31 CFR §357.15

1. (a)The interest of a debtor may be reached by creditors only by legal process upon the Securities Intermediary with whom the debtor's securities account is maintained. Exception: If a Security Entitlement is maintained in the name of a secured party, the debtor's interest may be reached by legal process upon the secured party.
2. (b)These regulations do not state whether a Federal Reserve Bank is required to honor an order or other notice of attachment in any particular case or class of cases.