24 CFR §350.11

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. These regulations do not purport to establish whether a Federal Reserve Bank as Depository is required to honor an order or other notice of attachment in any particular case or class of cases.