Apple is working to fix an iPhone vulnerability that could allow an attacker to remotely install and run unsigned software code with root access to the phone.

The attack in question exploits a weakness in the way iPhones handle text messages received via SMS (Short Message Service).

The SMS vulnerability allows an attacker to run software code on the phone that is sent by SMS over a mobile operator's network. The malicious code could include commands to monitor the location of the phone using GPS, turn on the phone's microphone to eavesdrop on conversations, or make the phone join a distributed denial of service attack or a botnet, security researcher Charlie Miller said

Apple is working to patch the vulnerability and expects to have a fix ready later this month.

Despite the SMS vulnerability, the stripped-down version of MacOS X used in the iPhone makes it more secure than computers running the full-blown operating system, Miller said.

For starters, the stripped-down version of the OS presents fewer options for attackers, removing applications and features such as support for Adobe Flash and Java, which they might otherwise be able to exploit for vulnerabilities. In addition, the iPhone includes hardware protection for data stored in memory and the phone is designed to only run software code that has been digitally signed by Apple.

The iPhone also requires applications to run in a sandbox, a security feature that isolates them from other applications and limits their access to the phone's capabilities. But SMS offers a way for attackers to get greater access to the phone's capabilities, Miller said.

"SMS is a great vector to attack the iPhone," he said.

Most often used to send brief text messages between cell phones, SMS can also send binary code to an iPhone, which then processes the code without any user interaction. Each SMS message is limited to 140 bytes, but longer sequences can be sent to the phone as multiple messages that are automatically reassembled.

This feature allows larger programs to be delivered to a phone, Miller said.

In addition, vulnerabilities found in the iPhone's SMS function give an attacker root access to the handset, Miller said. That's not the case for the iPhone's other applications, such as its browser, where vulnerabilities only give an attacker access to the application's sandbox.

"The iPhone is more secure than OS X, but SMS could be a critical vulnerability," Miller said.