Miso is made from purely vegan products like cooked soybeans, koji and salt. Koji is usually made using rice. It can also be made using barley and wheat. The misos made using barley and wheat are respectively called barley and wheat miso. Koji can also be made from soybean (soybean miso). The base mixture for miso is fermented for a few weeks to months as per the desired taste and consistency of its final form. You get sweeter miso when the miso has high rice/barley/wheat to soybean ratio. It will also be lighter in color.
 
Miso has been popular as a plant-based protein source for several years in the Japanese cuisine. Although miso is purely vegan, restaurants may add non-vegan ingredients to their miso dishes like soups. Before you order miso dishes from a restaurant, make sure that they do not use non-vegetarian ingredients like fish or meat-based items.