4 C.F.R. § 83.9 – Social Security number.

(a) GAO may not require individuals to disclose their Social Security Number (SSN) unless disclosure would be required—
1. (1) Under Federal statute; or
2. (2) Under any statute, executive order, or regulation that authorizes any Federal, State, or local agency maintaining a system of records that was in existence and operating prior to January 1, 1975, to request the SSN as a necessary means of verifying the identity of an individual.
(b) Individuals asked to voluntarily provide their SSN shall suffer no penalty or denial of benefits for refusing to provide it.
(c) When GAO requests an individual to disclose his or her SSN, it shall inform that individual whether that disclosure is mandatory or voluntary, by what statutory or other authority such number is solicited, and what uses will be made of it.