Diagnosis
That's a clogged defrost drain. When the freezer goes through a defrost cycle, melted water should drain down a tube into a pan at the bottom. If that tube gets blocked with gunk or ice, the water backs up and overflows into the fridge.
How to Fix It
You'll need to clear the drain tube. First, unplug the fridge. Locate the drain hole – it's in the back wall of the freezer compartment, near the bottom. You'll likely see ice around it. 1) Thaw it out with a hairdryer (on low, keep it moving) or hot water in a turkey baster until the ice plug melts. 2) Once clear, pour a cup of warm water mixed with a little baking soda down the hole to flush it and help clear any sludge. It should flow freely to the drip pan below.
If That Didn't Work
If water still won't drain, the drain tube itself might be kinked or crushed where it runs behind the liner. That's a more involved repair.
Heads Up
Be patient thawing the ice. Don't use sharp objects to poke the drain – you can puncture the drain line or liner. After clearing it, keep an eye on it. If it clogs again quickly, you might have a bigger defrost system problem.