CKEditor4 is an open source what-you-see-is-what-you-get HTML editor. A cross-site scripting vulnerability has been discovered in the core HTML parsing module in versions of CKEditor4 prior to 4.24.0-lts. It may affect all editor instances that enabled full-page editing mode or enabled CDATA elements in Advanced Content Filtering configuration (defaults to `script` and `style` elements). The vulnerability allows attackers to inject malformed HTML content bypassing Advanced Content Filtering mechanism, which could result in executing JavaScript code. An attacker could abuse faulty CDATA content detection and use it to prepare an intentional attack on the editor. A fix is available in version 4.24.0-lts.