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CVE-2021-21391

Regular expression Denial of Service in multiple packages

6.5 MediumFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

1.67%Down 1.1 points since Apr 29, 2021
90%50%10%1%

What each source reports

CVE List Score used 6.5

Regular expression Denial of Service in multiple packages

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.

Record
CVE-2021-21391
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Weakness
CWE-400
Updated
2026-11-02
EUVDEUVD-2021-0749 6.5

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.

Record
EUVD-2021-0749
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Updated
2026-11-03
GitHub AdvisoriesGHSA-3rh3-wfr4-76mj 6.5

Regular expression Denial of Service in multiple packages

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 [email protected] 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.

Record
GHSA-3rh3-wfr4-76mj
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Weakness
CWE-400
Updated
2026-11-02
NVD 6.5

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.

Record
CVE-2021-21391
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
CVSS 2.0
4.3 MediumAV:N/AC:M/Au:N/C:N/I:N/A:P
Weakness
CWE-400
Updated
2026-11-02
OSV 6.5

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.

Record
CVE-2021-21391
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVGHSA-3rh3-wfr4-76mj 6.5

Regular expression Denial of Service in multiple packages

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 [email protected] 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.

Record
GHSA-3rh3-wfr4-76mj
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Weakness
CWE-400
Updated
2026-11-02
OSVUBUNTU-CVE-2021-21391 6.5

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.

Record
UBUNTU-CVE-2021-21391
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
@ckeditor/ckeditor5-engine
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-font
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-image
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-list
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-markdown-gfm
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-media-embed
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-paste-from-office
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-widget
npm
 through 26.0.0
27.0.0
@ckeditor/ckeditor5-engine
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-font
npm
 before 27.0.0
27.0.0
Show 10 moreShow less
Affected packages
PackageAffectedFixed
@ckeditor/ckeditor5-image
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-list
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-markdown-gfm
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-media-embed
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-paste-from-office
npm
 before 27.0.0
27.0.0
@ckeditor/ckeditor5-widget
npm
 before 27.0.0
27.0.0
ckeditor3
Ubuntu:18.04:LTS
 All versions, plus listed versions
3.6.6.1+dfsg-1
 None known
ckeditor3
Ubuntu:20.04:LTS
 All versions, plus listed versions
3.6.6.1+dfsg-33.6.6.1+dfsg-4
 None known
ckeditor3
Ubuntu:22.04:LTS
 All versions, plus listed versions
3.6.6.1+dfsg-7
 None known
ckeditor3
Ubuntu:24.04:LTS
 All versions, plus listed versions
3.6.6.1+dfsg-7
 None known

History

History
RecordedChangedSources
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-21391 is a medium vulnerability (CVSS 6.5) in Ckeditor Ckeditor5, published April 6, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.67%, the 76th percentile. Fixed in @ckeditor/ckeditor5-engine 27.0.0.

References

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