You need to assign some height to make the overflow: auto; property work. For testing purpose, add height: 100px; and check. and also it will be better if you give overflow-y:auto; instead of overflow: auto;, because this makes the element to scroll only vertical but not horizontal.

If you don't know the height of the container and you want to show vertical scrollbar when the container reaches a fixed height say 100px, use max-height instead of height property.