Here's a Groovy script to reset all the passwords in our database to "". We used this when we went from Magnolia's native authentication to using LDAP.
import info.magnolia.jcr.util.NodeUtil import info.magnolia.jcr.predicate.NodeTypePredicate import info.magnolia.jcr.util.NodeTypes session = ctx.getJCRSession("users") users = NodeUtil.collectAllChildren(session.rootNode, new NodeTypePredicate(NodeTypes.User.NAME)) int count = 0 users.each() { if (it.name != "anonymous" && it.name != "superuser" ) { it.setProperty("pswd", "") it.save() count++ } } println("Reset password for " + count + " users.") println("Passwords for user \"anonymous\" and \"superuser\" remain unchanged.")