Find something you can stuff in that hole, put it on the end of a stick?

Like a wad of steel wool, if you’re worried about mice. Of course, a mousetrap could be slid back there with less effort.

Not a ”proper” repair, but is it worth removing the cabinet - I guess the question is, what harm is that hole doing?

I would be worried about why it is rotting in the first place. Is it under a sink, or by any water pipes, or next to a dishwasher? Something is obviously leaking and there could be a damage to floor joists, etc. Might be time to do a bit of investigating.

Be careful about sticking your face where there could be mice/rat pee and feces ... Hantavirus.

Thank you for the answers! Great questions. I am pretty sure it was old leaks before I bought the place. It is not near a sink at all. Of course, I have to replace the cabinet and the wall section, and who knows how much more damage. Don't have the money yet. In the meantime, just don't want a possible entrance for bugs, etc. I share the wall with a neighbor. I am going to remove as much as I can of the rotted wood. Trying to see how to cover the hole it in the meantime. Thank you!

You could also use some plastic clay or a big glop of plumbers putty to fill the hole. You’re just trying to stop critters from coming through, not make it pretty.

I’m going to guess a mouse made that.

Could there be a leak on your neighbors side, that is causing it?

To plug the hole, there is expanding spray foam that hardens, comes in a can with a extension tube for application.

If you use the foam, test spray some on a piece of cardboard or into a plastic bag so you can see the expansion.

Thank you so much for all the ideas! Very much appreciated!!