TinyMCE versions before 5.9.0 are affected by a stored cross-site scripting vulnerability. An unauthenticated and remote attacker could insert crafted HTML into the editor resulting in arbitrary JavaScript execution in another user's browser.
https://www.tiny.cloud/docs/release-notes/release-notes59/#securityfixes
https://vulncheck.com/advisories/vc-advisory-GHSA-5h9g-x5rv-25wg
https://github.com/tinymce/tinymce/security/advisories/GHSA-5h9g-x5rv-25wg