Craft CMS's mass assignment via id in newAttributes during bulk duplicate overwrites existing elements

Summary

There is a mass-assignment flaw in the bulk-duplicate element action. Alice, holding only the permission to duplicate an entry she owns, submits an arbitrary id through the newAttributes request parameter. The duplication routine overrides its own id = null reset with that value and writes Alice’s attributes into Bob’s existing entry row.

Details

ElementsController::beforeAction() (src/controllers/ElementsController.php:119-124) pulls the request body into $this->_attributes and rejects requests that ship an id or canonicalId key at the top level:

$this->_attributes = $this->request->getBodyParams();

// No funny business
if (isset($this->_attributes['id']) || isset($this->_attributes['canonicalId'])) {
    throw new BadRequestHttpException('Changing an element’s ID is not allowed.');
}

The check inspects only the top-level payload. actionBulkDuplicate() (src/controllers/ElementsController.php:1708-1749) reads a separate newAttributes array and passes it straight through to the service layer:

$elementInfo = $this->request->getRequiredBodyParam('elements');
$newAttributes = $this->request->getRequiredBodyParam('newAttributes');
...
$safeNewAttributes = Collection::make($newAttributes)
    ->only($element->safeAttributes())
    ->all();
...
$newElement = $elementsService->duplicateElement(
    $element,
    $safeNewAttributes + $element::baseBulkDuplicateAttributes(),
    false,
    checkAuthorization: true,
);

Elements::duplicateElement() (src/services/Elements.php:1814-1840) clones the source element, sets id to null, and then hands the attacker's array to Craft::configure():

$mainClone = clone $element;
$mainClone->id = null;
$mainClone->uid = StringHelper::UUID();
...
Craft::configure($mainClone, ArrayHelper::merge(
    $newAttributes,
    $siteAttributes[$mainClone->siteId] ?? [],
));

Craft::configure() overwrites the reset id with any numeric value inside $newAttributes. Yii's saveElement() then performs an UPDATE against the row with that primary key instead of an INSERT. Alice's title, slug, authorId, postDate, and UID land on Bob’s entry.

safeAttributes() on Entry includes id because the base element model exposes it, so the Collection::only() filter does not strip it.

Impact

A low-privileged author overwrites any other element (entries, categories, users that share the Entry element table inheritance) by predicting or enumerating element IDs. Content integrity on the entire install breaks. The attack requires only the ability to duplicate one entry Alice already owns.