[TOC] #Android Security Overview
Android is a modern mobile platform that was designed to be truly open. Android applications make use of advanced hardware and software, as well as local and served data, exposed through the platform to bring innovation and value to consumers. To protect that value, the platform must offer an application environment that ensures the security of users, data, applications, the device, and the network.
Securing an open platform requires a robust security architecture and rigorous security programs. Android was designed with multi-layered security that provides the flexibility required for an open platform, while providing protection for all users of the platform.
Android was designed with developers in mind. Security controls were designed to reduce the burden on developers. Security-savvy developers can easily work with and rely on flexible security controls. Developers less familiar with security will be protected by safe defaults.
Android was designed with device users in mind. Users are provided visibility into how applications work, and control over those applications. This design includes the expectation that attackers would attempt to perform common attacks, such as social engineering attacks to convince device users to install malware, and attacks on third-party applications on Android. Android was designed to both reduce the probability of these attacks and greatly limit the impact of the attack in the event it was successful.
This document outlines the goals of the Android security program, describes the fundamentals of the Android security architecture, and answers the most pertinent questions for system architects and security analysts. This document focuses on the security features of Android's core platform and does not discuss security issues that are unique to specific applications, such as those related to the browser or SMS application. Recommended best practices for building Android devices, deploying Android devices, or developing applications for Android are not the goal of this document and are provided elsewhere.
Android provides an open source platform and application environment for mobile devices.
The main Android platform building blocks are:
Device Hardware: Android runs on a wide range of hardware configurations including smart phones, tablets, and set-top-boxes. Android is processor-agnostic, but it does take advantage of some hardware-specific security capabilities such as ARM v6 eXecute-Never.
Android Operating System: The core operating system is built on top of the Linux kernel. All device resources, like camera functions, GPS data, Bluetooth functions, telephony functions, network connections, etc. are accessed through the operating system.
Android Application Runtime: Android applications are most often written in the Java programming language and run in the Dalvik virtual machine. However, many applications, including core Android services and applications are native applications or include native libraries. Both Dalvik and native applications run within the same security environment, contained within the Application Sandbox. Applications get a dedicated part of the filesystem in which they can write private data, including databases and raw files.
Android applications extend the core Android operating system. There are two primary sources for applications:
Pre-Installed Applications: Android includes a set of pre-installed applications including phone, email, calendar, web browser, and contacts. These function both as user applications and to provide key device capabilities that can be accessed by other applications. Pre-installed applications may be part of the open source Android platform, or they may be developed by an OEM for a specific device.
User-Installed Applications: Android provides an open development environment supporting any third-party application. Google Play offers users hundreds of thousands of applications.
Google provides a set of cloud-based services that are available to any compatible Android device. The primary services are:
Google Play: Google Play is a collection of services that allow users to discover, install, and purchase applications from their Android device or the web. Google Play makes it easy for developers to reach Android users and potential customers. Google Play also provides community review, application license verification, and other security services.
Android Updates: The Android update service delivers new capabilities and security updates to Android devices, including updates through the web or over the air (OTA).
These services are not part of the Android Open Source Project and are out of scope for this document. But they are relevant to the security of most Android devices, so a related security document titled “Google Services for Android: Security Overview” is available.
##Android Security Program Overview
Early on in development, the core Android development team recognized that a robust security model was required to enable a vigorous ecosystem of applications and devices built on and around the Android platform and supported by cloud services. As a result, through its entire development lifecycle, Android has been subjected to a professional security program. The Android team has had the opportunity to observe how other mobile, desktop, and server platforms prevented and reacted to security issues and built a security program to address weak points observed in other offerings.
The key components of the Android Security Program include:
##Android Platform Security Architecture
Android seeks to be the most secure and usable operating system for mobile platforms by re-purposing traditional operating system security controls to:
To achieve these objectives, Android provides these key security features:
The sections below describe these and other security features of the Android platform. Figure 1 summarizes the security components and considerations of the various levels of the Android software stack. Each component assumes that the components below are properly secured. With the exception of a small amount of Android OS code running as root, all code above the Linux Kernel is restricted by the Application Sandbox.
Figure 1: Android software stack.
#System and Kernel Level Security
At the operating system level, the Android platform provides the security of the Linux kernel, as well as a secure inter-process communication (IPC) facility to enable secure communication between applications running in different processes. These security features at the OS level ensure that even native code is constrained by the Application Sandbox. Whether that code is the result of included application behavior or a exploitation of an application vulnerability, the system would prevent the rogue application from harming other applications, the Android system, or the device itself.
The foundation of the Android platform is the Linux kernel. The Linux kernel itself has been in widespread use for years, and is used in millions of security-sensitive environments. Through its history of constantly being researched, attacked, and fixed by thousands of developers, Linux has become a stable and secure kernel trusted by many corporations and security professionals.
As the base for a mobile computing environment, the Linux kernel provides Android with several key security features, including:
As a multiuser operating system, a fundamental security objective of the Linux kernel is to isolate user resources from one another. The Linux security philosophy is to protect user resources from one another. Thus, Linux:
##The Application Sandbox
The Android platform takes advantage of the Linux user-based protection as a means of identifying and isolating application resources. The Android system assigns a unique user ID (UID) to each Android application and runs it as that user in a separate process. This approach is different from other operating systems (including the traditional Linux configuration), where multiple applications run with the same user permissions.
This sets up a kernel-level Application Sandbox. The kernel enforces security between applications and the system at the process level through standard Linux facilities, such as user and group IDs that are assigned to applications. By default, applications cannot interact with each other and applications have limited access to the operating system. If application A tries to do something malicious like read application B's data or dial the phone without permission (which is a separate application), then the operating system protects against this because application A does not have the appropriate user privileges. The sandbox is simple, auditable, and based on decades-old UNIX-style user separation of processes and file permissions.
Since the Application Sandbox is in the kernel, this security model extends to native code and to operating system applications. All of the software above the kernel in Figure 1, including operating system libraries, application framework, application runtime, and all applications run within the Application Sandbox. On some platforms, developers are constrained to a specific development framework, set of APIs, or language in order to enforce security. On Android, there are no restrictions on how an application can be written that are required to enforce security; in this respect, native code is just as secure as interpreted code.
In some operating systems, memory corruption errors generally lead to completely compromising the security of the device. This is not the case in Android due to all applications and their resources being sandboxed at the OS level. A memory corruption error will only allow arbitrary code execution in the context of that particular application, with the permissions established by the operating system.
Like all security features, the Application Sandbox is not unbreakable. However, to break out of the Application Sandbox in a properly configured device, one must compromise the security of the the Linux kernel.
##System Partition and Safe Mode
The system partition contains Android's kernel as well as the operating system libraries, application runtime, application framework, and applications. This partition is set to read-only. When a user boots the device into Safe Mode, only core Android applications are available. This ensures that the user can boot their phone into an environment that is free of third-party software.
In a UNIX-style environment, filesystem permissions ensure that one user cannot alter or read another user's files. In the case of Android, each application runs as its own user. Unless the developer explicitly exposes files to other applications, files created by one application cannot be read or altered by another application.
Android 3.0 and later provides full filesystem encryption, so all user data can be encrypted in the kernel using the dmcrypt implementation of AES128 with CBC and ESSIV:SHA256. The encryption key is protected by AES128 using a key derived from the user password, preventing unauthorized access to stored data without the user device password. To provide resistance against systematic password guessing attacks (e.g. “rainbow tables” or brute force), the password is combined with a random salt and hashed repeatedly with SHA1 using the standard PBKDF2 algorithm prior to being used to decrypt the filesystem key. To provide resistance against dictionary password guessing attacks, Android provides password complexity rules that can be set by the device administrator and enforced by the operating system. Filesystem encryption requires the use of a user password, pattern-based screen lock is not supported.
More details on implementation of filesystem encryption are available at [https://source.android.com/tech/encryption/android_crypto_implementation.html](/ tech/encryption/android_crypto_implementation.html)
Android can be configured to verify a user-supplied password prior to providing access to a device. In addition to preventing unauthorized use of the device, this password protects the cryptographic key for full filesystem encryption.
Use of a password and/or password complexity rules can be required by a device administrator.
Android 2.2 and later provide the Android Device Administration API, which provides device administration features at the system level. For example, the built-in Android Email application uses the APIs to improve Exchange support. Through the Email application, Exchange administrators can enforce password policies — including alphanumeric passwords or numeric PINs — across devices. Administrators can also remotely wipe (that is, restore factory defaults on) lost or stolen handsets.
In addition to use in applications included with the Android system, these APIs are available to third-party providers of Device Management solutions. Details on the API are provided here: [https://developer.android.com/guide/topics/admin/device-admin.html](https://devel oper.android.com/guide/topics/admin/device-admin.html).
##Memory Management Security Enhancements
Android includes many features that make common security issues harder to exploit. The Android SDK, compilers, and OS use tools to make common memory corruption issues significantly harder to exploit, including:
##Rooting of Devices
By default, on Android only the kernel and a small subset of the core applications run with root permissions. Android does not prevent a user or application with root permissions from modifying the operating system, kernel, and any other application. In general, root has full access to all applications and all application data. Users that change the permissions on an Android device to grant root access to applications increase the security exposure to malicious applications and potential application flaws.
The ability to modify an Android device they own is important to developers working with the Android platform. On many Android devices users have the ability to unlock the bootloader in order to allow installation of an alternate operating system. These alternate operating systems may allow an owner to gain root access for purposes of debugging applications and system components or to access features not presented to applications by Android APIs.
On some devices, a person with physical control of a device and a USB cable is able to install a new operating system that provides root privileges to the user. To protect any existing user data from compromise the bootloader unlock mechanism requires that the bootloader erase any existing user data as part of the unlock step. Root access gained via exploiting a kernel bug or security hole can bypass this protection.
Encrypting data with a key stored on-device does not protect the application data from root users. Applications can add a layer of data protection using encryption with a key stored off-device, such as on a server or a user password. This approach can provide temporary protection while the key is not present, but at some point the key must be provided to the application and it then becomes accessible to root users.
A more robust approach to protecting data from root users is through the use of hardware solutions. OEMs may choose to implement hardware solutions that limit access to specific types of content such as DRM for video playback, or the NFC-related trusted storage for Google wallet.
In the case of a lost or stolen device, full filesystem encryption on Android devices uses the device password to protect the encryption key, so modifying the bootloader or operating system is not sufficient to access user data without the user’s device password.
#Android Application Security
##Elements of Applications
Android provides an open source platform and application environment for mobile devices. The core operating system is based on the Linux kernel. Android applications are most often written in the Java programming language and run in the Dalvik virtual machine. However, applications can also be written in native code. Applications are installed from a single file with the .apk file extension.
The main Android application building blocks are:
AndroidManifest.xml: The [AndroidManifest.xml](https://developer.android.com/guide/topics/manifest/manifes t-intro.html) file is the control file that tells the system what to do with all the top-level components (specifically activities, services, broadcast receivers, and content providers described below) in an application. This also specifies which permissions are required.
Activities: An [Activity](https://developer.android.com/guide/topics/fundamentals/activities.htm l) is, generally, the code for a single, user-focused task. It usually includes displaying a UI to the user, but it does not have to -- some Activities never display UIs. Typically, one of the application's Activities is the entry point to an application.
Services: A Service is a body of code that runs in the background. It can run in its own process, or in the context of another application's process. Other components “bind” to a Service and invoke methods on it via remote procedure calls. An example of a Service is a media player: even when the user quits the media-selection UI, the user probably still intends for music to keep playing. A Service keeps the music going even when the UI has completed.
Broadcast Receiver: A [BroadcastReceiver](https://developer.android.com/reference/android/content/Broad castReceiver.html) is an object that is instantiated when an IPC mechanism known as an Intent is issued by the operating system or another application. An application may register a receiver for the low battery message, for example, and change its behavior based on that information.
##The Android Permission Model: Accessing Protected APIs
By default, an Android application can only access a limited range of system resources. The system manages Android application access to resources that, if used incorrectly or maliciously, could adversely impact the user experience, the network, or data on the device.
These restrictions are implemented in a variety of different forms. Some capabilities are restricted by an intentional lack of APIs to the sensitive functionality (e.g. there is no Android API for directly manipulating the SIM card). In some instances, separation of roles provides a security measure, as with the per-application isolation of storage. In other instances, the sensitive APIs are intended for use by trusted applications and protected through a security mechanism known as Permissions.
These protected APIs include:
These resources are only accessible through the operating system. To make use of the protected APIs on the device, an application must define the capabilities it needs in its manifest. When preparing to install an application, the system displays a dialog to the user that indicates the permissions requested and asks whether to continue the installation. If the user continues with the installation, the system accepts that the user has granted all of the requested permissions. The user can not grant or deny individual permissions -- the user must grant or deny all of the requested permissions as a block.
Once granted, the permissions are applied to the application as long as it is installed. To avoid user confusion, the system does not notify the user again of the permissions granted to the application, and applications that are included in the core operating system or bundled by an OEM do not request permissions from the user. Permissions are removed if an application is uninstalled, so a subsequent re-installation will again result in display of permissions.
Within the device settings, users are able to view permissions for applications they have previously installed. Users can also turn off some functionality globally when they choose, such as disabling GPS, radio, or wi-fi.
In the event that an application attempts to use a protected feature which has not been declared in the application's manifest, the permission failure will typically result in a security exception being thrown back to the application. Protected API permission checks are enforced at the lowest possible level to prevent circumvention. An example of the user messaging when an application is installed while requesting access to protected APIs is shown in Figure 2.
The system default permissions are described at https://developer.android.com/reference/android/Manifest.permission.html. Applications may declare their own permissions for other applications to use. Such permissions are not listed in the above location.
When defining a permission a protectionLevel attribute tells the system how the user is to be informed of applications requiring the permission, or who is allowed to hold a permission. Details on creating and using application specific permissions are described at [https://developer.android.com/guide/topics/security/security.html](https://develo per.android.com/guide/topics/security/security.html).
There are some device capabilities, such as the ability to send SMS broadcast intents, that are not available to third-party applications, but that may be used by applications pre-installed by the OEM. These permissions use the signatureOrSystem permission.
##How Users Understand Third-Party Applications
Android strives to make it clear to users when they are interacting with third-party applications and inform the user of the capabilities those applications have. Prior to installation of any application, the user is shown a clear message about the different permissions the application is requesting. After install, the user is not prompted again to confirm any permissions.
There are many reasons to show permissions immediately prior to installation time. This is when user is actively reviewing information about the application, developer, and functionality to determine whether it matches their needs and expectations. It is also important that they have not yet established a mental or financial commitment to the app, and can easily compare the application to other alternative applications.
Some other platforms use a different approach to user notification, requesting permission at the start of each session or while applications are in use. The vision of Android is to have users switching seamlessly between applications at will. Providing confirmations each time would slow down the user and prevent Android from delivering a great user experience. Having the user review permissions at install time gives the user the option to not install the application if they feel uncomfortable.
Also, many user interface studies have shown that over-prompting the user causes the user to start saying “OK” to any dialog that is shown. One of Android's security goals is to effectively convey important security information to the user, which cannot be done using dialogs that the user will be trained to ignore. By presenting the important information once, and only when it is important, the user is more likely to think about what they are agreeing to.
Some platforms choose not to show any information at all about application functionality. That approach prevents users from easily understanding and discussing application capabilities. While it is not possible for all users to always make fully informed decisions, the Android permissions model makes information about applications easily accessible to a wide range of users. For example, unexpected permissions requests can prompt more sophisticated users to ask critical questions about application functionality and share their concerns in places such as Google Play where they are visible to all users.
Processes can communicate using any of the traditional UNIX-type mechanisms. Examples include the filesystem, local sockets, or signals. However, the Linux permissions still apply.
Android also provides new IPC mechanisms:
Binder: A lightweight capability-based remote procedure call mechanism designed for high performance when performing in-process and cross-process calls. Binder is implemented using a custom Linux driver. See [https://developer.android.com/reference/android/os/Binder.html](https://developer .android.com/reference/android/os/Binder.html).
Services: Services (discussed above) can provide interfaces directly accessible using binder.
Intents: An Intent is a simple message object that represents an “intention” to do something. For example, if your application wants to display a web page, it expresses its “Intent” to view the URL by creating an Intent instance and handing it off to the system. The system locates some other piece of code (in this case, the Browser) that knows how to handle that Intent, and runs it. Intents can also be used to broadcast interesting events (such as a notification) system-wide. See [https://developer.android.com/reference/android/content/Intent.html](https://developer.android.com/reference/android/content/Intent.html.
ContentProviders: A ContentProvider is a data storehouse that provides access to data on the device; the classic example is the ContentProvider that is used to access the user's list of contacts. An application can access data that other applications have exposed via a ContentProvider, and an application can also define its own ContentProviders to expose data of its own. See https://developer.android.com/reference/android/content/ContentProvider.html.
While it is possible to implement IPC using other mechanisms such as network sockets or world-writable files, these are the recommended Android IPC frameworks. Android developers will be encouraged to use best practices around securing users' data and avoiding the introduction of security vulnerabilities.
A cost sensitive API is any function that might generate a cost for the user or the network. The Android platform has placed cost sensitive APIs in the list of protected APIs controlled by the OS. The user will have to grant explicit permission to third-party applications requesting use of cost sensitive APIs. These APIs include:
##SIM Card Access
Low level access to the SIM card is not available to third-party apps. The OS handles all communications with the SIM card including access to personal information (contacts) on the SIM card memory. Applications also cannot access AT commands, as these are managed exclusively by the Radio Interface Layer (RIL). The RIL provides no high level APIs for these commands.
Android has placed APIs that provide access to user data into the set of protected APIs. With normal usage, Android devices will also accumulate user data within third-party applications installed by users. Applications that choose to share this information can use Android OS permission checks to protect the data from third-party applications.
Figure 3: Access to sensitive user data is only available through protected APIs
System content providers that are likely to contain personal or personally identifiable information such as contacts and calendar have been created with clearly identified permissions. This granularity provides the user with clear indication of the types of information that may be provided to the application. During installation, a third-party application may request permission to access these resources. If permission is granted, the application can be installed and will have access to the data requested at any time when it is installed.
Any applications which collect personal information will, by default, have that data restricted only to the specific application. If an application chooses to make the data available to other applications though IPC, the application granting access can apply permissions to the IPC mechanism that are enforced by the operating system.
##Sensitive Data Input Devices
Android devices frequently provide sensitive data input devices that allow applications to interact with the surrounding environment, such as camera, microphone or GPS. For a third-party application to access these devices, it must first be explicitly provided access by the user through the use of Android OS Permissions. Upon installation, the installer will prompt the user requesting permission to the sensor by name.
If an application wants to know the user‘s location, the application requires a permission to access the user’s location. Upon installation, the installer will prompt the user asking if the application can access the user‘s location. At any time, if the user does not want any application to access their location, then the user can run the “Settings” application, go to “Location & Security”, and uncheck the “Use wireless networks” and “Enable GPS satellites”. This will disable location based services for all applications on the user’s device.
Android also strives to restrict access to data that is not intrinsically sensitive, but may indirectly reveal characteristics about the user, user preferences, and the manner in which they use a device.
By default applications do not have access to operating system logs, browser history, phone number, or hardware / network identification information. If an application requests access to this information at install time, the installer will prompt the user asking if the application can access the information. If the user does not grant access, the application will not be installed.
Code signing allows developers to identify the author of the application and to update their application without creating complicated interfaces and permissions. Every application that is run on the Android platform must be signed by the developer. Applications that attempt to install without being signed will rejected by either Google Play or the package installer on the Android device.
On Google Play, application signing bridges the trust Google has with the developer and the trust the developer has with their application. Developers know their application is provided, unmodified to the Android device; and developers can be held accountable for behavior of their application.
On Android, application signing is the first step to placing an application in its Application Sandbox. The signed application certificate defines which user id is associated with which application; different applications run under different user IDs. Application signing ensures that one application cannot access any other application except through well-defined IPC.
When an application (APK file) is installed onto an Android device, the Package Manager verifies that the APK has been properly signed with the certificate included in that APK. If the certificate (or, more accurately, the public key in the certificate) matches the key used to sign any other APK on the device, the new APK has the option to specify in the manifest that it will share a UID with the other similarly-signed APKs.
Applications can be signed by a third-party (OEM, operator, alternative market) or self-signed. Android provides code signing using self-signed certificates that developers can generate without external assistance or permission. Applications do not have to be signed by a central authority. Android currently does not perform CA verification for application certificates.
Applications are also able to declare security permissions at the Signature protection level, restricting access only to applications signed with the same key while maintaining distinct UIDs and Application Sandboxes. A closer relationship with a shared Application Sandbox is allowed via the [shared UID feature](https://developer.android.com/guide/topics/manifest/manifest-element.htm l#uid) where two or more applications signed with same developer key can declare a shared UID in their manifest.
##Digital Rights Management
The Android platform provides an extensible DRM framework that lets applications manage rights-protected content according to the license constraints that are associated with the content. The DRM framework supports many DRM schemes; which DRM schemes a device supports is left to the device manufacturer.
The [Android DRM framework](https://developer.android.com/reference/android/drm/package-summary.ht ml) is implemented in two architectural layers (see figure below):
A DRM framework API, which is exposed to applications through the Android application framework and runs through the Dalvik VM for standard applications.
A native code DRM manager, which implements the DRM framework and exposes an interface for DRM plug-ins (agents) to handle rights management and decryption for various DRM schemes
Figure 4: Architecture of Digital Rights Management on Android platform
Android provides system updates for both security and feature related purposes.
There are two ways to update the code on most Android devices: over-the-air (OTA updates) or side-loaded updates. OTA updates can be rolled out over a defined time period or be pushed to all devices at once, depending on how the OEM and/or carrier would like to push the updates. Side-loaded updates can be provided from a central location for users to download as a zip file to their local desktop machine or directly to their handset. Once the update is copied or downloaded to the SD card on the device, Android will recognize the update, verify its integrity and authenticity, and automatically update the device.
If a dangerous vulnerability is discovered internally or responsibly reported to Google or the Android Open Source Project, the Android security team will start the following process.
The NDA is required to ensure that the security issue does not become public prior to availabilty of a fix and put users at risk. Many OHA members run their own code on Android devices such as the bootloader, wifi drivers, and the radio. Once the Android Security team is notified of a security issue in this partner code, they will consult with OHA partners to quickly find a fix for the problem at hand and similar problems. However, the OHA member who wrote the faulty code is ultimately responsible for fixing the problem.
If a dangerous vulnerability is not responsibly disclosed (e.g., if it is posted to a public forum without warning), then Google and/or the Android Open Source Project will work as quickly as possible to create a patch. The patch will released to the public (and any partners) when the patch is tested and ready for use.
At Google I/O 2011, many of the largest OHA partners committed to providing updates to devices for 18 months after initial shipment. This will provide users with access to the most recent Android features, as well as security updates.
Any developer, Android user, or security researcher can notify the Android security team of potential security issues by sending email to firstname.lastname@example.org. If desired, communication can be encrypted using the Android security team PGP key available here: [https://developer.android.com/security_at_android_dot_com.txt](https://develope r.android.com/security_at_android_dot_com.txt).
Information about the Android Open Source Project is available at https://source.android.com.
Information for Android application developers is here: https://developer.android.com.
The Android Security team can be reached at email@example.com.
Security information exists throughout the Android Open Source and Developer Sites. A good place to start is here: [https://developer.android.com/guide/topics/security/security.html](https://develo per.android.com/guide/topics/security/security.html).
A Security FAQ for developers is located here: [https://developer.android.com/resources/faq/security.html](https://developer.andr oid.com/resources/faq/security.html).
A community resource for discussion about Android security exists here: https://groups.google.com/forum/?fromgroups#!forum/android-security-discuss.