Android应用安全：构建防黑客攻击的安全应用

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"深入理解Android应用安全" 本书"Android Apps Security"由Sheran A. Gunasekera撰写，旨在帮助开发者创建安全、免受黑客攻击和安全漏洞的应用程序。书中详细探讨了Android系统的架构、信息基础、安全架构以及与网络安全相关的各种概念。在第一章"Android架构"中，作者会介绍Android操作系统的基础，包括其分层结构，如Linux内核、硬件抽象层、系统库、应用程序框架和应用程序本身。这些内容对于理解Android应用的安全性至关重要，因为每个层次都可能成为潜在的攻击面。第二章"信息：应用的基础"讨论了信息在应用程序中的重要性，以及如何保护敏感数据不被非法获取或滥用。这包括对数据加密、用户隐私和数据完整性策略的深入分析。第三章"Android安全架构"深入到Android系统的核心安全机制，如权限模型、签名系统和沙箱环境。读者将了解到如何设计应用程序以充分利用这些机制，防止恶意软件和未经授权的访问。第四章和第八章"概念在行动"部分通过实际示例展示了安全原则在开发过程中的应用，包括如何处理输入验证、数据传输安全和应用更新策略。第五章"数据存储和加密"探讨了在Android设备上安全存储数据的方法，包括使用SQLite数据库、文件系统和加密技术来保护用户数据不被窃取。第六章"与Web应用通信"关注的是应用与服务器之间的通信安全，包括HTTP和HTTPS协议、OAuth认证以及防止中间人攻击的策略。第七章"企业安全"针对企业级应用，讨论了如何在企业环境中实施安全策略，包括设备管理、应用策略和远程擦除功能。第九章"发布和销售你的应用"涵盖了应用上线的安全考虑，如应用商店的安全政策、审核流程和持续监控，以防止恶意修改和欺诈行为。第十章"恶意软件和间谍软件"专注于Android平台上的威胁，提供了识别和对抗恶意软件和间谍软件的策略和技术。附录A包含了Android权限常量的详细列表，这对于理解和管理应用权限至关重要。这本书是开发者和安全专家的宝贵资源，它提供了全面的指导，帮助他们在Android平台上构建更加安全的应用程序。

Chapter 2

Information: The Foundation

of an App

The basis of all meaningful applications is information, and we design and build applications

to exchange, create, or store it. Mobile applications are no different. In today’s well-connected

mobile landscape, information exchange is the name of the game. To illustrate this point,

imagine an Android phone without mobile network or WiFi coverage. While there would still be

uses for such a phone, you would have lost access to some of the more important applications

on your device. For example, e-mail, instant messaging, web browsing, and any other

application that require the Internet would now be nonfunctional.

In later chapters, we will focus our efforts on examining information in transit and how to secure

it. In this chapter, we will focus mostly on what happens to information that is stored.

Securing Your Application from Attacks

When created or received, data needs to be stored somewhere. How this information is stored

will ultimately reﬂect on how secure your application really is. Releasing your application to the

public should be approached with the same caution and paranoia as launching a website on the

Internet. You should assume that your application will be either directly or indirectly attacked at

some time and that the only thing standing between your end user’s privacy and data protection

is your application.

Indirect Attacks

As dramatic as that last sentence sounds, it is not without basis. Before we go further, let’s take

a look at whether my fear mongering is justiﬁed. In the latter part of 2010 and early 2011, two

vulnerabilities were discovered in Android versions 2.2 and 2.3, respectively. The vulnerability is

essentially the same one, in which an attacker can copy any ﬁle that is stored on the device’s SD

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CHAPTER 2: Information: The Foundation of an App14

Card without permission or even without a visible cue that this is happening. The vulnerability

works as shown in Figure 2-1.

The following are the most noteworthy points:

1. A user visits a malicious website hosting a ﬁle, such as evil.html.

2. Due to one part of the vulnerability, the evil.html ﬁle is downloaded and

saved to the device SD Card without prompting the user.

3. Due to another part of the vulnerability, the saved ﬁle can be made to

execute JavaScript code as soon as it is saved. Once again, there is no

prompt to the end user.

4. Due to the ﬁnal part of this vulnerability, the executed JavaScript from

the preceding point, because it is running under the “local” context of the

device, will have full access to upload ﬁles stored on the SD Card to a

website of the attacker’s choosing.

For the sake of argument, assume that your application writes all saved information to the SD

Card for storage under its own directory. Because of the vulnerability just discussed, the data

used by your application is at risk of being stolen. Any Android device that runs your application

and the vulnerable ﬁrmware versions poses a risk of data theft to its end user. This is an example

of an indirect attack on your application.

How vulnerable your application is to an indirect attack depends largely on how much effort you

put into architecting and considering security aspects before you begin writing a single line of

code. You may ask the question, “I’m just a small app developer planning to sell my app for a

low price online, so do I really need to waste time doing so much planning beforehand?” And I

would answer you with a resounding, “Yes!” Whether you are part of a team of thirty developers

or an individual working from home, a well-architected application is something you should

always strive to create. I hope that this is what you will learn from this book.

Malicious

Page

evil.html

auto saved

to SDCard.

.js

javascript

executed.

Full access

to SDCard.

prompt

Figure 2-1. Data theft vulnerabilities

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CHAPTER 2: Information: The Foundation of an App

Direct Attacks

Direct attacks are signiﬁcantly different and can take many different forms. A direct attack can

be classiﬁed as one that is targeted directly at your application. Thus, the attacker is looking to

leverage weaknesses in your application design to either collect sensitive information on your

application’s users or to attack the server that your application talks to. Take, for instance, a

mobile-banking application. An attacker may go after the mobile applications belonging to a

speciﬁc bank. If the application design is weak—for example, if that sensitive user data is stored

in clear text, or the communication between application and server is not secured by SSL—then

an attacker can craft special attacks that only target these weaknesses. This is a direct attack on

a speciﬁc application. I will cover direct attacks in more detail in Chapter 9 of this book.

Project 1:“Proxim” and Data Storage

Let’s get started with a simple example called Proxim. I’ve been contracted to write an

application that can send an SMS to speciﬁc, deﬁned contacts when a user is within certain

proximity to a set of GPS coordinates. For instance, with this application, a user can add his

wife as a contact and have the application SMS her every time he is within three miles of his

workplace and house. This way, she knows when he is close to home and the ofﬁce.

You can download and examine the entire source code for the Proxim application from the

Source Code/Download area of the Apress website (www.apress.com). For the sake of clarity, let’s

take a look at the most important areas.

The data-storage routine is shown in Listing 2-1.

Listing 2-1. The Save Routine, SaveController. java

package net.zenconsult.android.controller;

import java.io.File;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import net.zenconsult.android.model.Contact;

import net.zenconsult.android.model.Location;

import android.content.Context;

import android.os.Environment;

import android.util.Log;

public class SaveController {

private static ﬁnal String TAG = "SaveController";

public static void saveContact(Context context, Contact contact) {

if (isReadWrite()) {

try {

File outputFile = new File(context.getExternalFilesDir(null),contact.getFirstName());

FileOutputStream outputStream = new FileOutputStream(outputFile);

outputStream.write(contact.getBytes());

outputStream.close();

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CHAPTER 2: Information: The Foundation of an App16

} catch (FileNotFoundException e) {

Log.e(TAG,"File not found");

} catch (IOException e) {

Log.e(TAG,"IO Exception");

}

} else {

Log.e(TAG,"Error opening media card in read/write mode!");

}

public static void saveLocation(Context context, Location location) {

if (isReadWrite()) {

try {

File outputFile = new File(context.getExternalFilesDir(null),location.getIdentiﬁer());

FileOutputStream outputStream = new FileOutputStream(outputFile);

outputStream.write(location.getBytes());

outputStream.close();

} catch (FileNotFoundException e) {

Log.e(TAG,"File not found");

} catch (IOException e) {

Log.e(TAG,"IO Exception");

}

} else {

Log.e(TAG,"Error opening media card in read/write mode!");

}

private static boolean isReadOnly() {

Log.e(TAG,Environment

.getExternalStorageState());

return Environment.MEDIA_MOUNTED_READ_ONLY.equals(Environment

.getExternalStorageState());

}

private static boolean isReadWrite() {

Log.e(TAG,Environment

.getExternalStorageState());

return Environment.MEDIA_MOUNTED.equals(Environment

.getExternalStorageState());

}

Each time a user selects the Save Location button or the Save Contact button, it triggers the

preceding code. Let’s take a look at the Location (see Listing 2-2) and Contact (see Listing

2-3) classes in more detail. While we could implement one main save routine, I am keeping it

separate in case there is a need to act on different objects in a different manner.

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