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# iOS App Extensions **Content copied form** [**https://mobile-security.gitbook.io/mobile-security-testing-guide/ios-testing-guide/0x06h-testing-platform-interaction#app-extensions**](https://mobile-security.gitbook.io/mobile-security-testing-guide/ios-testing-guide/0x06h-testing-platform-interaction#app-extensions) App extensions let apps offer custom functionality and content to users while they’re interacting with other apps or the system. Some notable ones are: * **Custom Keyboard**: replaces the iOS system keyboard with a custom keyboard for use in all apps. * **Share**: post to a sharing website or share content with others. * **Today**: also called **widgets**, they offer content or perform quick tasks in the Today view of Notification Center. For example, the user selects text in the _host app_, clicks on the "Share" button and selects one "app" or action from the list. This triggers the _app extension_ of the _containing app_. The app extension displays its view within the context of the host app and uses the items provided by the host app, the selected text in this case, to perform a specific task (post it on a social network, for example). See this picture from the [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionOverview.html#//apple\_ref/doc/uid/TP40014214-CH2-SW13) which pretty good summarizes this: ![](https://gblobscdn.gitbook.com/assets%2F-LH00RC4WVf3-6Ou4e0l%2F-Lf1APQHyCHdAvoJSvc\_%2F-Lf1AQx9khfTwUwYuMti%2Fapp\_extensions\_communication.png?alt=media) ### **Security Considerations** From the security point of view it is important to note that: * An **app extension does never communicate directly with its containing app** (typically, it isn’t even running while the contained app extension is running). * An **app extension** and the **host app** **communicate** via **inter-process** communication. * An **app extension’s** containing app and the **host app don’t communicate** at all. * A **Today** **widget** (and no other app extension type) can ask the system to open its containing app by calling the `openURL:completionHandler:` method of the `NSExtensionContext` class. * Any **app extension** and its **containing app** can **access shared data in a privately** defined shared container. * App extensions **cannot access some APIs**, for example, HealthKit. * They **cannot receive data using AirDrop** but do can send data. * **No long-running background tasks** are allowed but uploads or downloads can be initiated. * App extensions **cannot access the camera or microphone on an iOS device** (except for iMessage app extensions). ### Static analysis #### **Verifying if the App Contains App Extensions** If you have the original source code you can search for all occurrences of `NSExtensionPointIdentifier` with Xcode (cmd+shift+f) or take a look into "Build Phases / Embed App extensions": ![](<../../.gitbook/assets/image (496).png>) There you can find the names of all embedded app extensions followed by `.appex`, now you can navigate to the individual app extensions in the project. If not having the original source code: Grep for `NSExtensionPointIdentifier` among all files inside the app bundle (IPA or installed app): ```bash $ grep -nr NSExtensionPointIdentifier Payload/Telegram\ X.app/ Binary file Payload/Telegram X.app//PlugIns/SiriIntents.appex/Info.plist matches Binary file Payload/Telegram X.app//PlugIns/Share.appex/Info.plist matches Binary file Payload/Telegram X.app//PlugIns/NotificationContent.appex/Info.plist matches Binary file Payload/Telegram X.app//PlugIns/Widget.appex/Info.plist matches Binary file Payload/Telegram X.app//Watch/Watch.app/PlugIns/Watch Extension.appex/Info.plist matches ``` You can also access per SSH, find the app bundle and list all inside PlugIns (they are placed there by default) or do it with objection: ```bash ph.telegra.Telegraph on (iPhone: 11.1.2) [usb] # cd PlugIns /var/containers/Bundle/Application/15E6A58F-1CA7-44A4-A9E0-6CA85B65FA35/ Telegram X.app/PlugIns ph.telegra.Telegraph on (iPhone: 11.1.2) [usb] # ls NSFileType Perms NSFileProtection Read Write Name ------------ ------- ------------------ ------ ------- ------------------------- Directory 493 None True False NotificationContent.appex Directory 493 None True False Widget.appex Directory 493 None True False Share.appex Directory 493 None True False SiriIntents.appex ``` We can see now the same four app extensions that we saw in Xcode before. #### **Determining the Supported Data Types** This is important for data being shared with host apps (e.g. via Share or Action Extensions). When the user selects some data type in a host app and it matches the data types define here, the host app will offer the extension. It is worth noticing the difference between this and data sharing via `UIActivity` where we had to define the document types, also using UTIs. An app does not need to have an extension for that. It is possible to share data using only `UIActivity`. Inspect the app extension's `Info.plist` file and search for `NSExtensionActivationRule`. That key specifies the data being supported as well as e.g. maximum of items supported. For example: ```markup NSExtensionAttributes NSExtensionActivationRule NSExtensionActivationSupportsImageWithMaxCount 10 NSExtensionActivationSupportsMovieWithMaxCount 1 NSExtensionActivationSupportsWebURLWithMaxCount 1 ``` Only the data types present here and not having `0` as `MaxCount` will be supported. However, more complex filtering is possible by using a so-called predicate string that will evaluate the UTIs given. Please refer to the [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple\_ref/doc/uid/TP40014214-CH21-SW8) for more detailed information about this. **Checking Data Sharing with the Containing App** Remember that app extensions and their containing apps do not have direct access to each other’s containers. However, data sharing can be enabled. This is done via ["App Groups"](https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/EntitlementKeyReference/Chapters/EnablingAppSandbox.html#//apple\_ref/doc/uid/TP40011195-CH4-SW19) and the [`NSUserDefaults`](https://developer.apple.com/documentation/foundation/nsuserdefaults) API. See this figure from [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple\_ref/doc/uid/TP40014214-CH21-SW11): ![](broken-reference) As also mentioned in the guide, the app must set up a shared container if the app extension uses the `NSURLSession` class to perform a background upload or download, so that both the extension and its containing app can access the transferred data. **Verifying if the App Restricts the Use of App Extensions** It is possible to reject a specific type of app extension by using the following method: * [`application:shouldAllowExtensionPointIdentifier:`](https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623122-application?language=objc) However, it is currently only possible for "custom keyboard" app extensions (and should be verified when testing apps handling sensitive data via the keyboard like e.g. banking apps). ### Dynamic Analysis For the dynamic analysis we can do the following to gain knowledge without having the source code: * Inspecting the items being shared * Identifying the app extensions involved **Inspecting the Items Being Shared** For this we should hook `NSExtensionContext - inputItems` in the data originating app. Following the previous example of Telegram we will now use the "Share" button on a text file (that was received from a chat) to create a note in the Notes app with it: ![](<../../.gitbook/assets/image (497).png>) If we run a trace, we'd see the following output: ```bash (0x1c06bb420) NSExtensionContext - inputItems 0x18284355c Foundation!-[NSExtension _itemProviderForPayload:extensionContext:] 0x1828447a4 Foundation!-[NSExtension _loadItemForPayload:contextIdentifier:completionHandler:] 0x182973224 Foundation!__NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S3__ 0x182971968 Foundation!-[NSXPCConnection _decodeAndInvokeMessageWithEvent:flags:] 0x182748830 Foundation!message_handler 0x181ac27d0 libxpc.dylib!_xpc_connection_call_event_handler 0x181ac0168 libxpc.dylib!_xpc_connection_mach_event ... RET: ( " - userInfo: { NSExtensionItemAttachmentsKey = ( " {types = (\n \"public.plain-text\",\n \"public.file-url\"\n)}" ); }" ) ``` Here we can observe that: * This occurred under-the-hood via XPC, concretely it is implemented via a `NSXPCConnection` that uses the `libxpc.dylib` Framework. * The UTIs included in the `NSItemProvider` are `public.plain-text` and `public.file-url`, the latter being included in `NSExtensionActivationRule` from the [`Info.plist` of the "Share Extension" of Telegram](https://github.com/TelegramMessenger/Telegram-iOS/blob/master/Telegram/Share/Info.plist). **Identifying the App Extensions Involved** You can also find out which app extension is taking care of your the requests and responses by hooking `NSExtension - _plugIn`: We run the same example again: ```bash (0x1c0370200) NSExtension - _plugIn RET: (0x1c0372300) -[NSExtension _plugIn] RET: ``` As you can see there are two app extensions involved: * `Share.appex` is sending the text file (`public.plain-text` and `public.file-url`). * `com.apple.mobilenotes.SharingExtension.appex` which is receiving and will process the text file. If you want to learn more about what's happening under-the-hood in terms of XPC, we recommend to take a look at the internal calls from "libxpc.dylib". For example you can use [`frida-trace`](https://www.frida.re/docs/frida-trace/) and then dig deeper into the methods that you find more interesting by extending the automatically generated stubs. ###
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