Passive Interception of Unencrypted Telemetry and Radio Signals

TLDR: Modern radio protocols often lack basic encryption, allowing anyone with a low-cost Software Defined Radio (SDR) setup to intercept sensitive aircraft telemetry and military communications. This research demonstrates how to use dipole antennas and SDR receivers to capture real-time data, including aircraft positioning and unencrypted radio traffic. Pentesters should recognize that air-gapped or proprietary radio systems are frequently vulnerable to passive signal analysis and eavesdropping.

Radio frequency signals are the invisible backbone of global infrastructure, yet they are treated with a level of trust that would be laughed out of any room if applied to web applications. While we obsess over TLS configurations and API authentication, critical telemetry and communication systems continue to transmit data in the clear. The reality is that if you have a line of sight to a transmitter, you have a potential entry point into the data stream.

The Mechanics of Passive Signal Interception

Software Defined Radio (SDR) has moved from a niche hobbyist pursuit to a standard component of an offensive security toolkit. The core of this technique involves using an SDR Blog V4 receiver paired with a dipole antenna to capture raw RF data. Once the signal is digitized, the heavy lifting shifts to software.

Tools like Gpredict allow researchers to track satellite passes and predict when specific signals will be available for capture. When a satellite or aircraft comes into range, the SDR captures the raw frequency data. This data is then processed through tools like SatDump, which automates the demodulation and decoding of the signal into usable formats, such as images or telemetry logs.

The vulnerability here is not a software bug in the traditional sense. It is a fundamental design flaw in legacy and even some modern radio protocols that prioritize availability and simplicity over confidentiality. Because these systems were often designed decades ago, they lack the cryptographic primitives required to prevent unauthorized interception.

From Aircraft Tracking to Military Eavesdropping

During the early stages of the conflict in Ukraine, researchers observed that Russian military communications were frequently transmitted over unencrypted radio channels. By tuning into these frequencies using web-based SDR interfaces, anyone with an internet connection could listen to tactical communications in real-time. This is a stark reminder that security through obscurity is not a strategy.

For a pentester, this highlights a massive blind spot in physical security assessments. If you are auditing a facility that relies on proprietary radio systems for internal logistics, security, or facility management, you should assume those communications are public. A simple SDR setup can reveal:

• Internal security patrol routes and schedules.
• Operational status of critical infrastructure.
• Unencrypted telemetry from IoT devices or industrial control systems.

The impact of this is significant. If an attacker can passively monitor the operational rhythm of a target, they can time their active engagement to coincide with shift changes or periods of low activity. This is the radio equivalent of sniffing unencrypted traffic on a local network, but with a much larger physical radius.

Technical Implementation and Signal Amplification

Capturing these signals effectively often requires more than just a basic antenna. To pull in weaker signals, such as those from high-altitude aircraft or distant satellites, an FM/AM amplifier is often necessary to boost the signal-to-noise ratio before it hits the SDR receiver.

The workflow for a researcher typically looks like this:

# Example of tuning to a specific frequency using rtl_fm
rtl_fm -f 433.92M -s 200k -g 30 - | aplay -r 200k -f S16_LE

This command captures raw audio from a 433MHz signal, which is common for many low-power telemetry devices. Once the signal is captured, the focus shifts to identifying the protocol. Many of these devices use simple ASK or FSK modulation, which can be easily decoded using tools like Universal Radio Hacker.

The Defensive Reality

Defending against passive interception is difficult because the attacker is not interacting with the system. They are merely listening. The only effective defense is the implementation of robust, end-to-end encryption at the application layer. If the data is encrypted before it is handed off to the radio transmitter, the raw signal becomes nothing more than noise to an eavesdropper.

Organizations must stop assuming that their radio-based communications are private. If your system transmits data over the air, treat that data as if it were being broadcast on an open channel. Conduct a signal audit to determine what information is leaking from your facility. If you find sensitive data being transmitted in the clear, your priority must be to either move that traffic to a secure, encrypted medium or implement a layer of encryption that is independent of the underlying radio protocol.

The next time you are on a physical penetration test, look for the antennas. They are not just part of the building's infrastructure; they are potential data leaks waiting to be exploited. Investigating the radio landscape of your target is no longer optional for a comprehensive security assessment. Start by identifying the frequencies in use and see what you can pull out of the air. You might be surprised at how much information is just waiting to be intercepted.