HOMEVULNERABILITIESCVE-2021-21254
MEDIUM

CVE-2021-21254

CWE-400Published: January 29, 2021· Updated: Jun 17, 2026

6.5
CVSS v3.1

Official Description

CKEditor 5 is an open source rich text editor framework with a modular architecture. The CKEditor 5 Markdown plugin (@ckeditor/ckeditor5-markdown-gfm) before version 25.0.0 has a regex denial of service (ReDoS) vulnerability. The vulnerability allowed to abuse link recognition regular expression, which could cause a significant performance drop resulting in browser tab freeze. It affects all users using CKEditor 5 Markdown plugin at version <= 24.0.0. The problem has been recognized and patched. The fix will be available in version 25.0.0.

NVD Source

Technical Analysis

CVE-2021-21254 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

Exploitation requires low privileges, which limits the exposure to scenarios where an attacker has already gained initial access.

A successful exploit results in availability disruption (denial of service), with a CVSS base score of 6.5.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityLow
Privileges Req.Low
User InteractionNone
ScopeUnchanged
Impact
ConfidentialityNone
IntegrityNone
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected Vendors & Products

ckeditor1 product(s)
ckeditor5
Source: NVD CPE · 1 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
External links open in a new tab. Always verify in a controlled environment before use.

Official Patches & Advisories

ADVISORYhttps://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0
ADVISORYhttps://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr
ADVISORYhttps://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0
ADVISORYhttps://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wr

All References (6)

→
https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0Release Notes · Third Party Advisory
→
https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wrThird Party Advisory
→
https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm
→
https://github.com/ckeditor/ckeditor5/releases/tag/v25.0.0Release Notes · Third Party Advisory
→
https://github.com/ckeditor/ckeditor5/security/advisories/GHSA-hgmg-hhc8-g5wrThird Party Advisory
→
https://www.npmjs.com/package/%40ckeditor/ckeditor5-markdown-gfm

Quick Facts

CVE IDCVE-2021-21254
CVSS Score6.5 / 10
SeverityMEDIUM
WeaknessCWE-400
CISA KEVNo
Affected1 vendor(s)
PublishedJan 29, 2021

Related CVEs (CWE-400)

CVE-2017-1000378
9.8
CVE-2017-9119
9.8
CVE-2026-30141
9.8
CVE-2022-48716
9.8
CVE-2024-39462
9.8
CVE-2018-16486
9.8

Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-21254 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWatch. CVE data is provided under the NVD usage policy.
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