Designing for Security

Android was designed so that most developers will be able to build applications using the default settings and not be confronted with difficult decisions about security. Android also has a number of security features built into the operating system that significantly reduce the frequency and impact of application security issues.

Some of the security features that help developers build secure applications include:

• The Android Application Sandbox that isolates data and code execution on a per-application basis.
• Android application framework with robust implementations of common security functionality such as cryptography, permissions, and secure IPC.
• Technologies like ASLR, NX, ProPolice, safe_iop, OpenBSD dlmalloc, OpenBSD calloc, and Linux mmap_min_addr to mitigate risks associated with common memory management errors
• An encrypted filesystem that can be enabled to protect data on lost or stolen devices.

Nevertheless, it is important for developers to be familiar with Android security best practices to make sure they take advantage of these capabilities and to reduce the likelihood of inadvertently introducing security issues that can affect their applications.

This document is organized around common APIs and development techniques that can have security implications for your application and its users. As these best practices are constantly evolving, we recommend you check back occasionally throughout your application development process.

Using Dalvik Code

Writing secure code that runs in virtual machines is a well-studied topic and many of the issues are not specific to Android. Rather than attempting to rehash these topics, we’d recommend that you familiarize yourself with the existing literature. Two of the more popular resources are:

• http://www.securingjava.com/toc.html
• https://www.owasp.org/index.php/Java_Security_Resources

This document is focused on the areas which are Android specific and/or different from other environments. For developers experienced with VM programming in other environments, there are two broad issues that may be different about writing apps for Android:

• Some virtual machines, such as the JVM or .net runtime, act as a security boundary, isolating code from the underlying operating system capabilities. On Android, the Dalvik VM is not a security boundary -- the application sandbox is implemented at the OS level, so Dalvik can interoperate with native code in the same application without any security constraints.
• Given the limited storage on mobile devices, it’s common for developers to want to build modular applications and use dynamic class loading. When doing this consider both the source where you retrieve your application logic and where you store it locally. Do not use dynamic class loading from sources that are not verified, such as unsecured network sources or external storage, since that code can be modified to include malicious behavior.

Using Native Code

In general, we encourage developers to use the Android SDK for most application development, rather than using native code. Applications built with native code are more complex, less portable, and more like to include common memory corruption errors such as buffer overflows.

Android is built using the Linux kernel and being familiar with Linux development security best practices is especially useful if you are going to use native code. This document is too short to discuss all of those best practices, but one of the most popular resources is “Secure Programming for Linux and Unix HOWTO”, available at http://www.dwheeler.com/secure-programs.

An important difference between Android and most Linux environments is the Application Sandbox. On Android, all applications run in the Application Sandbox, including those written with native code. At the most basic level, a good way to think about it for developers familiar with Linux is to know that every application is given a unique UID with very limited permissions. This is discussed in more detail in the Android Security Overview and you should be familiar with application permissions even if you are using native code.

Storing Data

Using internal files

By default, files created on internal storage are only accessible to the application that created the file. This protection is implemented by Android and is sufficient for most applications.

Use of world writable or world readable files for IPC is discouraged because it does not provide the ability to limit data access to particular applications, nor does it provide any control on data format. As an alternative, you might consider using a ContentProvider which provides read and write permissions, and can make dynamic permission grants on a case-by-case basis.

To provide additional protection for sensitive data, some applications choose to encrypt local files using a key that is not accessible to the application. (For example, a key can be placed in a KeyStore and protected with a user password that is not stored on the device). While this does not protect data from a root compromise that can monitor the user inputting the password, it can provide protection for a lost device without file system encryption.

Using external storage

Files created on external storage, such as SD Cards, are globally readable and writable. Since external storage can be removed by the user and also modified by any application, applications should not store sensitive information using external storage.

As with data from any untrusted source, applications should perform input validation when handling data from external storage (see Input Validation section). We strongly recommend that applications not store executables or class files on external storage prior to dynamic loading. If an application does retrieve executable files from external storage they should be signed and cryptographically verified prior to dynamic loading.

Using content providers

ContentProviders provide a structured storage mechanism that can be limited to your own application, or exported to allow access by other applications. By default, a ContentProvider is exported for use by other applications. If you do not intend to provide other applications with access to your ContentProvider, mark them as android:exported=false in the application manifest.

When creating a ContentProvider that will be exported for use by other applications, you can specify a single permission for reading and writing, or distinct permissions for reading and writing within the manifest. We recommend that you limit your permissions to those required to accomplish the task at hand. Keep in mind that it’s usually easier to add permissions later to expose new functionality than it is to take them away and break existing users.

If you are using a ContentProvider for sharing data between applications built by the same developer, it is preferable to use signature level permissions. Signature permissions do not require user confirmation, so they provide a better user experience and more controlled access to the ContentProvider.

ContentProviders can also provide more granular access by declaring the grantUriPermissions element and using the FLAG_GRANT_READ_URI_PERMISSION and FLAG_GRANT_WRITE_URI_PERMISSION flags in the Intent object that activates the component. The scope of these permissions can be further limited by the grant-uri-permission element.

When accessing a ContentProvider, use parameterized query methods such as query(), update(), and delete() to avoid potential SQL Injection from untrusted data. Note that using parameterized methods is not sufficient if the selection is built by concatenating user data prior to submitting it to the method.

Do not have a false sense of security about the write permission. Consider that the write permission allows SQL statements which make it possible for some data to be confirmed using creative WHERE clauses and parsing the results. For example, an attacker might probe for presence of a specific phone number in a call-log by modifying a row only if that phone number already exists. If the content provider data has predictable structure, the write permission may be equivalent to providing both reading and writing.

Using Interprocess Communication (IPC)

Some Android applications attempt to implement IPC using traditional Linux techniques such as network sockets and shared files. We strongly encourage the use of Android system functionality for IPC such as Intents, Binders, Services, and Receivers. The Android IPC mechanisms allow you to verify the identity of the application connecting to your IPC and set security policy for each IPC mechanism.

Many of the security elements are shared across IPC mechanisms. Broadcast Receivers, Activities, and Services are all declared in the application manifest. If your IPC mechanism is not intended for use by other applications, set the android:exported property to false. This is useful for applications that consist of multiple processes within the same UID, or if you decide late in development that you do not actually want to expose functionality as IPC but you don’t want to rewrite the code.

If your IPC is intended to be accessible to other applications, you can apply a security policy by using the Permission tag. If IPC is between applications built by the same developer, it is preferable to use signature level permissions. Signature permissions do not require user confirmation, so they provide a better user experience and more controlled access to the IPC mechanism.

One area that can introduce confusion is the use of intent filters. Note that Intent filters should not be considered a security feature -- components can be invoked directly and may not have data that would conform to the intent filter. You should perform input validation within your intent receiver to confirm that it is properly formatted for the invoked receiver, service, or activity.

Using intents

Intents are the preferred mechanism for asynchronous IPC in Android. Depending on your application requirements, you might use sendBroadcast(), sendOrderedBroadcast(), or direct an intent to a specific application component.

Note that ordered broadcasts can be “consumed” by a recipient, so they may not be delivered to all applications. If you are sending an Intent where delivery to a specific receiver is required, the intent must be delivered directly to the receiver.

Senders of an intent can verify that the recipient has a permission specifying a non-Null Permission upon sending. Only applications with that Permission will receive the intent. If data within a broadcast intent may be sensitive, you should consider applying a permission to make sure that malicious applications cannot register to receive those messages without appropriate permissions. In those circumstances, you may also consider invoking the receiver directly, rather than raising a broadcast.

Using binder and AIDL interfaces

Binders are the preferred mechanism for RPC-style IPC in Android. They provide a well-defined interface that enables mutual authentication of the endpoints, if required.

We strongly encourage designing interfaces in a manner that does not require interface specific permission checks. Binders are not declared within the application manifest, and therefore you cannot apply declarative permissions directly to a Binder. Binders generally inherit permissions declared in the application manifest for the Service or Activity within which they are implemented. If you are creating an interface that requires authentication and/or access controls on a specific binder interface, those controls must be explicitly added as code in the interface.

If providing an interface that does require access controls, use checkCallingPermission() to verify whether the caller of the Binder has a required permission. This is especially important before accessing a Service on behalf of the caller, as the identify of your application is passed to other interfaces. If invoking an interface provided by a Service, the bindService() invocation may fail if you do not have permission to access the given Service. If calling an interface provided locally by your own application, it may be useful to use the clearCallingIdentity() to satisfy internal security checks.

Using broadcast receivers

Broadcast receivers are used to handle asynchronous requests initiated via an intent.

By default, receivers are exported and can be invoked by any other application. If your BroadcastReceivers is intended for use by other applications, you may want to apply security permissions to receivers using the <receiver> element within the application manifest. This will prevent applications without appropriate permissions from sending an intent to the BroadcastReceivers.

Using Services

Services are often used to supply functionality for other applications to use. Each service class must have a corresponding declaration in its package's AndroidManifest.xml.

By default, Services are exported and can be invoked by any other application. Services can be protected using the android:permission attribute within the manifest’s <service> tag. By doing so, other applications will need to declare a corresponding <uses-permission> element in their own manifest to be able to start, stop, or bind to the service.

A Service can protect individual IPC calls into it with permissions, by calling checkCallingPermission() before executing the implementation of that call. We generally recommend using the declarative permissions in the manifest, since those are less prone to oversight.

Using Activities

Activities are most often used for providing the core user-facing functionality of an application. By default, Activities are exported and invokable by other applications only if they have an intent filter or binder declared. In general, we recommend that you specifically declare a Receiver or Service to handle IPC, since this modular approach reduces the risk of exposing functionality that is not intended for use by other applications.

If you do expose an Activity for purposes of IPC, the android:permission attribute in the <activity> declaration in the application manifest can be used to restrict access to only those applications which have the stated permissions.

Using Permissions

Requesting Permissions

We recommend minimizing the number of permissions requested by an application. Not having access to sensitive permissions reduces the risk of inadvertently misusing those permissions, can improve user adoption, and makes applications less attractive targets for attackers.

If it is possible to design your application in a way that does not require a permission, that is preferable. For example, rather than requesting access to device information to create an identifier, create a GUID for your application. (This specific example is also discussed in Handling User Data) Or, rather than using external storage, store data in your application directory.

If a permission is not required, do not request it. This sounds simple, but there has been quite a bit of research into the frequency of over-requesting permissions. If you’re interested in the subject you might start with this research paper published by U.C. Berkeley: http://www.eecs.berkeley.edu/Pubs/TechRpts/2011/EECS-2011-48.pdf

In addition to requesting permissions, your application can use permissions to protect IPC that is security sensitive and will be exposed to other applications -- such as a ContentProvider. In general, we recommend using access controls other than user confirmed permissions where possible since permissions can be confusing for users. For example, consider using the signature protection level on permissions for IPC communication between applications provided by a single developer.

Do not cause permission re-delegation. This occurs when an app exposes data over IPC that is only available because it has a specific permission, but does not require that permission of any clients of it’s IPC interface. More details on the potential impacts, and frequency of this type of problem is provided in this research paper published at USENIX: http://www.cs.be rkeley.edu/~afelt/felt_usenixsec2011.pdf

Creating Permissions

Generally, you should strive to create as few permissions as possible while satisfying your security requirements. Creating a new permission is relatively uncommon for most applications, since system-defined permissions cover many situations. Where appropriate, perform access checks using existing permissions.

If you must create a new permission, consider whether you can accomplish your task with a Signature permission. Signature permissions are transparent to the user and only allow access by applications signed by the same developer as application performing the permission check. If you create a Dangerous permission, then the user needs to decide whether to install the application. This can be confusing for other developers, as well as for users.

If you create a Dangerous permission, there are a number of complexities that you need to consider.

• The permission must have a string that concisely expresses to a user the security decision they will be required to make.
• The permission string must be localized to many different languages.
• Uses may choose not to install an application because a permission is confusing or perceived as risky.
• Applications may request the permission when the creator of the permission has not been installed.

Each of these poses a significant non-technical challenge for an application developer, which is why we discourage the use of Dangerous permission.

Using Networking

Using IP Networking

Networking on Android is not significantly different from Linux environments. The key consideration is making sure that appropriate protocols are used for sensitive data, such as HTTPS for web traffic. We prefer use of HTTPS over HTTP anywhere that HTTPS is supported on the server, since mobile devices frequently connect on networks that are not secured, such as public WiFi hotspots.

Authenticated, encrypted socket-level communication can be easily implemented using the SSLSocket class. Given the frequency with which Android devices connect to unsecured wireless networks using WiFi, the use of secure networking is strongly encouraged for all applications.

We have seen some applications use localhost network ports for handling sensitive IPC. We discourage this approach since these interfaces are accessible by other applications on the device. Instead, use an Android IPC mechanism where authentication is possible such as a Service and Binder. (Even worse than using loopback is to bind to INADDR_ANY since then your application may receive requests from anywhere. We’ve seen that, too.)

Also, one common issue that warrants repeating is to make sure that you do not trust data downloaded from HTTP or other insecure protocols. This includes validation of input in WebView and any responses to intents issued against HTTP.

Using Telephony Networking

SMS is the telephony protocol most frequently used by Android developers. Developers should keep in mind that this protocol was primarily designed for user-to-user communication and is not well-suited for some application purposes. Due to the limitations of SMS, we strongly recommend the use of C2DM and IP networking for sending data messages to devices.

Many developers do not realize that SMS is not encrypted or strongly authenticated on the network or on the device. In particular, any SMS receiver should expect that a malicious user may have sent the SMS to your application -- do not rely on unauthenticated SMS data to perform sensitive commands. Also, you should be aware that SMS may be subject to spoofing and/or interception on the network. On the Android-powered device itself, SMS messages are transmitted as Broadcast intents, so they may be read or captured by other applications that have the READ_SMS permission.

Dynamically Loading Code

We strongly discourage loading code from outside of the application APK. Doing so significantly increases the likelihood of application compromise due to code injection or code tampering. It also adds complexity around version management and application testing. Finally, it can make it impossible to verify the behavior of an application, so it may be prohibited in some environments.

If your application does dynamically load code, the most important thing to keep in mind about dynamically loaded code is that it runs with the same security permissions as the application APK. The user made a decision to install your application based on your identity, and they are expecting that you provide any code run within the application, including code that is dynamically loaded.

The major security risk associated with dynamically loading code is that the code needs to come from a verifiable source. If the modules are included directly within your APK, then they cannot be modified by other applications. This is true whether the code is a native library or a class being loaded using DexClassLoader. We have seen many instances of applications attempting to load code from insecure locations, such as downloaded from the network over unencrypted protocols or from world writable locations such as external storage. These locations could allow someone on the network to modify the content in transit, or another application on a users device to modify the content, respectively.

Using WebView

Since WebView consumes web content that can include HTML and JavaScript, improper use can introduce common web security issues such as cross-site-scripting (JavaScript injection). Android includes a number of mechanisms to reduce the scope of these potential issues by limiting the capability of WebView to the minimum functionality required by your application.

If your application does not directly use JavaScript within a WebView, do not call setJavaScriptEnabled(). We have seen this method invoked in sample code that might be repurposed in production application -- so remove it if necessary. By default, WebView does not execute JavaScript so cross-site-scripting is not possible.

Use addJavaScriptInterface() with particular care because it allows JavaScript to invoke operations that are normally reserved for Android applications. Only expose addJavaScriptInterface() to sources from which all input is trustworthy. If untrusted input is allowed, untrusted JavaScript may be able to invoke Android methods. In general, we recommend only exposing addJavaScriptInterface() to JavaScript that is contained within your application APK.

Do not trust information downloaded over HTTP, use HTTPS instead. Even if you are connecting only to a single website that you trust or control, HTTP is subject to MiTM attacks and interception of data. Sensitive capabilities using addJavaScriptInterface() should not ever be exposed to unverified script downloaded over HTTP. Note that even with the use of HTTPS, addJavaScriptInterface() increases the attack surface of your application to include the server infrastructure and all CAs trusted by the Android-powered device.

If your application accesses sensitive data with a WebView, you may want to use the clearCache() method to delete any files stored locally. Server side headers like no-cache can also be used to indicate that an application should not cache particular content.

Performing Input Validation

Insufficient input validation is one of the most common security problems affecting applications, regardless of what platform they run on. Android does have platform-level countermeasures that reduce the exposure of applications to input validation issues, you should use those features where possible. Also note that selection of type-safe languages tends to reduce the likelihood of input validation issues. We strongly recommend building your applications with the Android SDK.

If you are using native code, then any data read from files, received over the network, or received from an IPC has the potential to introduce a security issue. The most common problems are buffer overflows, use after free, and off-by-one errors. Android provides a number of technologies like ASLR and DEP that reduce the exploitability of these errors, but they do not solve the underlying problem. These can be prevented by careful handling of pointers and managing of buffers.

Dynamic, string based languages such as JavaScript and SQL are also subject to input validation problems due to escape characters and script injection.

If you are using data within queries that are submitted to SQL Database or a Content Provider, SQL Injection may be an issue. The best defense is to use parameterized queries, as is discussed in the ContentProviders section. Limiting permissions to read-only or write-only can also reduce the potential for harm related to SQL Injection.

If you are using WebView, then you must consider the possibility of XSS. If your application does not directly use JavaScript within a WebView, do not call setJavaScriptEnabled() and XSS is no longer possible. If you must enable JavaScript then the WebView section provides other security best practices.

If you cannot use the security features above, we strongly recommend the use of well-structured data formats and verifying that the data conforms to the expected format. While blacklisting of characters or character-replacement can be an effective strategy, these techniques are error-prone in practice and should be avoided when possible.

Handling User Data

In general, the best approach is to minimize use of APIs that access sensitive or personal user data. If you have access to data and can avoid storing or transmitting the information, do not store or transmit the data. Finally, consider if there is a way that your application logic can be implemented using a hash or non-reversible form of the data. For example, your application might use the hash of an an email address as a primary key, to avoid transmitting or storing the email address. This reduces the chances of inadvertently exposing data, and it also reduces the chance of attackers attempting to exploit your application.

If your application accesses personal information such as passwords or usernames, keep in mind that some jurisdictions may require you to provide a privacy policy explaining your use and storage of that data. So following the security best practice of minimizing access to user data may also simplify compliance.

You should also consider whether your application might be inadvertently exposing personal information to other parties such as third-party components for advertising or third-party services used by your application. If you don't know why a component or service requires a personal information, don’t provide it. In general, reducing the access to personal information by your application will reduce the potential for problems in this area.

If access to sensitive data is required, evaluate whether that information must be transmitted to a server, or whether the operation can be performed on the client. Consider running any code using sensitive data on the client to avoid transmitting user data.

Also, make sure that you do not inadvertently expose user data to other application on the device through overly permissive IPC, world writable files, or network sockets. This is a special case of permission redelegation, discussed in the Requesting Permissions section.

If a GUID is required, create a large, unique number and store it. Do not use phone identifiers such as the phone number or IMEI which may be associated with personal information. This topic is discussed in more detail in the Android Developer Blog.

Application developers should be careful writing to on-device logs. In Android, logs are a shared resource, and are available to an application with the READ_LOGS permission. Even though the phone log data is temporary and erased on reboot, inappropriate logging of user information could inadvertently leak user data to other applications.

Handling Credentials

In general, we recommend minimizing the frequency of asking for user credentials -- to make phishing attacks more conspicuous, and less likely to be successful. Instead use an authorization token and refresh it.

Where possible, username and password should not be stored on the device. Instead, perform initial authentication using the username and password supplied by the user, and then use a short-lived, service-specific authorization token.

Services that will be accessible to multiple applications should be accessed using AccountManager. If possible, use the AccountManager class to invoke a cloud-based service and do not store passwords on the device.

After using AccountManager to retrieve an Account, check the CREATOR before passing in any credentials, so that you do not inadvertently pass credentials to the wrong application.

If credentials are to be used only by applications that you create, then you can verify the application which accesses the AccountManager using checkSignature(). Alternatively, if only one application will use the credential, you might use a KeyStore for storage.

Using Cryptography

In addition to providing data isolation, supporting full-filesystem encryption, and providing secure communications channels Android provides a wide array of algorithms for protecting data using cryptography.

In general, try to use the highest level of pre-existing framework implementation that can support your use case. If you need to securely retrieve a file from a known location, a simple HTTPS URI may be adequate and require no knowledge of cryptography on your part. If you need a secure tunnel, consider using HttpsURLConnection or SSLSocket, rather than writing your own protocol.

If you do find yourself needing to implement your own protocol, we strongly recommend that you not implement your own cryptographic algorithms. Use existing cryptographic algorithms such as those in the implementation of AES or RSA provided in the Cipher class.

Use a secure random number generator ( SecureRandom) to initialize any cryptographic keys ( KeyGenerator). Use of a key that is not generated with a secure random number generator significantly weakens the strength of the algorithm, and may allow offline attacks.

If you need to store a key for repeated use, use a mechanism like KeyStore that provides a mechanism for long term storage and retrieval of cryptographic keys.

Conclusion

Android provides developers with the ability to design applications with a broad range of security requirements. These best practices will help you make sure that your application takes advantage of the security benefits provided by the platform.

You can receive more information on these topics and discuss security best practices with other developers in the Android Security Discuss Google Group