Practical Attacks Against Privacy and Availability in 4G/LTE Mobile Communication Systems
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Practical Attacks Against Privacy and Availability in 4G/LTE Mobile Communication Systems Altaf Shaik∗, Ravishankar Borgaonkary, N. Asokanz, Valtteri Niemix and Jean-Pierre Seifert∗ ∗Technische Universitat¨ Berlin and Telekom Innovation Laboratories Email: (altaf329, jpseifert) @sec.t-labs.tu-berlin.de yAalto University Email: ravishankar.borgaonkar@aalto.fi zAalto University and University of Helsinki Email: [email protected] xUniversity of Helsinki Email: valtteri.niemi@helsinki.fi Abstract—Mobile communication systems are now an essential communication is an important cornerstone in the lives of the part of life throughout the world. Fourth generation “Long vast majority of people and societies on this planet. The latest Term Evolution” (LTE) mobile communication networks are generation in this evolution, the fourth generation “Long Term being deployed. The LTE suite of specifications is considered Evolution” (4G/LTE) systems are being deployed widely. By to be significantly better than its predecessors not only in terms the end of 2015 the worldwide LTE subscriber base is expected of functionality but also with respect to security and privacy to be around 1.37 billion [1]. for subscribers. We carefully analyzed LTE access network protocol specifications and uncovered several vulnerabilities. Early 2G systems were known to have several vulnera- Using commercial LTE mobile devices in real LTE networks, bilities. For example, lack of mutual authentication between we demonstrate inexpensive, and practical attacks exploiting these vulnerabilities. Our first class of attacks consists of three mobile users and the network implied that it was possible for different ways of making an LTE device leak its location: In an attacker to set up fake base stations and convince legitimate our experiments, a semi-passive attacker can locate an LTE mobile devices to connect to it. In order to minimize exposure device within a 2 km2 area in a city whereas an active attacker of user identifiers (known as International Mobile Subscriber can precisely locate an LTE device using GPS co-ordinates Identifier or IMSI) in over-the-air signaling messages, 2G or trilateration via cell-tower signal strength information. Our systems introduced the use of temporary mobile subscriber second class of attacks can persistently deny some or all services identifiers. However, in the absence of mutual authentication, to a target LTE device. To the best of our knowledge, our work fake base stations were used as “IMSI catchers” to harvest constitutes the first publicly reported practical attacks against LTE IMSIs and to track movements of users. access network protocols. We present several countermeasures to resist our specific The evolution of these mobile communication systems attacks. We also discuss possible trade-off considerations that may specified by 3GPP (Third Generation Partnership Project) explain why these vulnerabilities exist. We argue that justification have not only incorporated improvements in functionality but for these trade-offs may no longer valid. We recommend that also strengthened security. 3G specifications introduced mutual safety margins introduced into future specifications to address authentication and the use of stronger and well-analyzed cryp- such trade-offs should incorporate greater agility to accommodate tographic algorithms. LTE specifications further strengthened subsequent changes in the trade-off equilibrium. signaling protocols by requiring authentication and encryption (referred to as “ciphering” in 3GPP terminology) in more arXiv:1510.07563v3 [cs.CR] 7 Aug 2017 I. INTRODUCTION situations than was previously required. Consequently, there is a general belief that LTE specifications provide strong privacy During the past two decades, mobile devices such as and availability guarantees to mobile users. Previously known smartphones have become ubiquitous. The reach of mobile attacks, such as the ability to track user movement were communication systems, starting from the second generation thought to be difficult in LTE. Global System for Mobile Communications (2G/GSM) and the third generation Universal Mobile Telecommunication Systems In this paper, we demonstrate the first practical attacks (3G/UMTS), has extended to every corner in the world. Mobile against LTE access network protocols. Our attacks are based on vulnerabilities we discovered during a careful analysis of LTE access network protocol specifications. They fall into two Permission to freely reproduce all or part of this paper for noncommercial classes: location leaks and denial of service. In the first class, purposes is granted provided that copies bear this notice and the full citation on the first page. Reproduction for commercial purposes is strictly prohibited we describe three different attacks that can force an LTE without the prior written consent of the Internet Society, the first-named author device (User Equipment or UE in 3GPP terminology) into (for reproduction of an entire paper only), and the author’s employer if the revealing its location. The first two allow a passive or semi- paper was prepared within the scope of employment. passive attacker to localize the target user within about a 2 NDSS ’16, 21-24 February 2016, San Diego, CA, USA 2 Copyright 2016 Internet Society, ISBN 1-891562-41-X km area in an urban setting which is a much finer granularity http://dx.doi.org/10.14722/ndss.2016.23236 than previously reported location leak attacks [2] against 2G devices, while still using similar techniques. Notably, we show II. OVERVIEW OF LTE ARCHITECTURE how popular social network messaging applications (e.g., Face- book messenger [3] and WhatsApp [4]) can be used in such We briefly describe LTE infrastructure as well as security attacks. Our third attack allows an active attacker exploiting and paging mechanisms to assist readers in understanding the vulnerabilities in the specification and implementation of LTE vulnerabilities and attacks we present in this paper. Radio Resource Control (RRC) protocol [5] to accurately pinpoint the target user via GPS co-ordinates or trilateration A. LTE infrastructure using base station signal strengths as observed by that UE. We We consider a simplified LTE architecture involving com- believe that all LTE devices in the market are vulnerable to ponents required to set up access network protocols between this attack. a base station and mobile devices. We hide other details of In the second class, we describe three further attacks where the architecture which are not relevant from the point of view an active attacker can cause persistent denial of service against of understanding our attacks. Figure 1 depicts this simplified a target UE. In the first, the target UE will be forced into using architecture which contains three main components: User 2G or 3G networks rather than LTE networks, which can then Equipment (UE), Evolved Universal Terrestrial Radio Access make it possible to mount 2G/3G-specific attacks against that Network (E-UTRAN), and Evolved Packet Core (EPC). All UE. In the second, the target UE will be denied access to all three components are collectively referred to as Evolved Packet networks. In the last attack, the attacker can selectively limit a System (EPS) according to 3GPP terminology. In the interest UE only to some types of services (e.g., no voice calls). The of simplicity, throughout this paper we refer to the whole attacks are persistent and silent: devices require explicit user system as LTE. The three components are described below action (such as rebooting the device) to recover. (A list of common acronyms related to LTE appear in the full version of this paper [6]). We have implemented all our attacks (except one) and confirmed their effectiveness using commercial LTE devices from several vendors and real LTE networks of several carriers. The equipment needed for the attacks is inexpensive and readily available. We reported our attacks to the manufacturers and carriers concerned as well as to the standardization body (3GPP). Remedial actions are under way while writing. Specification of a large system like LTE is a complex endeavor involving many trade-offs among conflicting require- ments. Rather than merely report on LTE vulnerabilities and attacks, we also discuss possible considerations that may have led to the vulnerabilities in the first place. Based on this we suggest some general guidelines for future standardization as Fig. 1. LTE system architecture well as specific fixes for our attacks. User Equipment: UE refers to the actual communication • Fine-grained location leaks: New passive and active device which can be, for example, a smartphone . A UE techniques to link users’ real identities to LTE tem- contains a USIM (Universal Subscriber Identity Module)[7], porary identities assigned to them and to track user which represents the IMSI and stores the corresponding au- locations and movements to much higher levels thentication credentials [8]. This IMSI is used to identify an of granularity than was previously thought possible. LTE user (generally referred to as “subscriber” in 3GPP ter- (Section V) minology) uniquely. The USIM participates in LTE subscriber • Denial-of-Service (DoS) Attacks: New active DoS authentication protocol and generates cryptographic keys that attacks that can silently and persistently down- form the basis for the key hierarchy subsequently used to grade LTE devices by preventing their access to protect signaling and user data communication between the LTE networks (limiting them to less