It was found that the Apache commons-collections library permitted code execution when deserializing objects involving a specially constructed chain of classes. A remote attacker could use this flaw to execute arbitrary code with the permissions of the application using the commons-collections library (CVE-2015-7501).
With this update, deserialization of certain classes in the commons-collections library is no longer allowed. Applications that require those classes to be deserialized can use the system property "org.apache.commons.collections.enableUnsafeSerialization" to re-enable their deserialization.