GHSA-7jx7-3846-m7w7

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...