How to slay a cellphone with a single text

Feature phone fuzzer

There's virtually no software for performing vulnerability analysis on feature phones, so the researchers created a crude fuzzer of sorts for monitoring the effect various SMS messages had on each handset model. They ordered dozens of used phones on eBay and connected them to their own 2G network that cost them about €3,000. To avoid running afoul of any laws, they isolated the network inside a faraday cage.

The researchers then subjected the phones to no fewer than 120,000 different variations of SMS messages and logged each response, which usually was limited to an acknowledgment, an error message or no response at all.

“If it doesn't arrive, we knew that something really went wrong, because there should be either an acknowledgment or an error.” Golde said.

They quickly built up a war chest of SMS texts that caused problems and did further analysis to isolate the ones that caused the handsets to crash.

Many of the SMS death messages contain special binary characters, while others have header information that is longer than specifications allow. In still other cases, the malicious messages had faulty information indicating they had been broken up into 10 pieces when in fact there were only seven, five, or some different number of pieces.

The worst of the malicious texts had the effect of creating disruptions that were hard or impossible to recover from. In some cases, the only way to break out of a repeated crash cycle was to remove the phone's SIM card and put it in a handset that wasn't vulnerable to the attack. In the most extreme cases, the attacked phones could no longer be reflashed and had to be put out to pasture.

None of the smartphones the researchers tested were susceptible to the messages. It turns out the common weakness among the less advanced phones was their simplicity.

“Feature phones normally run on just one chip, which runs all the radio communications and the built-in applications,” Mulliner explained. “So if we get just one small part of the phone to misbehave or crash or do something strange, probably the whole phone is going to be crashing, rebooting or doing weird things. Just by finding small bugs, you probably have a large impact.” ®