CVE-2021-21391

mediumpriority · track · 27EPSS 0.017 · 77 %ileEUUS
grimoire priorityP3 · Track
Exploitable?
—
no known exploitation
Likely / severe?
1.67% (77th) EPSS
CVSS 4.3 · medium
Is it mine?
17 products
Check your exposure
Who says so?
4 sources
cnnvd · enisa_euvd · nvd · osv

EPSS · scored 2026-10-27

CKEditor 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.

01

cvss

Technical severity, 0–10. One row per scoring source.

6.5
010

base score 0–10 · higher = more severe · hover a vector to decode

v2.04.3mediumAV:N/AC:M/Au:N/C:N/I:N/A:Pprimary · NVD / NIST USexploitability 8.6 · impact 2.9
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:Hprimary · NVD / NIST USexploitability 2.8 · impact 3.6
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:Hsecondary · ENISA EU
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:Hsecondary · security-advisories@github.comexploitability 2.8 · impact 3.6
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:Hsecondary · OSV
v3.16mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H/E:Usecondary · CVSS-BT (t0sche)
02

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-400USCWE-400US
03

affected products (17)

Products and version ranges known to be affected.

04

advisories (1)

Vendor and ecosystem advisories describing this.

  • GHSA-3rh3-wfr4-76mjRegular expression Denial of Service in multiple packages2021-04-06ghsa · osvUS

    ### 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.

05

also known as (3)

Other names and identifiers for this entity.

06

sources

Which sources assert this record, and when.

cnnvdenisa_euvdnvdosvNVDCVE.orgMITRE
published 2021-04-29modified 2026-06-17
07

references (28)

External citations — patches, advisories, analysis.

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