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The problem: outdoor stairs often rock back and forth. Wooden stringers are endgrain and won’t hold a screw and the triangle wants to break off with the grain anyway.
Solution: Stair Saver screws into the continuous part of the stringer and up into the step and riser. eliminating the triangle and locking down the stairs and riser.
These are Solid Steel1/2 inch square bars, if you ever want to replace the stairs entirely just re-use them to make new stringers.

Shipping is 10 dollars for any number

While Stair savers are built for a standard 7 inch rise and 11 inch run, it can adapt to whatever rise/run you have. For any loose step you will need at least two Stair savers one at each end.Stair saver is basically hand-built from solid half inch square steel. Cuts and holes are not perfect and it makes no difference. we barely spray them to prevent surface rust. See this 120 plus year old fence with very little paint left, solid steel will only rust on the surface So paint them more if you want but they're not going anywhere

Use a 1 5/8" deck screw to penetrate 1 inch into the step. Stair saver screws locking down the steps and risers are close to the wood of the stringer so use a long bit holder or two put together. If later you want to build a new staircase just remove the Stair savers and use them to make new stringers. My partner and i thought this was so clever we applied for a patent.