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 this issue. 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 those vulnerabilities because it uses TinyMCE 6.
You can find more information about the underlying vulnerabilities in those GitHub security advisories:
References
GuySartorelli Reporter
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 this issue. 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 those vulnerabilities because it uses TinyMCE 6.
You can find more information about the underlying vulnerabilities in those GitHub security advisories:
References