ckeditor/ckeditor5-markdown-gfm","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"24.0.0"},{"fixed":"25.0.0"}]}]},{"source":"osv","ecosystem":"npm","package":"@ckeditor/ckeditor5-markdown-gfm","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"25.0.0"}]}]}],"products":[{"vendor":"ckeditor","product":"ckeditor5"}],"references":[{"url":"https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr","tags":["x_refsource_CONFIRM","x_transferred","Third Party Advisory","advisory","web"]},{"url":"https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0","tags":["x_refsource_MISC","x_transferred","Release Notes","Third Party Advisory","advisory","web"]},{"url":"https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm","tags":["x_refsource_MISC","x_transferred","web"]},{"url":"https://www.npmjs.com/package/@ckeditor/ckeditor5-markdown-gfm","tags":["web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21254","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-hgmg-hhc8-g5wr"},{"url":"https://github.com/ckeditor/ckeditor5","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2021-21254","title":"Regular expression Denial of Service in Markdown plugin","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 \u003c= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.","status":"published","published":"2021-01-29T21:55:14Z","modified":"2024-08-03T18:09:14.916Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna"}],"cwes":["CWE-400"],"updated_at":"2026-11-02T14:33:00.137099Z"}],"euvd":[{"id":"EUVD-2021-0505","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 \u003c= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.","status":"published","published":"2021-01-29T21:55:14Z","modified":"2024-08-03T18:09:14Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-03T03:25:06.913643Z"}],"ghsa":[{"id":"GHSA-hgmg-hhc8-g5wr","title":"CKEditor 5 Markdown plugin Regular expression Denial of Service","description":"### 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.","status":"published","published":"2021-01-29T21:51:22Z","modified":"2023-01-30T05:02:55Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-400"],"updated_at":"2026-11-02T14:36:27.500048Z"}],"nvd":[{"id":"CVE-2021-21254","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 \u003c= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.","status":"published","published":"2021-01-29T22:15:14.797Z","modified":"2026-06-17T03:35:08.19Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"secondary"},{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"primary"},{"version":"2.0","score":4,"vector":"AV:N/AC:L/Au:S/C:N/I:N/A:P","level":"medium","assessor":"primary"}],"cwes":["CWE-400"],"updated_at":"2026-11-02T14:06:49.660551Z"}],"osv":[{"id":"CVE-2021-21254","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 \u003c= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.","status":"published","published":"2021-01-29T22:15:14.797Z","modified":"2026-10-20T10:54:27.78502353Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T15:44:18.004149Z"},{"id":"GHSA-hgmg-hhc8-g5wr","title":"CKEditor 5 Markdown plugin Regular expression Denial of Service","description":"### 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.","status":"published","published":"2021-01-29T21:51:22Z","modified":"2026-07-08T06:29:35.375771949Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-400"],"updated_at":"2026-11-02T15:46:49.28217Z"}]}}