It was possible to execute a ReDoS-type attack inside CKEditor 4 before 4.16 by persuading a victim to paste crafted text into the Styles input of specific dialogs (in the Advanced Tab for Dialogs plugin).

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-26271
• https://github.com/ckeditor/ckeditor4/blob/major/CHANGES.md#ckeditor-416
• https://www.oracle.com//security-alerts/cpujul2021.html
• https://www.oracle.com/security-alerts/cpuoct2021.html
• https://web.archive.org/web/20210128132707/https://ckeditor.com/blog/CKEditor-4.16-with-improved-image-pasting-High-Contrast-support-and-a-new-color-API/#security-comes-first