With the promulgation of the 1988 Federal Constitution, Social Security was conceived by the Original Constituent Power as an integrated set of actions at the initiative of the Public Authorities and society, aimed at guaranteeing rights relating to health, social security and social assistance. Among the objectives of social assistance set out in the Federal Constitution is the guarantee of a minimum monthly benefit for the disabled and the elderly who can prove that they do not have the means to provide for their own maintenance or to have it provided for by their family. This benefit is regulated by Law 8742/93 (LOAS), which will undergo significant changes if the text of PEC 287/2016 is approved. In this context, based on a brief analysis of the legal provisions that regulate the social assistance benefit and a prognosis of the legislative changes intended by the aforementioned PEC, this study aims to investigate the role of fundamental rights as limiters of Derivative Constituent Power. In order to be legitimised, these legislative changes need to be in line with the essential core of fundamental rights.