48 C.F.R. § 32.304-6
32.304-6 Other collateral security.
The following are examples of other forms of security that, although seldom invoked under guaranteed loans, may be required when considered necessary for protection of the Government interest:
(a) Mortgages on fixed assets.
(b) Liens against inventories.
(c) Endorsements.
(d) Guarantees.
(e) Subordinations or standbys of other indebtedness.