CVE-2021-21254
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedCKEditor 5 is an open source rich text editor framework with a modular architecture. The CKEditor 5 Markdown plugin (@ckeditor/ckeditor5-markdown-gfm) before version 25.0.0 has a regex denial of service (ReDoS) vulnerability. The vulnerability allowed to abuse link recognition regular expression, which could cause a significant performance drop resulting in browser tab freeze. It affects all users using CKEditor 5 Markdown plugin at version <= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe CKEditor 5 Markdown plugin (ckeditor5-markdown-gfm) versions 24.0.0 and earlier contain a ReDoS vulnerability in the regular expression used for link recognition. Crafted markdown input can trigger catastrophic backtracking in the regex, causing significant performance degradation and freezing the browser tab.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 25.0.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check the installed version of the markdown-gfm packageRun 'npm list @ckeditor/ckeditor5-markdown-gfm' or check package.json dependencies for '@ckeditor/ckeditor5-markdown-gfm'Affected if The installed version is 24.0.0 or earlier (any version below 25.0.0)
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Verify the CKEditor 5 core versionRun 'npm list ckeditor5' or check the 'ckeditor5' entry in package.jsonAffected if The ckeditor5 core version is below 25.0.0
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Confirm the Markdown GFM plugin is loadedInspect the CKEditor configuration for 'markdownGfm' in the plugins array, or check if the editor instance includes the GFM pluginAffected if The markdownGfm plugin is included in the editor configuration and processes user input
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Check if the application processes markdown with linksReview application code or user input flows to see if markdown containing links (such as [text](url) format) is passed to the CKEditor instanceAffected if Markdown input containing links is processed by the vulnerable regex in the editor
A user is affected if the @ckeditor/ckeditor5-markdown-gfm package version is below 25.0.0 and the markdown GFM plugin is enabled to process user-supplied markdown with link syntax.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor data25.0.0
Upgrade the @ckeditor/ckeditor5-markdown-gfm package to version 25.0.0 or later to receive the patched regular expression.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2021-21254 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-21254 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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