what is the ruling on this unregistered marriage?

In our country, if the husband dies, the wife is entitled to receive benefits called a pension. But if she gets married to someone else, the state will stop paying her this pension. Some widows resort to not registering their new marriage, so that they can keep the pension from the first husband. This unregistered marriage is done with a guardian and witnesses (it is done openly), but it does not give any rights to the wife according to the state. So if her husband dies, she cannot inherit from him, and if he divorces her, she has no right to maintenance. The state does not recognize this marriage except in the case that the couple have children. We are not living in a tribal society where the tribe will guarantee people’s rights. I know that the pension that the wife is receiving is haraam, but what is the ruling on this unregistered marriage?