For several decades, the transformations of the welfare state associated with an increasing casualization of the society profoundly changed the shape of social work. These developments expose the professionals involved to new challenges that put tension on the ethical dimension of their intervention. The social support of undocumented migrant families is particularly affected in these issues. Based on a sociological survey conducted in an association which provides accommodation for these families, this article sets the limits of this form of social action. Indeed, it is to help a population which is denied the right to stay in France. This mandate, somewhat paradoxical, submits stakeholders to strong contradictions which make them rethink the terms of their engagement with the public. At the crossroad of political, ethical and institutional issues, then emerges a new moral economy of the intervention, whose terms lead to build the meaning of action and recompose the professional practice.