If the KDC server hostname used in krb5.conf is defined as an IPv6 address, then the com.ibm.security.auth.module.Krb5LoginModule fails because it wrongly parses the value. It uses only the part before the first colon as the hostname. For instance krb5.conf contains: [realms] JBOSS.ORG = { kdc = [2620:52:0:2804:5425:b1c4:c62d:82f4]:6088 } and the Krb5LoginModule tries to connect to a host "[2620". Exception stack trace: javax.security.auth.login.FailedLoginException: Login error: java.net.UnknownHostException: [2620 at com.ibm.security.jgss.i18n.I18NException.throwFailedLoginException(I18NException.java:29) at com.ibm.security.auth.module.Krb5LoginModule.j(Krb5LoginModule.java:128) ...