If the door pictured below is locked, and someone pulls quite hard on the outside handle, the door jamb will flex inward. There must not be good blocking behind the catch. I haven't tested to the extreme, but a hard enough pull might well cause enough flex to open the door.

IMG_4381.jpg

I would like to eliminate, or at least greatly reduce, this flex without disassembling the door frame. Any suggestions? For example, is there something that could be sprayed or injected through a small hole and would then harden sufficiently?