14 CFR § 1212.604 - Social security numbers.
(1) The disclosure is required by law; or
(2) The disclosure is from a system of records in existence and operating before January 1, 1975, and was required under statute or regulation adopted before that date to verify the identity of the individual(s).
(b) Any time individuals are requested to disclose their social security numbers, NASA shall indicate whether that disclosure is mandatory or voluntary, by what authority the numbers are requested, and what uses will be made of them.
(c) When sending physical mail, NASA will adhere to the following:
(1) Social Security account numbers shall not be visible on the outside of any package sent by mail.
(2) A document sent by mail may only include the Social Security account number of an individual if it is determined by the Administrator that the inclusion of a Social Security account number is necessary.
(3) The inclusion of a Social Security account number of an individual on a document sent by mail is necessary when—
(i) Required by law; or
(ii) Necessary to identify a specific individual and no adequate substitute is available.
(4) Social Security account numbers must be partially redacted in documents sent by mail whenever feasible.