CVE-2026-50281

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.

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.

Kodem intelligence

1. What is CVE-2026-50281? CVE-2026-50281 is a high-severity security vulnerability in craftcms/cms (composer), affecting versions >= 5.7.0, < 5.9.21. It is fixed in 5.9.21.
2. Which versions of craftcms/cms are affected by CVE-2026-50281? craftcms/cms (composer) versions >= 5.7.0, < 5.9.21 is affected.
3. Is there a fix for CVE-2026-50281? Yes. CVE-2026-50281 is fixed in 5.9.21. Upgrade to this version or later.
4. Is CVE-2026-50281 exploitable, and should I be worried? Whether CVE-2026-50281 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-50281 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-50281? Upgrade craftcms/cms to 5.9.21 or later.