31 CFR §357.25

1. (a)General. The Department will not recognize any notice or claim of a lien, encumbrance, or security interest of any kind, including a pledge, in a security in Legacy Treasury Direct ® except as provided in § 357.23 and in paragraph (b) of this section.
2. (b)Security for the performance of duty or obligation under Federal law. The Department will accept and hold pursuant to the provisions of 31 U.S.C. 9303, book-entry bonds, notes or bills submitted in lieu of a surety bond as security for the performance of a duty or obligation required by Federal law in accordance with said section.