Charlie Miller - iOS Hackers Handbook

Chapter 1

iOS Security Basics

If you're like us, every time you get your hands on a new device you wonder how secure it is. The iPhone was no exception. Here was a device that had jumped across the threshold from being a phone that might have a small web browser to a device that was more like your computer than your old phone. Surely there were going to be similar security issues in these (and future) devices to the issues that were already occurring on desktop computers. What precautions and security mechanisms had Apple built into these devices to prevent compromises? Here was a chance to start a whole new branch of computing, from the beginning. How important was security going to be for these emerging smart devices?

This chapter answers these questions for iOS devices. It begins by looking at the hardware seen for various iOS devices and then quickly moves into describing the security architecture of iOS 5. This includes highlighting the many layers of defense built into current devices to make attacks by malware and exploitation by attackers difficult. It then illustrates how these defenses have held up (or not) in the real world by showing some attacks that have occurred against iOS devices. This section on iOS attacks takes a historical approach starting from attacks against the very first iPhone and ending with attacks against iOS 5 devices. Along the way you will notice how much the security of iOS devices has improved. Whereas the very first versions of iOS had almost no security, the most recent versions of iOS 5 have quite a strong and robust security posture.

As iOS evolved during the years, so did the hardware inside the various Apple devices. When smartphones and tablets became widespread among users, people started to feel the need to have powerful devices at their disposal. In a way, the expectation was to have a computer in their pocket.

The first step in that direction was the creation of the iPad. The original iPad had an ARM Cortex-A8 CPU, which, compared to the CPU present on the original iPhone, was roughly twice as fast.

Another big step forward was the iPad 2 and the iPhone 4S. They both feature ARM Cortex-A9 dual-core processors, which are 20 percent faster compared to the A8 in terms of CPU operations. Even more astonishing is the fact that the GPU of the A9 is nine times faster compared to the A8.

From a security perspective, the biggest hardware differences came with the iPhone 3GS and the iPad 2. The iPhone 3GS was the first one to support the Thumb2 instruction set. The new instruction set changed the way ROP payloads needed to be created. Most code sequences present in previous versions of the device were suddenly different on the 3GS.

The iPad 2, on the other hand, introduced dual-core processors, which in turn enabled the iOS allocator to work in full swing. This has had a huge impact on exploit development because exploits become much less reliable in a multi-processor environment.

Another relevant hardware component from a security point of view is the baseband. In fact, in most countries the iDevices are bound to a carrier (locked).

To unlock iPhones, most exploits use bugs in the baseband component inside the phone. Both devices have historically used Infineon baseband firmwares. Only recently with the iPhone 4 CDMA and iPhone 4S has Apple moved to Qualcomm.

A number of exploits have been published on the various Infineon firmwares, but none yet on the Qualcomm ones.

One of the things that makes iOS devices so great is the number of applications, or apps, that are available to run on them. These apps can be found in Apple's App Store. There have been more than 18 billion downloads from the App Store, and at least 500,000 different apps are available (see ).

A users' view of the App Store.

Apps are developed using Xcode and the iOS SDK on Mac OS X computers. The built apps can run in an iOS simulator or can be put on real devices for testing. The apps are then sent to Apple for review. If approved, they are signed by Apple's private key and pushed out to the App Store for download. Apps must be signed by a trusted party, such as Apple, or they will not run on the devices because of the Mandatory Code-Signing requirement in iOS (see Chapter 4 for more details). Enterprises can also distribute apps to their employees using a similar system, but the employees' phones must be configured to accept apps that are signed by the enterprise as well as by Apple.

Of course, once you could download new apps to iOS devices, it opened up the possibility for malware. Apple has tried to reduce this risk with code signing and the App Store review process. Additionally, App Store apps run in a sandbox at a low privilege level to reduce the damage they can cause. You see more on this in a bit.

This book is about iOS security how it works and how to break it. To fully understand the decisions made by Apple in trying to secure its devices, it is first necessary to think about the different types of threats that the device might face.

At a high level, iOS devices face many of the same types of attacks that any desktop computer faces. These types of attacks can be split into two broad categories: malware and exploits. Malware has been around for decades on personal computers and is starting to become a menace for mobile devices as well. In general, malware is any software that does something bad when it is installed and run on a device. This malware might be bundled with software the user wants, or it might disguise itself as something the user wants. In either case, the user downloads and installs the malware and when the malware is executed, it performs its malicious actions. These actions might include sending e-mails, allowing remote access to an attacker, installing a keylogger, and so on. All general-purpose computing devices are susceptible at some level to malware. Computers are designed to run software; they do what they are told. If the user tells it to run something that turns out to be malicious, the computing device will happily comply. There is no real vulnerability with the computer; it is just not in a position to know which programs it should run and which it should not. The typical way to protect devices from malware is with antivirus (AV) software. It is the AV's job to determine which software is safe to run and which is not safe to run.

On the other hand, exploits take advantage of some underlying defect of the software on the device to run its code. A user might be innocently surfing a web page, reading an e-mail, or doing absolutely nothing at all, when all of a sudden some malicious code (perhaps in the form of a web page, e-mail, or text message) takes advantage of a vulnerability to run code on the device. Such attacks are sometimes called drive-by-downloads because, unlike the malware example, the user is mostly an innocent victim and wasn't trying to install any code, but just trying to use his or her device! The exploit might run some code inside the compromised process, or it might download some software, install it, and run it. The victim might have no idea that anything out of the ordinary has happened.

Exploitation such as this requires two ingredients. The first is a flaw or vulnerability in the software on the device. The second is a way to leverage this vulnerability to get attacker-controlled code to run on the device. Because of this two-step process, you have two main ways to protect against this kind of attack. The first involves making it harder to find vulnerabilities. This might mean exposing less code to the attacker (reducing the attack surface) or cleaning up and removing as many flaws as possible in the code. The problem with this approach is that some code must always be exposed to the attacker or the device cannot interact with the outside world. Furthermore, it is very difficult to find all (or even most) of the vulnerabilities lurking deep in a code base. If it were easy, there would be no book like this one or any jailbreaks, for that matter!