Cardboard boxes will deteriorate due to heat and water and animals and bugs will snack on them in the meantime.
Boxes in attic.
Whether a birds or moths in the attic a raccoon in the garage or mice in the basement critters aren t concerned with taking care of your belongings as a general rule of thumb nothing in your attic.
As a very general guide if the joists are only 2x4s don t plan on storing much in the attic other than very lightweight items.
Heat can damage things such as photographs and wood.
Think empty boxes and suitcases.
Boxes also break down and disintegrate which will attract pests even more.
Use the attic to store anything that is impervious to heat.
A few more universal.
If they are 2x6 you can probably get away with some boxes filled with relatively light stuff.
Joists that are 2x8 or larger can likely support more weight.
Pests are attracted to the cardboard and to the glue used in construction of the box says santoro.