For the most part, traditional sourdough bread is made with just 3 simple ingredients; flour, salt and water. So if the bread in question is the traditional simple sourdough bread, then it it is definitely vegan friendly.
Even though the bread doesn’t contain any added ingredients, if it has been baked in a loaf tin, you should check what the loaf tin has been greased with.
Some Sourdough Breads are Sweetened with Non-Vegan IngredientsDifferent sweeteners can be used, but the most common non vegan one to look out for is honey.
Sourdough starter is maintained on the most part by using just flour and water, but some starters are artificially boosted with other ingredients such as pineapple juice, honey, milk or other ingredients.
If your vegan diet consists of whole plant based foods only, there are some additional factors to take into considration. White sourdough bread is NOT considered a whole food