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Fencer Vulns
medium 6.5

CVE-2021-21391 json

Regular expression Denial of Service in multiple packages

published 2021-04-06 5 y ago modified 2026-07-08 4 mo ago also GHSA-3rh3-wfr4-76mj , EUVD-2021-0749

cvss
6.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.7%
EPSS 1.7% chance of exploitation in 30 days, percentile 76%
public exploits
0
none known

Summary

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.

Patch / fix

  • npm @ckeditor/ckeditor5-engine 27.0.0
  • npm @ckeditor/ckeditor5-font 27.0.0
  • npm @ckeditor/ckeditor5-image 27.0.0
  • npm @ckeditor/ckeditor5-list 27.0.0
  • npm @ckeditor/ckeditor5-markdown-gfm 27.0.0
  • npm @ckeditor/ckeditor5-media-embed 27.0.0
  • npm @ckeditor/ckeditor5-paste-from-office 27.0.0
  • npm @ckeditor/ckeditor5-widget 27.0.0
  • ckeditor ckeditor5-engine 27.0.0
  • ckeditor ckeditor5-font 27.0.0
  • ckeditor ckeditor5-image 27.0.0
  • ckeditor ckeditor5-list 27.0.0
  • ckeditor ckeditor5-markdown-gfm 27.0.0
  • ckeditor ckeditor5-media-embed 27.0.0
  • ckeditor ckeditor5-paste-from-office 27.0.0
  • ckeditor ckeditor5-widget 27.0.0

Exploit prediction

1.67% chance of exploitation in the next 30 days, higher than 76% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:N/A:P
GitHub 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
npm @ckeditor/ckeditor5-engine
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-font
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-image
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-list
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-markdown-gfm
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-media-embed
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-paste-from-office
<27.0.0
27.0.0
npm @ckeditor/ckeditor5-widget
<27.0.0
27.0.0

Affected products

  • ckeditor ckeditor5

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.