12 CFR §1511.8

The interest of a debtor in a Security Entitlement may be reached by a creditor only by legal process upon the Securities Intermediary with whom the debtor's securities account is maintained, except where a Security Entitlement is maintained in the name of a secured party, in which case the debtor's interest may be reached by legal process upon the secured party. The regulations in this part do not purport to establish whether a Federal Reserve Bank is required to honor an order or other notice of attachment in any particular case or class of cases.