Dissecting VoLTE: Exploiting Free Data Channels and Security Problems

Presented at 32C3 (2015), Dec. 28, 2015, 9:45 p.m. (60 minutes)

Newly adopted VoLTE requires changes in all associated parties, such as 3GPP standard, device, operating system, and cellular core networks. Therefore, it is not too surprising that it has security problems. However, it turns out that it has way too many problems. In this talk, we introduce how you can freely send data in the cellular network, and how an attacker can perform caller spoofing and denial of service attacks on calls to disable the target’s calling. Furthermore, we explain how small implementation glitch on VoLTE may lead to break the whole cellular network down. Voice-over-LTE (VoLTE) is a newly adopted voice technology in the LTE network, whose functionality is similar to VoIP. Even though VoLTE works similar to VoIP, implementing it on the cellular network is not an easy problem because it needs many changes at each component of LTE. If these changes are not securely considered, this may lead to several security problems. In the legacy 3G network, as data and voice are separate, the accounting policies are also different: data is charged based on byte usage, and voice, on time usage. However, in VoLTE, even though voice is delivered as a packet, it is still charged by time usage. Therefore, this strange accounting policy might open free data channels. Another point is that voice signaling for VoLTE is not handled as in the legacy 3G network. Basically, a phone has two processors: an application processor (AP) which runs mobile OSes such as Android and a communication processor (CP) which manages digital signal processing and radio access. In 3G, voice signaling is handled in CP which makes an attacker hard to manipulate it. However, in VoLTE, because voice signaling is handled in AP, an attacker can easily analyze or modify the call flow. Furthermore, this new change can cause problems to the mobile OS. To scrutinize these two points, we analyzed 5 operators, two in the U.S and three in South Korea. As a result, we found four free data channels. For free data channels, an attacker can inject data in the call signaling procedure or voice data transmission. Additionally, the attacker can freely send data to the Internet or to another phone in the cellular network through the VoLTE interface. Furthermore, we discovered five security problems which include no encryption of voice packets, no authentication of call signaling, no call session management, IMS bypassing, and permission model mismatch in Android. We responsibly disclosed all the vulnerabilities to US/KR CERTs and Google in May. We suggest mitigations for each vulnerability, and further propose possible attack vectors that researchers can study on.

Presenters:

  • Hongil Kim
    Hongil Kim is a Ph.D. candidate in System Security Laboratory from Korea Advanced Institute of Science and Technology. He received his M.S. and B.S. in electrical engineering from Korea Advanced Institute of Science and Technology. He has broad interests in system security. Especially, He is mainly working on cellular network system and mobile device security.
  • Dongkwan
    Dongkwan Kim is a student in a master's degree in the Department of Electrical Engineering at KAIST. He is interested in various fields of security: cellular network, embedded devices, sensing and actuation systems. He is now working on designing secure architecture of cellular network, and building a spoofing detection and prevention framework for sensing and actuation systems. He has been working on several embedded devices such as automobiles, smart TVs, network routers, and femtocells. He participated in several hacking CTFs (DEFCON, Codegate, Whitehat Contest, HDCON) as a member of KAIST GoN. He holds a BS from KAIST (2014) in CS.

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