Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
A Fatwa has already been published about the permissibility of participating in social security if it is free of the following dangers:
1) Social security money should not be saved in banks which deal with usury and interest, be it in a current account or a saving account, except of necessity when this money can be saved in a current account in the absence of Islamic banks.
2) This money should not be invested (all or part of it) in Haram or doubtful fields.
3) To abide by the requirements of the Islamic Shariah and its rules, regarding his rights and duties.
4) The social security contract should not contain usury, interest, gambling or cheating.
We draw the attention of the brother asking the question that it is not permissible for anybody to join (subscribe to) this system if it contains a religiously dangerous things as already mentioned or any other religious inconvenience, except in case of an obligation, and leaving it would cause a greater danger.
So, in this case it would be permissible to join these social securities and the sin is on whoever obliges him to do so. He has to get rid of the illegible amount of money after taking his right determined in the contract to declare his innocence.
Allah knows best.