MIL wants my sons ssn. Won't tell me why

Amber • Wife. Mommy to 3.
My in laws recently visited and asked for my son's social security number. When asked why she said "because I wanted to get it". I never said anything back to her. A few hours later she asked my husband for it and when he asked why, she gave the same "because I wanted to get it" answer. I told her she wasn't getting it without telling us why and that it's just not something we hand out. My own mother doesn't even have it. What makes her think I'd give it to her? She's not even his biological grandmother and I do not trust her any further than I can throw her. My husband doesn't consider her to be his mother nor does he trust her. Was I over reacting? Would you give it to her? 
She still refuses to tell us what it was for. They only say that we can forget about it now. Pretty sure they hate me now...