Although it doesn't necessarily signal a problem, squeaky stairs can be very annoying. It's easy enough to ignore, but wouldn't you rather fix it for good? Luckily, the fix is fairly simple for any home DIY'er.
Not able to access the underside of the stairs? You will have to insert screws in the top side of the tread into the riser. Start by drilling a pilot hole for several screws. It is best to drill these holes at an angle. Ensure that this hole is smaller than the screw so the threads have materials to dig into.
Next drill a small counterbore over the top of each pilot hole so that the head of the screw can be buried below the surface of the wood. Cover up the screw head by applying wood plastic or putty over the top. Try to stain the putty the same color as the rest of the wood. It is unlikely that it will blend in perfectly, but it will be less noticeable.