xref: /frameworks/base/docs/html/guide/topics/security/security.jd

page.title=Designing for Security
@jd:body

<div id="qv-wrapper">
<div id="qv">
<h2>In this document</h2>
<ol>
<li><a href="#Dalvik">Using Davlik Code</a></li>
<li><a href="#Native">Using Native Code</a></li>
<li><a href="#Data">Storing Data</a></li>
<li><a href="#IPC">Using IPC</a></li>
<li><a href="#Permissions">Using Permissions</a></li>
<li><a href="#Networking">Using Networking</a></li>
<li><a href="#DynamicCode">Dynamically Loading Code</a></li>
<li><a href="#Input">Performing Input Validation</a></li>
<li><a href="#UserData">Handling User Data</a></li>
<li><a href="#Crypto">Using Cryptography</a></li>
</ol>
<h2>See also</h2>
<ol>
<li><a href="http://source.android.com/tech/security/index.html">Android
Security Overview</a></li>
<li><a href="{@docRoot}guide/topics/security/security.html">Android Security
And Permissions</a></li>
</ol>
</div></div>
<p>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.</p>

<p>Some of the security features that help developers build secure applications
include:
<ul>
<li>The Android Application Sandbox that isolates data and code execution on a
per-application basis.</li>
<li>Android application framework with robust implementations of common
security functionality such as cryptography, permissions, and secure IPC.</li>
<li>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</li>
<li>An encrypted filesystem that can be enabled to protect data on lost or
stolen devices.</li>
</ul></p>

<p>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.</p>

<p>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.</p>

<a name="Dalvik"></a>
<h2>Using Dalvik Code</h2>
<p>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:
<ul>
<li><a href="http://www.securingjava.com/toc.html">
http://www.securingjava.com/toc.html</a></li>
<li><a
href="https://www.owasp.org/index.php/Java_Security_Resources">
https://www.owasp.org/index.php/Java_Security_Resources</a></li>
</ul></p>

<p>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:
<ul>
<li>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.</li>
<li>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.</li>
</ul></p>

<a name="Native"></a>
<h2>Using Native Code</h2>

<p>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.</p>

<p>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 <a
href="http://www.dwheeler.com/secure-programs">
http://www.dwheeler.com/secure-programs</a>.</p>

<p>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 <a
href="http://source.android.com/tech/security/index.html">Android Security
Overview</a> and you should be familiar with application permissions even if
you are using native code.</p>

<a name="Data"></a>
<h2>Storing Data</h2>

<h3>Using internal files</h3>

<p>By default, files created on <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal
storage</a> are only accessible to the application that created the file. This
protection is implemented by Android and is sufficient for most
applications.</p>

<p>Use of <a
href="{@docRoot}reference/android/content/Context.html#MODE_WORLD_WRITEABLE">
world writable</a> or <a
href="{@docRoot}reference/android/content/Context.html#MODE_WORLD_READABLE">world
readable</a> 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.</p>

<p>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 {@link java.security.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 <a
href="http://source.android.com/tech/encryption/index.html">file system
encryption</a>.</p>

<h3>Using external storage</h3>

<p>Files created on <a
href="{@docRoot}guide/topics/data/data-storage.html#filesExternal">external
storage</a>, 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.</p>

<p>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.</p>

<h3>Using content providers</h3>

<p>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 <code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code> is
<a href="{@docRoot}guide/topics/manifest/provider-element.html#exported">exported
</a> for use by other applications.  If you do not intend to provide other
applications with access to your<code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code>, mark them as <code><a
href="{@docRoot}guide/topics/manifest/provider-element.html#exported">
android:exported=false</a></code> in the application manifest.</p>

<p>When creating a <code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">ContentProvider
</a></code> that will be exported for use by other applications, you can specify
a single
<a href="{@docRoot}guide/topics/manifest/provider-element.html#prmsn">permission
</a> 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.</p>

<p>If you are using a <code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code> for sharing data between applications built by the
same developer, it is preferable to use
<a href="{@docRoot}guide/topics/manifest/permission-element.html#plevel">signature
level permissions</a>.  Signature permissions do not require user confirmation,
so they provide a better user experience and more controlled access to the
<code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code>.</p>

<p>ContentProviders can also provide more granular access by declaring the <a
href="{@docRoot}guide/topics/manifest/provider-element.html#gprmsn">
grantUriPermissions</a> element and using the <code><a
href="{@docRoot}reference/android/content/Intent.html#FLAG_GRANT_READ_URI_PERMISSION">FLAG_GRANT_READ_URI_PERMISSION</a></code>
and <code><a
href="{@docRoot}reference/android/content/Intent.html#FLAG_GRANT_WRITE_URI_PERMISSION">FLAG_GRANT_WRITE_URI_PERMISSION</a></code>
flags in the Intent object
that activates the component.  The scope of these permissions can be further
limited by the <code><a
href="{@docRoot}guide/topics/manifest/grant-uri-permission-element.html">
grant-uri-permission element</a></code>.</p>

<p>When accessing a <code>
<a href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code>, use parameterized query methods such as <code>
<a href="{@docRoot}reference/android/content/ContentProvider.html#query(android.net.Uri,%20java.lang.String[],%20java.lang.String,%20java.lang.String[],%20java.lang.String)">query()</a></code>, <code><a
href="{@docRoot}reference/android/content/ContentProvider.html#update(android.net.Uri,%20android.content.ContentValues,%20java.lang.String,%20java.lang.String[])">update()</a></code>, and <code><a
href="{@docRoot}reference/android/content/ContentProvider.html#delete(android.net.Uri,%20java.lang.String,%20java.lang.String[])">delete()</a></code> to avoid
potential <a href="http://en.wikipedia.org/wiki/SQL_injection">SQL
Injection</a> from untrusted data. Note that using parameterized methods is not
sufficient if the <code>selection</code> is built by concatenating user data
prior to submitting it to the method.</p>

<p>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 <code>WHERE</code> 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.</p>

<a name="IPC"></a>
<h2>Using Interprocess Communication (IPC)</h2>

<p>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.</p>

<p>Many of the security elements are shared across IPC mechanisms. <a
href="{@docRoot}reference/android/content/BroadcastReceiver.html">
Broadcast Receivers</a>, <a
href="{@docRoot}reference/android/R.styleable.html#AndroidManifestActivity">
Activities</a>, and <a
href="{@docRoot}reference/android/R.styleable.html#AndroidManifestService">
Services</a> are all declared in the application manifest.  If your IPC mechanism is
not intended for use by other applications, set the <a
href="{@docRoot}guide/topics/manifest/service-element.html#exported">{@code android:exported}</a>
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.</p>

<p>If your IPC is intended to be accessible to other applications, you can
apply a security policy by using the <a
href="{@docRoot}reference/android/R.styleable.html#AndroidManifestPermission">
Permission</a> tag. If IPC is between applications built by the same developer,
it is preferable to use <a
href="{@docRoot}guide/topics/manifest/permission-element.html#plevel">signature
level permissions</a>.  Signature permissions do not require user confirmation,
so they provide a better user experience and more controlled access to the IPC
mechanism.</p>

<p>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.</p>

<h3>Using intents</h3>

<p>Intents are the preferred mechanism for asynchronous IPC in Android.
Depending on your application requirements, you might use <code><a
href="{@docRoot}reference/android/content/Context.html#sendBroadcast(android.content.Intent)">sendBroadcast()</a></code>,
<code><a
href="{@docRoot}reference/android/content/Context.html#sendOrderedBroadcast(android.content.Intent,%20java.lang.String)">sendOrderedBroadcast()</a></code>,
or direct an intent to a specific application component.</p>

<p>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.</p>

<p>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.</p>

<h3>Using binder and AIDL interfaces</h3>

<p><a href="{@docRoot}reference/android/os/Binder.html">Binders</a> 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.</p>

<p>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.</p>

<p>If providing an interface that does require access controls, use <code><a
href="{@docRoot}reference/android/content/Context.html#checkCallingPermission(java.lang.String)">checkCallingPermission()</a></code>
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 <code><a
href="{@docRoot}reference/android/content/Context.html#bindService(android.content.Intent,%20android.content.ServiceConnection,%20int)">bindService()</a></code>
 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 <code><a
href="{@docRoot}reference/android/os/Binder.html#clearCallingIdentity()">
clearCallingIdentity()</a></code> to satisfy internal security checks.</p>

<h3>Using broadcast receivers</h3>

<p>Broadcast receivers are used to handle asynchronous requests initiated via
an intent.</p>

<p>By default, receivers are exported and can be invoked by any other
application. If your <code><a
href="{@docRoot}reference/android/content/BroadcastReceiver.html">
BroadcastReceivers</a></code> is intended for use by other applications, you
may want to apply security permissions to receivers using the <code><a
href="{@docRoot}guide/topics/manifest/receiver-element.html">
&lt;receiver&gt;</a></code> element within the application manifest.  This will
prevent applications without appropriate permissions from sending an intent to
the <code><a
href="{@docRoot}reference/android/content/BroadcastReceiver.html">
BroadcastReceivers</a></code>.</p>

<h3>Using Services</h3>

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

<p>By default, Services are exported and can be invoked by any other
application.  Services can be protected using the <a
href="{@docRoot}guide/topics/manifest/service-element.html#prmsn">{@code android:permission}</a>
attribute
within the manifest’s <code><a
href="{@docRoot}guide/topics/manifest/service-element.html">
&lt;service&gt;</a></code> tag. By doing so, other applications will need to declare
a corresponding <code><a
href="{@docRoot}guide/topics/manifest/uses-permission-element.html">&lt;uses-permission&gt;</a>
</code> element in their own manifest to be
able to start, stop, or bind to the service.</p>

<p>A Service can protect individual IPC calls into it with permissions, by
calling <code><a
href="{@docRoot}reference/android/content/Context.html#checkCallingPermission(java.lang.String)">checkCallingPermission()</a></code>
before executing
the implementation of that call.  We generally recommend using the
declarative permissions in the manifest, since those are less prone to
oversight.</p>

<h3>Using Activities</h3>

<p>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.</p>

<p>If you do expose an Activity for purposes of IPC, the  <code><a
href="{@docRoot}guide/topics/manifest/activity-element.html#prmsn">android:permission</a></code>
attribute in the  <code><a
href="{@docRoot}guide/topics/manifest/activity-element.html">
&lt;activity&gt;</a></code> declaration in the application manifest can be used to
restrict access to only those applications which have the stated
permissions.</p>

<a name="Permissions"></a>
<h2>Using Permissions</h2>

<h3>Requesting Permissions</h3>

<p>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.</p>

<p>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 <a
href="{@docRoot}reference/java/util/UUID.html">GUID</a> 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.</p>

<p>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: <a
href="http://www.eecs.berkeley.edu/Pubs/TechRpts/2011/EECS-2011-48.pdf">
http://www.eecs.berkeley.edu/Pubs/TechRpts/2011/EECS-2011-48.pdf</a></p>

<p>In addition to requesting permissions, your application can use <a
href="{@docRoot}guide/topics/manifest/permission-element.html">permissions</a>
to protect IPC that is security sensitive and will be exposed to other
applications -- such as a <code><a
href="{@docRoot}reference/android/content/ContentProvider.html">
ContentProvider</a></code>.  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 <a
href="{@docRoot}guide/topics/manifest/permission-element.html#plevel">signature
protection level</a> on permissions for IPC communication between applications
provided by a single developer.</p>

<p>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: <a
href="http://www.cs.berkeley.edu/~afelt/felt_usenixsec2011.pdf">http://www.cs.be
rkeley.edu/~afelt/felt_usenixsec2011.pdf</a></p>

<h3>Creating Permissions</h3>

<p>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 <a
href="{@docRoot}reference/android/Manifest.permission.html">system-defined
permissions</a> cover many situations.  Where appropriate,
perform access checks using existing permissions.</p>

<p>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.</p>

<p>If you create a Dangerous permission, there are a number of complexities
that you need to consider.
<ul>
<li>The permission must have a string that concisely expresses to a user the
security decision they will be required to make.</li>
<li>The permission string must be localized to many different languages.</li>
<li>Uses may choose not to install an application because a permission is
confusing or perceived as risky.</li>
<li>Applications may request the permission when the creator of the permission
has not been installed.</li>
</ul></p>

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

<a name="Networking"></a>
<h2>Using Networking</h2>

<h3>Using IP Networking</h3>

<p>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 <a
href="{@docRoot}reference/javax/net/ssl/HttpsURLConnection.html">HTTPS</a> 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.</p>

<p>Authenticated, encrypted socket-level communication can be easily
implemented using the <code><a
href="{@docRoot}reference/javax/net/ssl/SSLSocket.html">SSLSocket</a></code>
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.</p>

<p>We have seen some applications use <a
href="http://en.wikipedia.org/wiki/Localhost">localhost</a> 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.)</p>

<p>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 <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code> and
any responses to intents issued against HTTP.</p>

<h3>Using Telephony Networking</h3>

<p>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 <a
href="http://code.google.com/android/c2dm/">C2DM</a> and IP networking for
sending data messages to devices.</p>

<p>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.</p>

<a name="DynamicCode"></a>
<h2>Dynamically Loading Code</h2>

<p>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.</p>

<p>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.</p>

<p>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
<a href="{@docRoot}reference/dalvik/system/DexClassLoader.html">
<code>DexClassLoader</code></a>.  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.</p>


<h3>Using WebView</h3>

<p>Since WebView consumes web content that can include HTML and JavaScript,
improper use can introduce common web security issues such as <a
href="http://en.wikipedia.org/wiki/Cross_site_scripting">cross-site-scripting</a
> (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.</p>

<p>If your application does not directly use JavaScript within a <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code>, do
not call
<a href="{@docRoot}reference/android/webkit/WebSettings.html#setJavaScriptEnabled(boolean)">
<code>setJavaScriptEnabled()</code></a>. We have seen this method invoked
in sample code that might be repurposed in production application -- so
remove it if necessary. By default, <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code> does
not execute JavaScript so cross-site-scripting is not possible.</p>

<p>Use <code><a
href="{@docRoot}reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object,%20java.lang.String)">addJavaScriptInterface()</a></code> with
particular care because it allows JavaScript to invoke operations that are
normally reserved for Android applications.  Only expose <code><a
href="{@docRoot}reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object,%20java.lang.String)">addJavaScriptInterface()</a></code> 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 <code><a
href="{@docRoot}reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object,%20java.lang.String)">addJavaScriptInterface()</a></code> to
JavaScript that is contained within your application APK.</p>

<p>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 <a
href="http://en.wikipedia.org/wiki/Man-in-the-middle_attack">MiTM</a> attacks
and interception of data.  Sensitive capabilities using <code><a
href="{@docRoot}reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object,%20java.lang.String)">addJavaScriptInterface()</a></code> should
not ever be exposed to unverified script downloaded over HTTP. Note that even
with the use of HTTPS,
<code><a
href="{@docRoot}reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object,%20java.lang.String)">addJavaScriptInterface()</a></code>
increases the attack surface of your application to include the server
infrastructure and all CAs trusted by the Android-powered device.</p>

<p>If your application accesses sensitive data with a <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code>, you
may want to use the <code><a
href="{@docRoot}reference/android/webkit/WebView.html#clearCache(boolean)">
clearCache()</a></code> 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.</p>

<a name="Input"></a>
<h2>Performing Input Validation</h2>

<p>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.</p>

<p>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 <a
href="http://en.wikipedia.org/wiki/Buffer_overflow">buffer overflows</a>, <a
href="http://en.wikipedia.org/wiki/Double_free#Use_after_free">use after
free</a>, and <a
href="http://en.wikipedia.org/wiki/Off-by-one_error">off-by-one errors</a>.
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.</p>

<p>Dynamic, string based languages such as JavaScript and SQL are also subject
to input validation problems due to escape characters and <a
href="http://en.wikipedia.org/wiki/Code_injection">script injection</a>.</p>

<p>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.</p>

<p>If you are using <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code>, then
you must consider the possibility of XSS.  If your application does not
directly use JavaScript within a <code><a
href="{@docRoot}reference/android/webkit/WebView.html">WebView</a></code>, do
not call setJavaScriptEnabled() and XSS is no longer possible. If you must
enable JavaScript then the WebView section provides other security best
practices.</p>

<p>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.</p>

<a name="UserData"></a>
<h2>Handling User Data</h2>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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 <a
href="http://android-developers.blogspot.com/2011/03/identifying-app-installations.html">Android Developer Blog</a>.</p>

<p>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
<a href="{@docRoot}reference/android/Manifest.permission.html#READ_LOGS">
<code>READ_LOGS</code></a> 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.</p>


<h3>Handling Credentials</h3>

<p>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.</p>

<p>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.</p>

<p>Services that will be accessible to multiple applications should be accessed
using <code>
<a href="{@docRoot}reference/android/accounts/AccountManager.html">
AccountManager</a></code>. If possible, use the <code><a
href="{@docRoot}reference/android/accounts/AccountManager.html">
AccountManager</a></code> class to invoke a cloud-based service and do not store
passwords on the device.</p>

<p>After using <code><a
href="{@docRoot}reference/android/accounts/AccountManager.html">
AccountManager</a></code> to retrieve an Account, check the <code><a
href="{@docRoot}reference/android/accounts/Account.html#CREATOR">CREATOR</a>
</code> before passing in any credentials, so that you do not inadvertently pass
credentials to the wrong application.</p>

<p>If credentials are to be used only by applications that you create, then you
can verify the application which accesses the <code><a
href="{@docRoot}reference/android/accounts/AccountManager.html">
AccountManager</a></code> using <code><a
href="{@docRoot}reference/android/content/pm/PackageManager.html#checkSignatures(java.lang.String,%20java.lang.String)">checkSignature()</a></code>.
Alternatively, if only one application will use the credential, you might use a
{@link java.security.KeyStore} for
storage.</p>

<a name="Crypto"></a>
<h2>Using Cryptography</h2>

<p>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.</p>

<p>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
<a href="{@docRoot}reference/javax/net/ssl/HttpsURLConnection.html">
<code>HttpsURLConnection</code></a> or <code><a
href="{@docRoot}reference/javax/net/ssl/SSLSocket.html">SSLSocket</a></code>,
rather than writing your own protocol.</p>

<p>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 <code><a
href="{@docRoot}reference/javax/crypto/Cipher.html">Cipher</a></code> class.</p>

<p>Use a secure random number generator (
<a href="{@docRoot}reference/java/security/SecureRandom.html">
<code>SecureRandom</code></a>) to initialize any cryptographic keys (<a
href="{@docRoot}reference/javax/crypto/KeyGenerator.html">
<code>KeyGenerator</code></a>). 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.</p>

<p>If you need to store a key for repeated use, use a mechanism like {@link java.security.KeyStore} that
provides a mechanism for long term storage and retrieval of cryptographic
keys.</p>

<h2>Conclusion</h2>

<p>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.</p>

<p>You can receive more information on these topics and discuss security best
practices with other developers in the <a
href="http://groups.google.com/group/android-security-discuss">Android Security
Discuss</a> Google Group</p>