What your phone won’t tell you

Uncovering fake base stations on iOS devices

Lukas Arnold

Playlists: '37c3' videos starting here / audio

Your phone’s internal communication contains precious data. It can be analyzed to detect fake base stations used in cellular attacks. For that, we reverse-engineered a proprietary communication channel between the phone’s OS and modem.

Connecting to cellular networks around the world is a highly complex task. iPhones contain a baseband chip (also referred to as a modem) for that purpose. It communicates via a high-level interface with the smartphone’s application processor running iOS. So far, Apple hasn’t been able to build such basebands in-house. Instead, starting from the iPhone 12, they exclusively rely on Qualcomm basebands.

Qualcomm’s basebands use a proprietary protocol for external communication, the Qualcomm MSM Interface. We reverse-engineered its iOS implementation and built a framework to extract the protocol’s packet structures from iOS firmware. Our iOS Wireshark dissector uses these packet structures and enables us to monitor the flow of packets between the baseband and iOS. This allows us to gain new insights into the iPhone’s wireless communication infrastructure, including its satellite connectivity. Our tooling also provides a novel way to directly interact with the baseband chip in jailbroken iPhones, bypassing iOS and unlocking hidden capabilities of the baseband.

Fake or Rouge base stations can be set up by individuals using readily available software-defined radios. Adversaries can utilize them to capture IMSIs of nearby smartphones, track their location, or exploit vulnerable basebands. iPhone users usually don’t notice such attacks, and there are (almost) no protection mechanisms implemented in iOS.

During our research, we discovered Apple’s internal cell location database, which is intended for determining approximate positions. Our CellGuard iOS app combines this database with the QMI analysis framework to monitor various parameters of connected cells, verify their authenticity, and alert users in case there’s suspicious activity. The app even works on non-jailbroken iPhones. We evaluated the app in a lab environment with SDRs and real-world tests since February 2023 and are steadily improving it for a release next year.


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