Permissions
Google I/O 2015—Android M Permissions: Best Practices for Developers
Android is a privilege-separated operating system, in which each application runs with a distinct system identity (Linux user ID and group ID). Parts of the system are also separated into distinct identities. Linux thereby isolates applications from each other and from the system.
Additional finer-grained security features are provided through a "permission" mechanism that enforces restrictions on the specific operations that a particular process can perform, and per-URI permissions for granting ad hoc access to specific pieces of data.
This document describes how application developers can use the security features provided by Android. A more general Android Security Overview is provided in the Android Open Source Project.
A central design point of the Android security architecture is that no application, by default, has permission to perform any operations that would adversely impact other applications, the operating system, or the user. This includes reading or writing the user's private data (such as contacts or emails), reading or writing another application's files, performing network access, keeping the device awake, and so on.
Because each Android application operates in a process sandbox, applications must explicitly share resources and data. They do this by declaring the permissions they need for additional capabilities not provided by the basic sandbox. Applications statically declare the permissions they require, and the Android system prompts the user for consent.
The application sandbox does not depend on the technology used to build an application. In particular the Dalvik VM is not a security boundary, and any app can run native code (see the Android NDK). All types of applications — Java, native, and hybrid — are sandboxed in the same way and have the same degree of security from each other.
All APKs ({@code .apk} files) must be signed with a certificate whose private key is held by their developer. This certificate identifies the author of the application. The certificate does not need to be signed by a certificate authority; it is perfectly allowable, and typical, for Android applications to use self-signed certificates. The purpose of certificates in Android is to distinguish application authors. This allows the system to grant or deny applications access to signature-level permissions and to grant or deny an application's request to be given the same Linux identity as another application.
At install time, Android gives each package a distinct Linux user ID. The identity remains constant for the duration of the package's life on that device. On a different device, the same package may have a different UID; what matters is that each package has a distinct UID on a given device.
Because security enforcement happens at the
process level, the code of any two packages cannot normally
run in the same process, since they need to run as different Linux users.
You can use the {@link android.R.attr#sharedUserId} attribute in the
AndroidManifest.xml's
{@link android.R.styleable#AndroidManifest manifest} tag of each package to
have them assigned the same user ID. By doing this, for purposes of security
the two packages are then treated as being the same application, with the same
user ID and file permissions. Note that in order to retain security, only two applications
signed with the same signature (and requesting the same sharedUserId) will
be given the same user ID.
Any data stored by an application will be assigned that application's user ID, and not normally accessible to other packages. When creating a new file with {@link android.content.Context#getSharedPreferences}, {@link android.content.Context#openFileOutput}, or {@link android.content.Context#openOrCreateDatabase}, you can use the {@link android.content.Context#MODE_WORLD_READABLE} and/or {@link android.content.Context#MODE_WORLD_WRITEABLE} flags to allow any other package to read/write the file. When setting these flags, the file is still owned by your application, but its global read and/or write permissions have been set appropriately so any other application can see it.
A basic Android application has no permissions associated with it by default,
meaning it cannot do anything that would adversely impact the user experience
or any data on the device. To make use of protected features of the device,
you must include one or more
<uses-permission>
tags in your app
manifest.
For example, an application that needs to monitor incoming SMS messages would specify:
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.android.app.myapp" >
<uses-permission android:name="android.permission.RECEIVE_SMS" />
...
</manifest>
For more information about the different protection levels for permissions, see Normal and Dangerous Permissions.
If your app lists normal permissions in its manifest (that is, permissions that don't pose much risk to the user's privacy or the device's operation), the system automatically grants those permissions. If your app lists dangerous permissions in its manifest (that is, permissions that could potentially affect the user's privacy or the device's normal operation), the system asks the user to explicitly grant those permissions. The way Android makes the requests depends on the system version, and the system version targeted by your app:
targetSdkVersion
is 23 or higher, the app requests permissions from the user at run-time.
The user can revoke the permissions at any time, so the app needs to
check whether it has the permissions every time it runs.
For more information about requesting permissions in your app, see the
Working with System
Permissions training guide.
targetSdkVersion
is 22 or lower, the system asks the user to grant the permissions when the
user installs the app. If you add a new permission to an updated version of
the app, the system asks the user to grant that permission when the user
updates the app. Once the user installs the app, the only way they can
revoke the permission is by uninstalling the app.
Often times a permission failure will result in a {@link java.lang.SecurityException} being thrown back to the application. However, this is not guaranteed to occur everywhere. For example, the {@link android.content.Context#sendBroadcast} method checks permissions as data is being delivered to each receiver, after the method call has returned, so you will not receive an exception if there are permission failures. In almost all cases, however, a permission failure will be printed to the system log.
The permissions provided by the Android system can be found at {@link android.Manifest.permission}. Any application may also define and enforce its own permissions, so this is not a comprehensive list of all possible permissions.
A particular permission may be enforced at a number of places during your program's operation:
Over time, new restrictions may be added to the platform such that, in order to use certain APIs, your app must request a permission that it previously did not need. Because existing apps assume access to those APIs is freely available, Android may apply the new permission request to the app's manifest to avoid breaking the app on the new platform version. Android makes the decision as to whether an app might need the permission based on the value provided for the {@code targetSdkVersion} attribute. If the value is lower than the version in which the permission was added, then Android adds the permission.
For example, the {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE} permission was added in API level 4 to restrict access to the shared storage space. If your {@code targetSdkVersion} is 3 or lower, this permission is added to your app on newer versions of Android.
Caution: If a permission is automatically added to your app, your app listing on Google Play lists these additional permissions even though your app might not actually require them.
To avoid this and remove the default permissions you don't need, always update your {@code targetSdkVersion} to be as high as possible. You can see which permissions were added with each release in the {@link android.os.Build.VERSION_CODES} documentation.
System permissions are divided into several protection levels. The two most important protection levels to know about are normal and dangerous permissions:
There are a couple of permissions that don't behave like normal and dangerous permissions. {@link android.Manifest.permission#SYSTEM_ALERT_WINDOW SYSTEM_ALERT_WINDOW} and {@link android.Manifest.permission#WRITE_SETTINGS WRITE_SETTINGS} are particularly sensitive, so most apps should not use them. If an app needs one of these permissions, it must declare the permission in the manifest, and send an intent requesting the user's authorization. The system responds to the intent by showing a detailed management screen to the user.
For details on how to request these permissions, see the {@link android.Manifest.permission#SYSTEM_ALERT_WINDOW SYSTEM_ALERT_WINDOW} and {@link android.Manifest.permission#WRITE_SETTINGS WRITE_SETTINGS} reference entries.
All dangerous Android system permissions belong to permission groups.
If the device is running Android 6.0 (API level 23) and the app's targetSdkVersion is 23 or higher, the following system
behavior applies when your app requests a dangerous permission:
Any permission can belong to a permission group, including normal permissions and permissions defined by your app. However, a permission's group only affects the user experience if the permission is dangerous. You can ignore the permission group for normal permissions.
If the device is running Android 5.1 (API level 22) or lower, or the app's
targetSdkVersion is 22 or lower, the system asks the user
to grant the permissions at install time. Once again, the system just tells
the user what permission groups the app needs, not the individual
permissions.
Table 1. Dangerous permissions and permission groups.
| Permission Group | Permissions |
|---|---|
| {@link android.Manifest.permission_group#CALENDAR CALENDAR} |
|
| {@link android.Manifest.permission_group#CAMERA CAMERA} |
|
| {@link android.Manifest.permission_group#CONTACTS CONTACTS} |
|
| {@link android.Manifest.permission_group#LOCATION LOCATION} |
|
| {@link android.Manifest.permission_group#MICROPHONE MICROPHONE} |
|
| {@link android.Manifest.permission_group#PHONE PHONE} |
|
| {@link android.Manifest.permission_group#SENSORS SENSORS} |
|
| {@link android.Manifest.permission_group#SMS SMS} |
|
| {@link android.Manifest.permission_group#STORAGE STORAGE} |
|
To enforce your own permissions, you must first declare them in your
AndroidManifest.xml using one or more <permission>
elements.
For example, an application that wants to control who can start one of its activities could declare a permission for this operation as follows:
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.myapp" >
<permission android:name="com.example.myapp.permission.DEADLY_ACTIVITY"
android:label="@string/permlab_deadlyActivity"
android:description="@string/permdesc_deadlyActivity"
android:permissionGroup="android.permission-group.COST_MONEY"
android:protectionLevel="dangerous" />
...
</manifest>
Note: The system does not allow multiple packages to declare
a permission with the same name, unless all the packages are signed with the
same certificate. If a package declares a permission, the system does not permit
the user to install other packages with the same permission name, unless
those packages are signed with the same certificate as the first package. To
avoid naming collisions, we recommend using reverse-domain-style naming for custom
permissions, for example com.example.myapp.ENGAGE_HYPERSPACE.
The {@link android.R.styleable#AndroidManifestPermission_protectionLevel protectionLevel} attribute is required, telling the system how the user is to be informed of applications requiring the permission, or who is allowed to hold that permission, as described in the linked documentation.
The android:permissionGroup
attribute is optional, and only used to help the system display permissions
to the user. In most cases you will want to set this to a standard system
group (listed in {@link android.Manifest.permission_group
android.Manifest.permission_group}), although you can define a group yourself.
It is preferable to use an existing group, as this simplifies the
permission UI shown to the user.
You need to supply both a label and description for the
permission. These are string resources that the user can see when
they are viewing a list of permissions
({@link android.R.styleable#AndroidManifestPermission_label android:label})
or details on a single permission (
{@link android.R.styleable#AndroidManifestPermission_description android:description}).
The label should be short; a few words
describing the key piece of functionality the permission is protecting. The
description should be a couple of sentences describing what the permission allows
a holder to do. Our convention is a two-sentence description:
the first sentence describes the permission, and the second sentence warns the
user of the type of things that can go wrong if an application is granted the
permission.
Here is an example of a label and description for the CALL_PHONE permission:
<string name="permlab_callPhone">directly call phone numbers</string>
<string name="permdesc_callPhone">Allows the application to call
phone numbers without your intervention. Malicious applications may
cause unexpected calls on your phone bill. Note that this does not
allow the application to call emergency numbers.</string>
You can view at the permissions currently defined in the system using the
Settings app and the shell command adb shell pm list permissions.
To use the Settings app, go to Settings > Applications. Pick an app and
scroll down to see the permissions that the app uses. For developers, the adb '-s'
option displays the permissions in a form similar to how the user will see them:
$ adb shell pm list permissions -s All Permissions: Network communication: view Wi-Fi state, create Bluetooth connections, full Internet access, view network state Your location: access extra location provider commands, fine (GPS) location, mock location sources for testing, coarse (network-based) location Services that cost you money: send SMS messages, directly call phone numbers ...
Apps can define their own custom permissions and request custom permissions
from other apps by defining <uses-permission> elements.
However, you should carefully assess whether it is necessary for your app to
do so.
<uses-permission>
element.
You can apply high-level permissions restricting access to entire components
of the system or application through your
AndroidManifest.xml. To do this, include an {@link
android.R.attr#permission android:permission} attribute on the desired
component, naming the permission that controls access to
it.
{@link android.app.Activity} permissions (applied to the {@link android.R.styleable#AndroidManifestActivity <activity>} tag) restrict who can start the associated activity. The permission is checked during {@link android.content.Context#startActivity Context.startActivity()} and {@link android.app.Activity#startActivityForResult Activity.startActivityForResult()}; if the caller does not have the required permission then {@link java.lang.SecurityException} is thrown from the call.
{@link android.app.Service} permissions (applied to the {@link android.R.styleable#AndroidManifestService <service>} tag) restrict who can start or bind to the associated service. The permission is checked during {@link android.content.Context#startService Context.startService()}, {@link android.content.Context#stopService Context.stopService()} and {@link android.content.Context#bindService Context.bindService()}; if the caller does not have the required permission then {@link java.lang.SecurityException} is thrown from the call.
{@link android.content.BroadcastReceiver} permissions (applied to the {@link android.R.styleable#AndroidManifestReceiver <receiver>} tag) restrict who can send broadcasts to the associated receiver. The permission is checked after {@link android.content.Context#sendBroadcast Context.sendBroadcast()} returns, as the system tries to deliver the submitted broadcast to the given receiver. As a result, a permission failure will not result in an exception being thrown back to the caller; it will just not deliver the intent. In the same way, a permission can be supplied to {@link android.content.Context#registerReceiver(android.content.BroadcastReceiver, android.content.IntentFilter, String, android.os.Handler) Context.registerReceiver()} to control who can broadcast to a programmatically registered receiver. Going the other way, a permission can be supplied when calling {@link android.content.Context#sendBroadcast(Intent, String) Context.sendBroadcast()} to restrict which BroadcastReceiver objects are allowed to receive the broadcast (see below).
{@link android.content.ContentProvider} permissions (applied to the {@link android.R.styleable#AndroidManifestProvider <provider>} tag) restrict who can access the data in a {@link android.content.ContentProvider}. (Content providers have an important additional security facility available to them called URI permissions which is described later.) Unlike the other components, there are two separate permission attributes you can set: {@link android.R.attr#readPermission android:readPermission} restricts who can read from the provider, and {@link android.R.attr#writePermission android:writePermission} restricts who can write to it. Note that if a provider is protected with both a read and write permission, holding only the write permission does not mean you can read from a provider. The permissions are checked when you first retrieve a provider (if you don't have either permission, a SecurityException will be thrown), and as you perform operations on the provider. Using {@link android.content.ContentResolver#query ContentResolver.query()} requires holding the read permission; using {@link android.content.ContentResolver#insert ContentResolver.insert()}, {@link android.content.ContentResolver#update ContentResolver.update()}, {@link android.content.ContentResolver#delete ContentResolver.delete()} requires the write permission. In all of these cases, not holding the required permission results in a {@link java.lang.SecurityException} being thrown from the call.
In addition to the permission enforcing who can send Intents to a registered {@link android.content.BroadcastReceiver} (as described above), you can also specify a required permission when sending a broadcast. By calling {@link android.content.Context#sendBroadcast(android.content.Intent,String) Context.sendBroadcast()} with a permission string, you require that a receiver's application must hold that permission in order to receive your broadcast.
Note that both a receiver and a broadcaster can require a permission. When this happens, both permission checks must pass for the Intent to be delivered to the associated target.
Arbitrarily fine-grained permissions can be enforced at any call into a service. This is accomplished with the {@link android.content.Context#checkCallingPermission Context.checkCallingPermission()} method. Call with a desired permission string and it will return an integer indicating whether that permission has been granted to the current calling process. Note that this can only be used when you are executing a call coming in from another process, usually through an IDL interface published from a service or in some other way given to another process.
There are a number of other useful ways to check permissions. If you have the pid of another process, you can use the Context method {@link android.content.Context#checkPermission(String, int, int) Context.checkPermission(String, int, int)} to check a permission against that pid. If you have the package name of another application, you can use the direct PackageManager method {@link android.content.pm.PackageManager#checkPermission(String, String) PackageManager.checkPermission(String, String)} to find out whether that particular package has been granted a specific permission.
The standard permission system described so far is often not sufficient when used with content providers. A content provider may want to protect itself with read and write permissions, while its direct clients also need to hand specific URIs to other applications for them to operate on. A typical example is attachments in a mail application. Access to the mail should be protected by permissions, since this is sensitive user data. However, if a URI to an image attachment is given to an image viewer, that image viewer will not have permission to open the attachment since it has no reason to hold a permission to access all e-mail.
The solution to this problem is per-URI permissions: when starting an activity or returning a result to an activity, the caller can set {@link android.content.Intent#FLAG_GRANT_READ_URI_PERMISSION Intent.FLAG_GRANT_READ_URI_PERMISSION} and/or {@link android.content.Intent#FLAG_GRANT_WRITE_URI_PERMISSION Intent.FLAG_GRANT_WRITE_URI_PERMISSION}. This grants the receiving activity permission access the specific data URI in the Intent, regardless of whether it has any permission to access data in the content provider corresponding to the Intent.
This mechanism allows a common capability-style model where user interaction (opening an attachment, selecting a contact from a list, etc) drives ad-hoc granting of fine-grained permission. This can be a key facility for reducing the permissions needed by applications to only those directly related to their behavior.
The granting of fine-grained URI permissions does, however, require some cooperation with the content provider holding those URIs. It is strongly recommended that content providers implement this facility, and declare that they support it through the {@link android.R.styleable#AndroidManifestProvider_grantUriPermissions android:grantUriPermissions} attribute or {@link android.R.styleable#AndroidManifestGrantUriPermission <grant-uri-permissions>} tag.
More information can be found in the {@link android.content.Context#grantUriPermission Context.grantUriPermission()}, {@link android.content.Context#revokeUriPermission Context.revokeUriPermission()}, and {@link android.content.Context#checkUriPermission Context.checkUriPermission()} methods.