Craft CMS is Vulnerable to Authenticated Remote Code Execution via Malicious Attached Behavior

Summary

A Remote Code Execution (RCE) vulnerability exists in Craft CMS 5.x and 4.x that bypasses the security fixes for GHSA-7jx7-3846-m7w7 and GHSA-255j-qw47-wjh5. This vulnerability can be exploited by any authenticated user with control panel access.

The existing patches add cleanseConfig() to assembleLayoutFromPost() and various FieldsController actions to strip Yii2 behavior/event injection keys (as and on prefixed keys). However, the fieldLayouts parameter in ElementIndexesController::actionFilterHud() is passed directly to FieldLayout::createFromConfig() without any sanitization, enabling the same behavior injection attack chain.

Impact

• Attack Type: Remote Code Execution (RCE)
• Authentication Required: Authenticated user with control panel access (accessCp permission)

Vulnerability Details

Root Cause

In ElementIndexesController::actionFilterHud() (line 493-494), the fieldLayouts body parameter is passed to FieldLayout::createFromConfig() without cleanseConfig():

// ElementIndexesController.php:485-494
if ($conditionConfig) {
    $conditionConfig = Component::cleanseConfig($conditionConfig); // conditionConfig IS cleansed
    $condition = $conditionsService->createCondition($conditionConfig);
} else {
    $condition = $this->elementType()::createCondition();
}

if (!empty($fieldLayouts)) {
    // fieldLayouts is NOT cleansed!
    $condition->setFieldLayouts(array_map(
        fn(array $config) => FieldLayout::createFromConfig($config),
        $fieldLayouts
    ));
}

Note the inconsistency: conditionConfig is sanitized with cleanseConfig(), but fieldLayouts is not.

Attack Chain

1. Send a fieldLayouts array containing config with "as <name>" prefixed keys
2. FieldLayout::createFromConfig($config) -> new self($config) -> Model::__construct($config)
3. App::configure($this, $config) processes each key
4. "as rce" key -> Component::__set("as rce", $value) -> Yii::createObject($value) -> instantiates AttributeTypecastBehavior and attaches it to the FieldLayout