Summary
Workarounds
The user can work around the issue by:
- Upgrading CKEditor 5 to version 25.0.0.
- Disabling the Markdown plugin.
More information
If you have any questions or comments about this advisory:
- Email us at [email protected]
Acknowledgements
The CKEditor 5 team would like to thank Erik Krogh Kristensen from the GitHub team for recognizing this vulnerability and
Alvaro Muñoz from GitHub for reporting it.
Impact
A regular expression denial of service (ReDoS) vulnerability has been discovered in the CKEditor 5 Markdown plugin code. The vulnerability allowed to abuse a link recognition regular expression, which could cause a significant performance drop resulting in a browser tab freeze. It affects all users using the CKEditor 5 Markdown plugin at version <= 24.0.0.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2021-21254 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and no user interaction. 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 (25.0.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
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.
Remediation advice
The problem has been recognized and patched. The fix will be available in version 25.0.0.
Frequently Asked Questions
- What is CVE-2021-21254? CVE-2021-21254 is a medium-severity uncontrolled resource consumption vulnerability in @ckeditor/ckeditor5-markdown-gfm (npm), affecting versions <= 24.0.0. It is fixed in 25.0.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2021-21254? CVE-2021-21254 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.
- Which versions of @ckeditor/ckeditor5-markdown-gfm are affected by CVE-2021-21254? @ckeditor/ckeditor5-markdown-gfm (npm) versions <= 24.0.0 is affected.
- Is there a fix for CVE-2021-21254? Yes. CVE-2021-21254 is fixed in 25.0.0. Upgrade to this version or later.
- Is CVE-2021-21254 exploitable, and should I be worried? Whether CVE-2021-21254 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
- What actually determines whether CVE-2021-21254 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.
- How do I fix CVE-2021-21254? Upgrade
@ckeditor/ckeditor5-markdown-gfmto 25.0.0 or later.