Silverstripe Elemental extends a page type to swap the content area for a list of manageable elements to compose a page out of rather than a single text field. An elemental block can include an XSS payload, which can be executed when viewing the "Content blocks in use" report. The vulnerability is specific to that report and is a result of failure to cast input prior to including it in the grid field. This vulnerability is fixed in 5.3.12.

Silverstripe cross-site scripting (XSS) attack in elemental "Content blocks in use" report