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176 lines
11 KiB
Markdown
176 lines
11 KiB
Markdown
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# iOS App Extensions
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**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)\*\*\*\*
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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:
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* **Custom Keyboard**: replaces the iOS system keyboard with a custom keyboard for use in all apps.
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* **Share**: post to a sharing website or share content with others.
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* **Today**: also called **widgets**, they offer content or perform quick tasks in the Today view of Notification Center.
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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:
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### **Security Considerations**
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From the security point of view it is important to note that:
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* An **app extension does never communicate directly with its containing app** \(typically, it isn’t even running while the contained app extension is running\).
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* An **app extension** and the **host app** **communicate** via **inter-process** communication.
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* An **app extension’s** containing app and the **host app don’t communicate** at all.
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* 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.
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* Any **app extension** and its **containing app** can **access shared data in a privately** defined shared container.
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* App extensions **cannot access some APIs**, for example, HealthKit.
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* They **cannot receive data using AirDrop** but do can send data.
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* **No long-running background tasks** are allowed but uploads or downloads can be initiated.
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* App extensions **cannot access the camera or microphone on an iOS device** \(except for iMessage app extensions\).
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### Static analysis
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#### **Verifying if the App Contains App Extensions**
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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":
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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.
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If not having the original source code:
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Grep for `NSExtensionPointIdentifier` among all files inside the app bundle \(IPA or installed app\):
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```bash
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$ grep -nr NSExtensionPointIdentifier Payload/Telegram\ X.app/
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Binary file Payload/Telegram X.app//PlugIns/SiriIntents.appex/Info.plist matches
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Binary file Payload/Telegram X.app//PlugIns/Share.appex/Info.plist matches
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Binary file Payload/Telegram X.app//PlugIns/NotificationContent.appex/Info.plist matches
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Binary file Payload/Telegram X.app//PlugIns/Widget.appex/Info.plist matches
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Binary file Payload/Telegram X.app//Watch/Watch.app/PlugIns/Watch Extension.appex/Info.plist matches
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```
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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:
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```bash
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ph.telegra.Telegraph on (iPhone: 11.1.2) [usb] # cd PlugIns
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/var/containers/Bundle/Application/15E6A58F-1CA7-44A4-A9E0-6CA85B65FA35/
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Telegram X.app/PlugIns
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ph.telegra.Telegraph on (iPhone: 11.1.2) [usb] # ls
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NSFileType Perms NSFileProtection Read Write Name
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------------ ------- ------------------ ------ ------- -------------------------
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Directory 493 None True False NotificationContent.appex
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Directory 493 None True False Widget.appex
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Directory 493 None True False Share.appex
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Directory 493 None True False SiriIntents.appex
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```
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We can see now the same four app extensions that we saw in Xcode before.
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#### **Determining the Supported Data Types**
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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`.
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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:
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```markup
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<key>NSExtensionAttributes</key>
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<dict>
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<key>NSExtensionActivationRule</key>
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<dict>
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<key>NSExtensionActivationSupportsImageWithMaxCount</key>
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<integer>10</integer>
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<key>NSExtensionActivationSupportsMovieWithMaxCount</key>
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<integer>1</integer>
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<key>NSExtensionActivationSupportsWebURLWithMaxCount</key>
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<integer>1</integer>
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</dict>
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</dict>
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```
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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.
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**Checking Data Sharing with the Containing App**
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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):
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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.
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**Verifying if the App Restricts the Use of App Extensions**
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It is possible to reject a specific type of app extension by using the following method:
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* [`application:shouldAllowExtensionPointIdentifier:`](https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623122-application?language=objc)
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However, it is currently only possible for "custom keyboard" app extensions \(and should be verified when testing apps handling sensitive data via the keyboard like e.g. banking apps\).
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### Dynamic Analysis
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For the dynamic analysis we can do the following to gain knowledge without having the source code:
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* Inspecting the items being shared
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* Identifying the app extensions involved
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**Inspecting the Items Being Shared**
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For this we should hook `NSExtensionContext - inputItems` in the data originating app.
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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:
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If we run a trace, we'd see the following output:
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```bash
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(0x1c06bb420) NSExtensionContext - inputItems
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0x18284355c Foundation!-[NSExtension _itemProviderForPayload:extensionContext:]
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0x1828447a4 Foundation!-[NSExtension _loadItemForPayload:contextIdentifier:completionHandler:]
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0x182973224 Foundation!__NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S3__
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0x182971968 Foundation!-[NSXPCConnection _decodeAndInvokeMessageWithEvent:flags:]
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0x182748830 Foundation!message_handler
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0x181ac27d0 libxpc.dylib!_xpc_connection_call_event_handler
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0x181ac0168 libxpc.dylib!_xpc_connection_mach_event
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...
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RET: (
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"<NSExtensionItem: 0x1c420a540> - userInfo:
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{
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NSExtensionItemAttachmentsKey = (
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"<NSItemProvider: 0x1c46b30e0> {types = (\n \"public.plain-text\",\n \"public.file-url\"\n)}"
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);
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}"
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)
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```
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Here we can observe that:
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* This occurred under-the-hood via XPC, concretely it is implemented via a `NSXPCConnection` that uses the `libxpc.dylib` Framework.
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* 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).
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**Identifying the App Extensions Involved**
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You can also find out which app extension is taking care of your the requests and responses by hooking `NSExtension - _plugIn`:
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We run the same example again:
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```bash
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(0x1c0370200) NSExtension - _plugIn
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RET: <PKPlugin: 0x1163637f0 ph.telegra.Telegraph.Share(5.3) 5B6DE177-F09B-47DA-90CD-34D73121C785
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1(2) /private/var/containers/Bundle/Application/15E6A58F-1CA7-44A4-A9E0-6CA85B65FA35
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/Telegram X.app/PlugIns/Share.appex>
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(0x1c0372300) -[NSExtension _plugIn]
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RET: <PKPlugin: 0x10bff7910 com.apple.mobilenotes.SharingExtension(1.5) 73E4F137-5184-4459-A70A-83
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F90A1414DC 1(2) /private/var/containers/Bundle/Application/5E267B56-F104-41D0-835B-F1DAB9AE076D
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/MobileNotes.app/PlugIns/com.apple.mobilenotes.SharingExtension.appex>
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```
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As you can see there are two app extensions involved:
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* `Share.appex` is sending the text file \(`public.plain-text` and `public.file-url`\).
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* `com.apple.mobilenotes.SharingExtension.appex` which is receiving and will process the text file.
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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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###
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