CVE-2021-21391
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 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.
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 confidenceA Regular Expression Denial of Service (ReDoS) vulnerability in CKEditor 5 packages (ckeditor5-engine, ckeditor5-font, ckeditor5-image, ckeditor5-list, ckeditor5-markdown-gfm, ckeditor5-media-embed, ckeditor5-paste-from-office, ckeditor5-widget) allows abuse of specific regular expressions causing significant performance degradation and browser tab freeze.
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< 27.0.0< 27.0.0< 27.0.0< 27.0.0< 27.0.0< 27.0.0< 27.0.0< 27.0.0CVSS 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
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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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Locate CKEditor 5 package definitionsSearch your project for package.json files or look in node_modules for folders named ckeditor5-*Affected if Any ckeditor5 package folders exist in your project dependencies
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Identify installed CKEditor 5 versionsRun 'npm list --depth=0' or inspect the dependencies section of your package.json file to list all installed ckeditor5-* packages and their versionsAffected if Any of these packages are listed: ckeditor5-engine, ckeditor5-font, ckeditor5-image, ckeditor5-list, ckeditor5-markdown-gfm, ckeditor5-media-embed, ckeditor5-paste-from-office, ckeditor5-widget
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Compare versions against vulnerable rangeFor each ckeditor5-* package found, check the version number shown (e.g., 26.0.0, 27.1.0) against the threshold 27.0.0Affected if Any affected package has a version lower than 27.0.0
You are affected if any of the eight CKEditor 5 packages (engine, font, image, list, markdown-gfm, media-embed, paste-from-office, widget) are installed at a version below 27.0.0.
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 data27.0.0
Upgrade CKEditor 5 packages to version 27.0.0 or later to patch the vulnerable regular expressions.
- Consultation2.0 h
- Implementation3.0 h
- Testing6.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,520.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-21391 — 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-2021-21391 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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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- 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
- No spam, self-promotion, credentials, or personal data