The vulnerability stems from improper access control in z3c.form's widget traversal mechanism. The CVE description explicitly references 'widget_traversal.py' in Plone's security advisory 20121106/07, indicating traversal-related exposure of default values. The getitem method in widget traversal is typically responsible for resolving traversal steps, and lack of authorization here would let attackers access hidden form field metadata by constructing URLs targeting specific widget element IDs. The patch in Plone 4.2.3/4.3b1 likely added security checks to this traversal pathway.