Diagnosis
That's a common wear point. The door handles are typically held on by a single screw or bolt from the inside of the door. That connection can work loose over time with regular opening and closing.
How to Fix It
You can fix this without any special tools. 1) Open the door fully. 2) Look at the inside of the door, right behind where the exterior handle attaches. You should see a small plastic cover or a single Phillips-head screw. 3) If there's a cover, gently pry it off with a flat-head screwdriver. 4) Use a Phillips screwdriver to firmly tighten the screw you find there. Don't overtighten and strip it—just snug it up. 5) Replace the cover if there was one. Test the handle; it should feel solid now.
If That Didn't Work
If the screw is already tight but the handle is still loose, the plastic post on the handle itself or the receiving part on the door might be cracked. You'd need to order a replacement handle kit from the manufacturer.
Heads Up
Never lift or carry the oven by its door handles. They're designed for opening and closing, not for supporting the weight of the entire appliance. Lifting it by the handles is a sure way to break the mounting points.