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.

Impact

A low-privileged, authenticated user (view-only on a category group) can persistently alter the sibling ordering and parent/child nesting of the category taxonomy. Because a category’s URI is derived from its position in the structure (ancestor slugs), moving a category changes its URL and the URLs of its descendants, and can corrupt any navigation/menus built from the category tree. This is an integrity/broken access-control issue: content that the user has no permission to modify is being modified. No confidentiality impact and no RCE; scope is content/taxonomy integrity.