CVE-2021-21254
MEDIUMCVE-2021-21254 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in @ckeditor/ckeditor5-markdown-gfm. O3 Security confirms whether CVE-2021-21254 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
CKEditor 5 Markdown plugin Regular expression Denial of Service
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
@ckeditor/ckeditor5-markdown-gfmnpmDescription
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @ckeditor/ckeditor5-markdown-gfm | all versions | 25.0.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @ckeditor/ckeditor5-markdown-gfm. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update @ckeditor/ckeditor5-markdown-gfm to 25.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21254 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-21254 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-21254. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21254 in your dependencies?
O3 detects CVE-2021-21254 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.