CVE-2021-21391
6.5 · MEDIUMOverview
CVE-2021-21391 is a medium-severity vulnerability affecting ckeditor ckeditor5-engine. It was published on April 29, 2021 and has a CVSS 3.1 base score of 6.5 (MEDIUM).
This vulnerability has a CVSS 3.1 base score of 6.5, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
Technical Description
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.
Remediation
Check the references section for vendor advisories and patches from ckeditor. Update ckeditor5-engine to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
Affected Products
| Vendor | Product | Versions | Status |
|---|---|---|---|
| ckeditor | ckeditor5-engine | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-font | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-image | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-list | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-markdown-gfm | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-media-embed | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-paste-from-office | >= 0, < 27.0.0 | Affected |
| ckeditor | ckeditor5-widget | >= 0, < 27.0.0 | Affected |
References
Frequently Asked Questions
What is CVE-2021-21391?
CVE-2021-21391 is a medium-severity vulnerability affecting ckeditor ckeditor5-engine. It was published on April 29, 2021 and has a CVSS 3.1 base score of 6.5 (MEDIUM).
How severe is CVE-2021-21391?
This vulnerability has a CVSS 3.1 base score of 6.5, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
How do I fix or remediate CVE-2021-21391?
Check the references section for vendor advisories and patches from ckeditor. Update ckeditor5-engine to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
How can CyberStrike help with CVE-2021-21391?
CyberStrike's AI-powered security agents can automatically detect CVE-2021-21391 across your infrastructure using autonomous pentesting, DAST scanning, and HackBrowser. The platform continuously monitors for known vulnerabilities and provides actionable remediation guidance prioritized by real-world exploitability.
How CyberStrike Helps
AI agents map your attack surface to find vulnerabilities like this one.
Automated penetration testing that runs continuously, not just quarterly.
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Browser-based exploitation validates findings with real proof-of-concept.
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