GHSA-52cw-pvq9-9m5v: Silverstripe uses TinyMCE which allows svg files linked in object tags

TinyMCE v6 has a configuration value convert_unsafe_embeds set to false which allows svg files containing javascript to be used in <object> or <embed> tags, which can be used as a vector for XSS attacks.

Note that <embed> tags are not allowed by default.

After patching the default value of convert_unsafe_embeds will be set to true. This means that <object> tags will be converted to iframes instead the next time the page is saved, which may break any pages that rely upon previously saved <object> tags. Developers can override this configuration if desired to revert to the original behaviour.

We reviewed the potential impact of this vulnerability within the context of Silverstripe CMS. We concluded this is a medium impact vulnerability given how TinyMCE is used by Silverstripe CMS.