TinyMCE 4.x is vulnerable to several XSS vectors, which had been patched in later versions. Two of these have been identified as affecting silverstripe/admin. Only Silverstripe CMS 4 is affected by these vulnerabilities. It's not possible to upgrade Silverstripe CMS 4 to use a more recent release of TinyMCE without introducing breaking changes. Instead, the security patches that shipped in later releases of TinyMCE have been backported to the TinyMCE version bundled in silverstripe/admin. Silverstripe CMS 5 is not affected by these vulnerabilities because it uses TinyMCE 6.