Undertow allows you to share sessions between wars in an ear, if it is explicitly configured to do so. Note that if you use this feature your applications may not be portable, as this is not a standard servlet feature.
In order to enable this you must include a shared-session-config element in the jboss-all.xml file in the META-INF directory of the ear:
This element is used to configure the shared session manager that will be used by all wars in the ear. For full details of all the options provided by this file please see the schema at https://github.com/wildfly/wildfly/blob/master/undertow/src/main/resources/schema/shared-session-config_1_0.xsd, however in general it mimics the options that are available in jboss-web.xml for configuring the session.