CVE-2021-21391
MEDIUMCVE-2021-21391 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in @ckeditor/ckeditor5-engine. O3 Security confirms whether CVE-2021-21391 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Regular expression Denial of Service in multiple packages
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
@ckeditor/ckeditor5-enginenpm@ckeditor/ckeditor5-fontnpm@ckeditor/ckeditor5-imagenpmDescription
CKEditor 5 provides a WYSIWYG editing solution. This CVE affects the following npm packages: ckeditor5-engine, ckeditor5-font, ckeditor5-image, ckeditor5-list, ckeditor5-markdown-gfm, ckeditor5-media-embed, ckeditor5-paste-from-office, and ckeditor5-widget. Following an internal audit, a regular expression denial of service (ReDoS) vulnerability has been discovered in multiple CKEditor 5 packages. The vulnerability allowed to abuse particular regular expressions, which could cause a significant performance drop resulting in a browser tab freeze. It affects all users using the CKEditor 5 packages listed above at version <= 26.0.0. The problem has been recognized and patched. The fix will be available in version 27.0.0.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @ckeditor/ckeditor5-engine | all versions | 27.0.0 |
| 📦npm | @ckeditor/ckeditor5-font | all versions | 27.0.0 |
| 📦npm | @ckeditor/ckeditor5-image | all versions | 27.0.0 |
| 📦npm | @ckeditor/ckeditor5-list | all versions | 27.0.0 |
| 📦npm | @ckeditor/ckeditor5-markdown-gfm | all versions | 27.0.0 |
| 📦npm | @ckeditor/ckeditor5-media-embed | all versions | 27.0.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @ckeditor/ckeditor5-engine. 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-engine to 27.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21391 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-2021-21391 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-2021-21391. 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-2021-21391 in your dependencies?
O3 detects CVE-2021-21391 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.