Social Security funds are invested. They are not just sitting there- they must acquire interest money. They are invested in interest-bearing federal securities. Who buys these?

Extracted from Social Security Online; Trust Fund Data.

1. Special issues- Investment securities available only to trust funds.
a. Certificates of indebtedness that are issued daily for the investment of receipts not required to meet current expenditures, and they mature on the next June 30 following the date of issue.
b. Special issue bonds. Maturities from one to fifteen years.

2. Special issues available to the general public (marketable securities).
Not issued at present, but they have been issued in the past, and could be again.