Craft CMS Vulnerable to potential authenticated Remote Code Execution via malicious attached Behavior

Relationship to Previously Patched Vulnerability

This vulnerability is in addition to the RCE vulnerability patched in GHSA-255j-qw47-wjh5. That advisory addressed a similar RCE vulnerability that affected two specific routes:

• /index.php?p=admin%2Factions%2Ffields%2Fapply-layout-element-settings
• /index.php?p=admin%2Factions%2Ffields%2Frender-card-preview

This one addresses some additional endpoints that were not covered in the https://github.com/craftcms/cms/security/advisories/GHSA-255j-qw47-wjh5.

The patched vulnerability used a malicious AttributeTypecastBehavior with a wildcard event listener ("on *": "self::beforeSave") and __construct() syntax to trigger RCE via the typecastBeforeSave callback. The fix was implemented in commits:

• 6e608a1
• 27f5588
• ec43c49

This vulnerability follows the same attack pattern (behavior injection via "as <behavior>" syntax) but affects a different code path (assembleLayoutFromPost() in Fields.php) that was not patched in those commits. The attack vector uses typecastAfterValidate instead of typecastBeforeSave and does not require the wildcard event listener syntax, demonstrating that multiple entry points exist for this type of vulnerability.

Executive Summary

A Remote Code Execution (RCE) vulnerability exists in Craft CMS where the assembleLayoutFromPost() function in src/services/Fields.php fails to sanitize user-supplied configuration data before passing it to Craft::createObject(). This allows authenticated administrators to inject malicious Yii2 behavior configurations that execute arbitrary system commands on the server. This vulnerability represents an unpatched variant of the behavior injection vulnerability addressed in GHSA-255j-qw47-wjh5, affecting different endpoints through a separate code path.

Vulnerability Details

Attack Prerequisites

• Authentication: Admin-level access required
• Network Access: Access to admin panel (/admin)

Location

• File: src/services/Fields.php
• Function: assembleLayoutFromPost() (lines 1125-1143)
• Root Cause: Missing cleanseConfig() call on user-supplied fieldLayout POST parameter

Vulnerable Code Path

// src/services/Fields.php:1125-1133
public function assembleLayoutFromPost(?string $namespace = null): FieldLayout
{
    $paramPrefix = $namespace ? rtrim($namespace, '.') . '.' : '';
    $request = Craft::$app->getRequest();
    $config = JsonHelper::decode($request->getBodyParam("{$paramPrefix}fieldLayout"));
    // ... additional config values added ...
    $layout = $this->createLayout($config);  // <-- No cleanseConfig() call!
    // ...
}

// src/services/Fields.php:1089-1093
public function createLayout(array $config): FieldLayout
{
    $config['class'] = FieldLayout::class;
    return Craft::createObject($config);  // <-- Untrusted data passed directly
}

Attack Chain

The exploitation leverages Yii2's object configuration system and behavior attachment mechanism:

1. Behavior Injection: Attacker includes 'as rce' key in the fieldLayout JSON POST parameter
2. Object Creation: Craft::createObject() processes the config through Yii2's BaseYii::configure()
3. Behavior Attachment: Yii2's Component::__set() detects the 'as ' prefix and attaches the behavior
4. RCE Trigger: When validate() is called on the model, EVENT_AFTER_VALIDATE fires
5. Command Execution: AttributeTypecastBehavior calls the configured typecast function (ConsoleProcessus::execute) with the uid attribute value as the command

RCE Gadget Chain

FieldLayout POST parameter
    → Craft::createObject()
    → Yii2 Component::__set() with 'as rce' key
    → AttributeTypecastBehavior attached
    → Model::validate() called
    → EVENT_AFTER_VALIDATE triggered
    → typecastAfterValidate → typecastAttributes()
    → call_user_func(['Psy\Readline\Hoa\ConsoleProcessus', 'execute'], $command)
    → Shell command execution

Affected Controllers

The assembleLayoutFromPost() function is called by multiple admin controllers:

References

• https://github.com/craftcms/cms/commit/395c64f0b80b507be1c862a2ec942eaacb353748
• GHSA-255j-qw47-wjh5 - Previously patched RCE vulnerability via behavior injection (affecting different endpoints)
• CVE-2024-4990 - Related vulnerability that inspired the behavior injection attack pattern
• Yii2 GHSA-gcmh-9pjj-7fp4 - Original Yii framework report (framework team declined to fix at framework level)