Unless otherwise noted, an object has immunity to the following: necrotic damage poison damage psychic damage any attack that targets WillThe DM might decide that a particular object is immune to other damage types. For instance, a magic altar might be immune to fire.Objects don’t have any universal resistances or vulnerabilities, but the DM might rule that some kinds of damage are particularly effective against a certain object—the object has vulnerability to that damage. For instance, a gauzy curtain or a pile of dry papers might have vulnerable 5 fire because any spark is likely to destroy it. Similarly, the DM might occasionally decide that an unusual material resists a type of damage, typically having resist 5 or 10 to that damage.
Published in Rules Compendium, page(s) 177.