GHSA-xxpx-f366-4xpq: Craft CMS: Authorization bypass: view-only Categories user can modify category structure via structures/move-element

A control-panel user who holds only the viewCategories permission for a category group (and not saveCategories) can permanently modify that group’s category structure — reordering and re-parenting categories via the structures/move-element action.

A read-time authorization grant that a write endpoint later trusts. For categories, the structureEditable flag is computed from the view permission (src/elements/Category.php:205) instead of the save permission (entries correctly use saveEntries — src/elements/Entry.php:341). When the read-only category index renders, craft\base\Element::indexHtml() calls Craft::$app->getSession()->authorize('editStructure:<structureId>'); StructuresController then authorizes the structure-mutating action solely on that session grant, with no canSave re-check.

Verified on Craft CMS 5.10.5. Same class as the moderate-severity authorization bypasses fixed in 5.10.3 and 5.10.5; this is a distinct, unpatched instance.