CVE-2021-21391 — @ckeditor/ckeditor5-engine
MEDIUMCVE-2021-21391 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in @ckeditor/ckeditor5-engine. A fix is available for @ckeditor/ckeditor5-engine — see the affected versions and patch details below.
Regular expression Denial of Service in multiple packages
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21391 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@ckeditor/ckeditor5-enginenpm@ckeditor/ckeditor5-fontnpm@ckeditor/ckeditor5-imagenpmDescription
Impact
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.
Patches
The problem has been recognized and patched. The fix will be available in version 27.0.0.
For more information
Email us at [email protected] if you have any questions or comments about this advisory.
Acknowledgements
The CKEditor 5 team would like to thank Yeting Li for recognizing and reporting these vulnerabilities.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @ckeditor/ckeditor5-engine | all versions | 27.0.0npm install @ckeditor/ckeditor5-engine@27.0.0 |
| 📦npm | @ckeditor/ckeditor5-font | all versions | 27.0.0npm install @ckeditor/ckeditor5-font@27.0.0 |
| 📦npm | @ckeditor/ckeditor5-image | all versions | 27.0.0npm install @ckeditor/ckeditor5-image@27.0.0 |
| 📦npm | @ckeditor/ckeditor5-list | all versions | 27.0.0npm install @ckeditor/ckeditor5-list@27.0.0 |
| 📦npm | @ckeditor/ckeditor5-markdown-gfm | all versions | 27.0.0npm install @ckeditor/ckeditor5-markdown-gfm@27.0.0 |
| 📦npm | @ckeditor/ckeditor5-media-embed | all versions | 27.0.0npm install @ckeditor/ckeditor5-media-embed@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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21391 can be triaged on real exposure rather than presence alone.
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 Security finds CVE-2021-21391 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.