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Introduction:
The modern cellular-connected IoT device—ranging from smart cameras to vehicle trackers—is built on a dangerous assumption: that the internal communication link between its main processor and cellular modem is trusted. New research by Rapid7’s Deral Heiland and Carlota Bindner reveals that this link is often transmitted in plaintext over exposed USB or UART interfaces. With just minutes of physical access, an attacker can tap this unencrypted channel, hijack the modem, and route malicious traffic through the device’s legitimate cellular connection, turning a simple sensor into an undetectable gateway into your corporate network and cloud environments.
Learning Objectives:
- Understand the technical mechanics of CPU-to-modem interchip attacks on cellular IoT devices.
- Learn how attackers leverage AT commands to build SOCKS5 proxies, port scanners, and cloud storage enumerators.
- Acquire practical mitigation commands and hardware-level security checks to defend against physical compromise.
You Should Know:
1. Understanding the Attack Surface: USB/UART Interchip Communication
Cellular IoT devices—such as GPS trackers, remote cameras, and medical equipment—typically connect to the internet via an embedded cellular module. The device’s main processor communicates with this module using either USB (Universal Serial Bus) or UART (Universal Asynchronous Receiver-Transmitter) interfaces. In many cases, both interfaces are physically present on the circuit board, but only one is actively used by the device’s firmware. This leaves the unused interface completely exposed and accessible to an attacker with physical access.
Once an attacker gains physical access to the device (e.g., by stealing it, tampering with it in the field, or intercepting it during shipping), they can connect to the exposed interface using simple hardware tools like a USB-to-TTL serial adapter. The communication between the CPU and modem often transmits sensitive data in plaintext—including APN settings, authentication certificates, and device credentials. By observing this traffic, an attacker can harvest credentials and then send their own AT commands directly to the modem, effectively taking control of the cellular connection.
To assess your own devices for this vulnerability, you can perform a basic reconnaissance check. On Linux, connect to the device’s serial console over UART using a tool like `screen` or minicom. First, identify the serial device:
List all serial devices ls -la /dev/ttyUSB /dev/ttyACM /dev/ttyS Connect to the serial console (replace with your device) sudo screen /dev/ttyUSB0 115200
Once connected, test if the modem responds to standard AT commands:
AT Expected response: OK AT+CGMI Returns modem manufacturer information AT+CGSN Returns the device's IMEI number
On Windows, you can use PuTTY or PowerShell to connect to the serial interface:
List available COM ports
Get-WmiObject Win32_SerialPort | Select-Object DeviceID, Description
Connect via PowerShell to COM3 at 115200 baud
$port = new-Object System.IO.Ports.SerialPort COM3,115200,None,8,one
$port.Open()
$port.WriteLine("AT")
$port.ReadLine()
If the modem responds to `AT` commands, it confirms that the interface is active and unauthenticated—a critical security gap.
- Turning a Modem into a Malicious Proxy: AT Commands in Action
Once an attacker gains command-line access to the cellular modem via USB or UART, they can leverage standard AT commands to turn the device into a fully functional network proxy. Cellular modems typically support a rich set of AT commands that allow raw socket operations, HTTP requests, and TCP tunneling. The researchers developed proof-of-concept tools including a TCP port scanner, an S3 bucket enumerator, a SOCKS5 proxy, and a Metasploit integration module.
To demonstrate how this works in practice, here is a step-by-step guide to establishing a basic TCP tunnel through a compromised modem using AT commands. This example assumes you have already connected to the modem’s serial interface.
First, configure the modem to activate a PDP context (the data connection to the cellular network):
Set APN (example for a generic carrier) AT+CGDCONT=1,"IP","internet" Activate the PDP context AT+CGACT=1,1
Next, open a TCP socket to a remote server:
Open a TCP socket to port 80 at 192.168.1.100 AT+QISOPEN=1,"TCP","192.168.1.100",80 Send an HTTP GET request AT+QISEND=1,20 <blockquote> GET / HTTP/1.1\r\n
For more advanced operations, an attacker can establish a SOCKS5 proxy. Using the Metasploit module developed by Rapid7, the attacker can route all their traffic through the compromised device, making it appear as legitimate device traffic to backend security controls.
To detect unauthorized AT command activity on your network, you can monitor serial traffic using a logic analyzer or a simple Python script that logs all commands sent to the modem:
import serial
import logging
logging.basicConfig(filename='modem_audit.log', level=logging.INFO)
ser = serial.Serial('/dev/ttyUSB0', 115200, timeout=1)
while True:
if ser.in_waiting:
cmd = ser.readline().decode().strip()
logging.info(f"AT command: {cmd}")
Alert on suspicious commands
if cmd.startswith("AT+QIS") or cmd.startswith("ATSG"):
print(f"ALERT: Suspicious command detected: {cmd}")
3. Cloud Pivoting and Trust Exploitation
Once the cellular module is compromised, the attacker inherits all the trust relationships that the legitimate device has established with cloud and backend systems. IoT devices typically authenticate to cloud services using credentials embedded in the device—such as API tokens, X.509 certificates, or device identifiers—and are often granted broad access to data and services. Devices that use private APNs (Access Point Names) are particularly high-risk, as they can provide direct access to internal corporate network infrastructure.
The Rapid7 researchers demonstrated that an attacker can use the compromised modem to enumerate cloud storage resources. For example, if the device has permission to access an S3 bucket, the attacker can list and exfiltrate bucket contents using AT commands to send authenticated HTTP requests directly through the modem.
To test whether your cloud-connected IoT devices are vulnerable to such pivoting, you can perform a simple network segmentation check. On Linux, use `tcpdump` to monitor traffic originating from the device’s cellular interface:
Identify the cellular network interface (usually wwan0 or usb0)
ip link show
Capture all traffic on the cellular interface
sudo tcpdump -i wwan0 -n -c 1000 -w iot_traffic.pcap
Analyze the capture for unexpected outbound connections
sudo tcpdump -r iot_traffic.pcap -n | awk '{print $5}' | cut -d. -f1-4 | sort | uniq -c
On Windows, you can use `netsh` to enable tracing on the cellular interface:
Start a trace on the cellular interface netsh trace start capture=yes report=disabled provider=Microsoft-Windows-Dhcp-Client level=5 Stop the trace after collecting data netsh trace stop Analyze the .etl file using Message Analyzer or convert to pcap
4. Hardware-Level Mitigation: Disabling Unused Interfaces
The most effective defense against this attack vector is to prevent physical access to the internal interfaces altogether. Manufacturers should disable or physically remove any unused USB or UART interfaces before shipping devices. For existing deployments, organizations can implement hardware-level tamper protection.
One practical mitigation is to apply a conformal coating or epoxy potting over critical circuit board areas, including the modem’s debug ports. This makes it significantly more difficult for an attacker to probe or connect to the interface without destroying the device.
For security teams testing their own devices, you can verify whether the UART interface is accessible by attempting to connect to it after applying a temporary physical barrier:
After applying a physical barrier, verify that serial communication is blocked Use a multimeter to check for continuity on UART TX/RX pins Expected result: No continuity (open circuit)
If you cannot physically modify the device, network-level controls become essential. Configure your APN to enforce strict outbound access controls, allowing only known-good destinations and protocols. For example, on a Linux-based cellular gateway, you can use iptables to restrict outbound traffic:
Allow only outbound connections to specific cloud endpoints sudo iptables -A OUTPUT -o wwan0 -d 192.168.1.100 -j ACCEPT sudo iptables -A OUTPUT -o wwan0 -d 10.0.0.0/8 -j ACCEPT sudo iptables -A OUTPUT -o wwan0 -j DROP Log all dropped packets for monitoring sudo iptables -A OUTPUT -o wwan0 -j LOG --log-prefix "BLOCKED: "
5. End-to-End Encryption: Closing the Plaintext Gap
The core of this vulnerability is that data transmitted from the device’s processor to the cellular module often remains unencrypted, even if the cellular network itself encrypts traffic over the air. This means that an attacker tapping the internal interface can see everything—including credentials, API tokens, and sensitive sensor data—before it ever reaches the cellular network.
To close this gap, organizations must implement end-to-end encryption at the application layer, before data ever leaves the device’s main processor. This means using TLS for all outbound connections, even to internal services. For constrained IoT devices, lightweight alternatives like DTLS (Datagram TLS) or MQTT with TLS can be used.
Here is an example of how to enforce TLS 1.2 for all outbound HTTP requests from a Linux-based IoT device using curl:
Enforce TLS 1.2 and verify the server certificate
curl --tlsv1.2 --cacert /etc/ssl/certs/ca-certificates.crt https://api.example.com/data
For applications using Python's requests library
python3 -c "import requests; requests.post('https://api.example.com/data', verify='/etc/ssl/certs/ca-certificates.crt')"
On Windows IoT devices, you can use PowerShell to enforce TLS:
Enforce TLS 1.2 system-wide Make a secure web request Invoke-WebRequest -Uri "https://api.example.com/data" -Method POST -Body $jsonBody
Additionally, you should audit your device firmware to ensure that no plaintext credentials are stored or transmitted. Use `strings` on Linux to scan the firmware image for exposed secrets:
Extract and scan the firmware image for potential credentials binwalk -e firmware.bin strings firmware.bin | grep -E "(api_key|token|password|secret)"
What Undercode Say:
- Physical access is a privilege, not a threat. Organizations must treat cellular IoT devices as privileged assets and implement hardware-level tamper protection as a baseline requirement.
- Encrypt early, encrypt often. Never rely on cellular network encryption alone; always implement end-to-end encryption from the application layer to protect data in transit across internal interfaces.
- Trust but verify. The implicit trust placed in cellular communication paths is a critical blind spot. Continuous monitoring of outbound traffic and strict APN controls are essential to detect and block anomalous behavior.
Prediction:
As the number of cellular-connected IoT devices continues to surge, this class of hardware-level attacks will become increasingly common. In the next 18 months, we expect to see a wave of real-world compromises targeting critical infrastructure sectors—including healthcare, transportation, and utilities—where physical access to devices is often feasible. Regulatory bodies will likely mandate hardware security requirements for cellular IoT devices, including mandatory encryption of interchip communications and tamper-evident design. Organizations that fail to address these risks now will face not only data breaches but also potential liability for enabling lateral movement into connected cloud and backend environments. The era of assuming “cellular equals secure” is over.
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IT/Security Reporter URL:
Reported By: Dmitry Kurbatov – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
