u/Funchalia

I recently bought this house, and unfortunately the inspector didn’t notice this mold issue. I started removing my kitchen cabinets to replace them, and when I took out the sink base cabinet, I found this behind the wall.
What would you recommend I do next? Should I cut out part of the wall to see how bad it is behind there, or would using something like Kilz Mold & Mildew primer be enough?
Any advice would be appreciated.