Image: Maruyama et al.

Modern Android smartphones are susceptible to a new type of attack named "Tap 'n Ghost" that can induce fake finger taps to take unwanted actions.

The attack exploits flaws at both the software and hardware level and has been proven to work even against the most recent smartphone models.

It works against most NFC-enabled smartphones with capacitive touchscreens -- which is the most common smartphone touchscreen technology today.

Generating fake screen taps

The Tap 'n Ghost attack -- discovered and documented by three academics from the Waseda University in Tokyo -- works using an attack rig that consists of a 5mm thick copper sheet connected to a DDS signal generator, a high-voltage transformer, a battery pack, NFC readers/writers, and a small computer (laptop, Raspberry Pi).

This rig might look bulky, but the research team says it can be embedded inside regular tables, coffee tables, or any other furniture object on which a victim might place their smartphone.

Image: Maruyama et al.

The attack itself consists of two steps. Once a user has placed their smartphone near the attack rig to be in the smartphone's NFC range (of 4 to 10cm), the NFC readers/writers can get basic info about a device and trigger one of three actions.

It can make the user's smartphone open and access a specific URL (doesn't require any interaction), it can ask the smartphone to pair a rogue Bluetooth device (requires interaction), or it can ask the user to connect to a malicious WiFi network (requires interaction).

This works because, by default, Android devices always look for nearby NFC transmissions, at all times.

At this point, the attack moves in the second phase where the attacker can use the copper plate to induce electrical disturbances into the touchscreen.

Because capacitive touchscreens are a collection of electrodes that exchange small currents between each other during a touch interaction, the extra induced noise can cause ghost taps on the screen, either on a vertical or horizontal axis.

Image: Maruyama et al.

These fake taps can be used to hijack a user's original tap on a "No" button and apply it on the "Yes" one, allowing the smartphone to connect to a rogue WiFi network, or approve a malicious Bluetooth connection.

The Waseda research team says it tested the Ghost 'n Tap attack on seven smartphone models and were successful on five.

Image: Maruyama et al.

The attack doesn't work only on smartphones, but also on any NFC-enabled device with a capacitive touchscreen, such as ATMs, voting machines, display screens, and others.

The research team says it worked with the Japan Computer Emergency Response Team (CERT) to notify the several smartphone manufacturers about this new attack vector.

"We demonstrated the attack to them and confirmed that the attack is applicable to their latest model," researchers said.

Not a universal threat

Fortunately, the Tap 'n Ghost attack isn't something that can be used against any user. First and foremost, the range of the attack is limited and requires that the user place their device(s) near a disguised attack rig.

Second, because each smartphone model uses different capacitive touchscreen technologies, special signals at different frequencies are needed per phone model. This means that the attacker needs to know a victim's smartphone model beforehand and configure the attack rig accordingly.

Furthermore, the Waseda team says the attack can be easily mitigated at both the software and hardware level. For example, the Android OS could be modified to introduce a popup that asks the user for permission before a device initiates any NFC operation. Second, signal noise protection can be added to capacitive touchscreen technologies.

More on this research can be found in a whitepaper named "Tap 'n Ghost: A Compilation of Novel Attack Techniques against Smartphone Touchscreens."

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