Mobile Network Operators (MNOs) are considering 3rd Generation Partnership Project (3GPP) Systems (including 5G system) to interwork with Wireless Local Area Network (WLAN; also referred as WiFi) as a cost-effective way to expand the wireless user data traffic capacity further. Achieving adequate level of security from WLAN is one of the main challenges to be addressed, from the 3GPP system perspective, to have effective interworking with the WLAN. Preventing User Equipment (UE) tracking and protecting subscriber's privacy is considered to be crucial requirement in the 3GPP systems, where as in WLAN it is never been considered as an important issue. Hence, combination of WLAN permanent identity and exposure of the 3GPP system permanent identity in the same context in the interworking scenario is identified to be a serious threat for user's privacy and for launching subscriber specific targeted attacks in the 3GPP network. A network harmonized security mechanism is needed to perform authorized tracking by the MNO and at the same time, it should prevent exposure of permanent identities to mitigate unauthorized tracking by the eavesdroppers. This paper details the user traceability threat identified in Long-Term Evolution (LTE)-WLAN Interworking (LWI) and proposes novel ideas to mitigate the identified threat by use of 3GPP network Locally administered Randomized Address (LRA) for the UE's WLAN MAC address instead of universally administrated MAC address. The LRA is generated using the 3GPP network assigned pseudonyms, so that the user's privacy (confidentiality of user and device identity) in the interworking scenario is as secure and efficient as the 3GPP systems. Among the detailed novel ideas, the LRA generated using the core network assigned pseudonyms addresses the user's privacy effectively and has less than 1% collision probability. © 2018 IEEE.