CVE-2021-21254Public exploitNetwork attack vector

Browser-tab freeze via backtracking link regular expression

Published Jan 29, 2021 · Updated Aug 3, 2024

Regular expression denial of service in CKSource CKEditor 5 Markdown plugin before 25.0.0 allows attackers to freeze a browser tab via link text. The Markdown exporter's link-recognition pattern lets a broad non-whitespace branch overlap its parenthesized-link branch, causing exponential backtracking on crafted URL-like input. The Markdown plugin must process attacker-controlled editor content; the resulting availability impact is limited to a browser-tab freeze, with no reported data leakage.

CVSS severity6.5
Medium
EPSS probability1.79%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in CKSource CKEditor 5 Markdown plugin before 25.0.0 allows attackers to freeze a browser tab via link text. The Markdown exporter's link-recognition pattern lets a broad non-whitespace branch overlap its parenthesized-link branch, causing exponential backtracking on crafted URL-like input. The Markdown plugin must process attacker-controlled editor content; the resulting availability impact is limited to a browser-tab freeze, with no reported data leakage.

The record

CVE
CVE-2021-21254
Published
Jan 29, 2021
Updated
Aug 3, 2024
Vendor
CKSource
Product
CKEditor 5
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 29, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 25.0.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

CKSource · CKEditor 5version=< 25.0.0

Published CVSS scores

6.5GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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