Is it permissible for a father to reject a suitor who is of good character and financially comfortable, merely because his father has a bad reputation due to his previous work in the State Security apparatus, without verifying that claim? And what is the legitimate (Shariah) criterion for choosing a suitor?

The legitimate standard for choosing a suitor is his religiosity and character, based on the Prophet, peace and blessings be upon him, who said: "If there comes to you one whose religiosity and character you are pleased with, then marry him." So, if the young man is religious and has good character, and you are pleased with him, your father does not have the right to refuse him, because Islamic law has guaranteed women the right to choose a suitable husband. The ill reputation of his father does not harm his reputation, as Allah Almighty says: "And no bearer of burdens will bear the burden of another." If the father insists on refusing despite your desire for him, then he is committing 'adl (preventing a woman from marrying a suitable suitor).