CVE-2021-21391

Vulnerability in @ckeditor/ckeditor5-engine (CVE-2021-21391)

Summary

Vulnerability in @ckeditor/ckeditor5-engine (CVE-2021-21391). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 27.0.0 or later.

Original description

The description as published by the source.

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.

Patches

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

For more information

Email us at security@cksource.com 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.

Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2021-21391 has been found in @ckeditor/ckeditor5-engine.
Risk of unauthorized operations or information disclosure.
What to do: upgrade @ckeditor/ckeditor5-engine to 27.0.0 or later.
If unsure, ask your IT team or search "@ckeditor/ckeditor5-engine CVE-2021-21391" on the vendor's site.

CVE-2021-21391 (@ckeditor/ckeditor5-engine) — CWE-400
Attack vector: remote (network-reachable) / unauthenticated
Patched: 27.0.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in @ckeditor/ckeditor5-engine.

📍 Affected scope

Check the vendor advisory for the affected versions of @ckeditor/ckeditor5-engine.

🔥 Severity

Severity: ?. Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 27.0.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r '@ckeditor/ckeditor5-engine' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

1
Identify exposure identify
```
grep -r '@ckeditor/ckeditor5-engine' . | grep -v node_modules
```
リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で @ckeditor/ckeditor5-engine を grep し、稼働しているサービス・バージョンを把握する。
2
Apply patch patch
```
Upgrade @ckeditor/ckeditor5-engine to 27.0.0
```
ステージング環境で 27.0.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
3
Post-deployment verification verify
```
Confirm patched version is live in production
```
パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。