For almost a decade, some divisions of the Department of Agriculture published the Social Security numbers of individuals who receive federal aid in a publicly available online database of government grants. The Farm Service Agency and at least one other agency within Agriculture included the nine digit numbers as part of the tracking number assigned to each recipient of government assistance, called a Federal Award ID.
Those tracking numbers were then published in the Federal Assistance Awards Database System (FAADS), an online compendium of "all types of financial assistance awards made by federal agencies to all types of recipients," which is updated quarterly. This database is generally used by experts and is not very user-friendly.
So far, it's not clear whether the inadvertent publication of individuals' Social Security numbers is limited to the two agencies in Agriculture, or whether the problem extends to other departments and agencies as well.