CVE-2021-21391

CVE-2021-21391 is a medium-severity uncontrolled resource consumption vulnerability in @ckeditor/ckeditor5-engine (npm), affecting versions <= 26.0.0. It is fixed in 27.0.0.

Summary

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.

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-21391 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (27.0.0); upgrading removes the vulnerable code path.

Affected versions

@ckeditor/ckeditor5-engine (<= 26.0.0) @ckeditor/ckeditor5-font (<= 26.0.0) @ckeditor/ckeditor5-image (<= 26.0.0) @ckeditor/ckeditor5-list (<= 26.0.0) @ckeditor/ckeditor5-markdown-gfm (<= 26.0.0) @ckeditor/ckeditor5-media-embed (<= 26.0.0) @ckeditor/ckeditor5-paste-from-office (<= 26.0.0) @ckeditor/ckeditor5-widget (<= 26.0.0)

Security releases

@ckeditor/ckeditor5-engine → 27.0.0 (npm) @ckeditor/ckeditor5-font → 27.0.0 (npm) @ckeditor/ckeditor5-image → 27.0.0 (npm) @ckeditor/ckeditor5-list → 27.0.0 (npm) @ckeditor/ckeditor5-markdown-gfm → 27.0.0 (npm) @ckeditor/ckeditor5-media-embed → 27.0.0 (npm) @ckeditor/ckeditor5-paste-from-office → 27.0.0 (npm) @ckeditor/ckeditor5-widget → 27.0.0 (npm)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

See it in your environment

Remediation advice

The problem has been recognized and patched. The fix will be available in version 27.0.0.

Frequently Asked Questions

  1. What is CVE-2021-21391? CVE-2021-21391 is a medium-severity uncontrolled resource consumption vulnerability in @ckeditor/ckeditor5-engine (npm), affecting versions <= 26.0.0. It is fixed in 27.0.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
  2. How severe is CVE-2021-21391? CVE-2021-21391 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which packages are affected by CVE-2021-21391?
    • @ckeditor/ckeditor5-engine (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-font (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-image (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-list (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-markdown-gfm (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-media-embed (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-paste-from-office (npm) (versions <= 26.0.0)
    • @ckeditor/ckeditor5-widget (npm) (versions <= 26.0.0)
  4. Is there a fix for CVE-2021-21391? Yes. CVE-2021-21391 is fixed in 27.0.0. Upgrade to this version or later.
  5. Is CVE-2021-21391 exploitable, and should I be worried? Whether CVE-2021-21391 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2021-21391 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2021-21391?
    • Upgrade @ckeditor/ckeditor5-engine to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-font to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-image to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-list to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-markdown-gfm to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-media-embed to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-paste-from-office to 27.0.0 or later
    • Upgrade @ckeditor/ckeditor5-widget to 27.0.0 or later

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