CVE-2021-33829

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 3.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2021-06-09 · updated 2026-06-17
Weakness: CWE-79

Description

A cross-site scripting (XSS) vulnerability in the HTML Data Processor in CKEditor 4 4.14.0 through 4.16.x before 4.16.1 allows remote attackers to inject executable JavaScript code through a crafted comment because --!> is mishandled.

Affected products

ckeditor4: from 4.14.0 before 4.16.1 — fixed in 4.16.1
Debian Linux: 9.0
Drupal: from 8.9.0 before 8.9.16; from 9.0.0 before 9.0.14; from 9.1.0 before 9.1.9 — fixed in 8.9.16, 9.0.14, 9.1.9
Fedoraproject Fedora: 33, 34, 35

How to fix

Upgrade to a fixed version: ckeditor4 4.16.1; Drupal 8.9.16, 9.0.14, 9.1.9. Check the vendor advisory for mitigations.

References

https://ckeditor.com/blog/ckeditor-4.16.1-with-accessibility-enhancements/#improvements-for-comments-in-html-parser
https://lists.debian.org/debian-lts-announce/2021/11/msg00007.html
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/NYA354LJP47KCVJMTUO77ZCX3ZK42G3T/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UVOYN2WKDPLKCNILIGEZM236ABQASLGW/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WAGNWHFIQAVCP537KFFS2A2GDG66J7XD/
https://www.drupal.org/sa-core-2021-003
https://ckeditor.com/blog/ckeditor-4.16.1-with-accessibility-enhancements/#improvements-for-comments-in-html-parser
https://lists.debian.org/debian-lts-announce/2021/11/msg00007.html
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/NYA354LJP47KCVJMTUO77ZCX3ZK42G3T/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UVOYN2WKDPLKCNILIGEZM236ABQASLGW/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WAGNWHFIQAVCP537KFFS2A2GDG66J7XD/
https://www.drupal.org/sa-core-2021-003

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