hacktricks/mobile-pentesting/ios-pentesting/ios-custom-uri-handlers-deeplinks-custom-schemes.md

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This is a sumary from the related information from [https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0075/](https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0075/)

## Basic Information

Custom URL schemes enable apps to communicate using a custom protocol, as detailed in the [Apple Developer Documentation](https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/Inter-AppCommunication/Inter-AppCommunication.html#//apple_ref/doc/uid/TP40007072-CH6-SW1). These schemes must be declared by the app, which then handles incoming URLs following those schemes. It's crucial to **validate all URL parameters** and **discard any malformed URLs** to prevent attacks through this vector.

An example is given where the URI `myapp://hostname?data=123876123` invokes a specific application action. A noted vulnerability was in the Skype Mobile app, which allowed unpermitted call actions via the `skype://` protocol. The registered schemes can be found in the app's `Info.plist` under `CFBundleURLTypes`. Malicious applications can exploit this by re-registering URIs to intercept sensitive information.

### Application Query Schemes Registration

From iOS 9.0, to check if an app is available, `canOpenURL:` requires declaring URL schemes in the `Info.plist` under `LSApplicationQueriesSchemes`. This limits the schemes an app can query to 50, enhancing privacy by preventing app enumeration.

```xml
<key>LSApplicationQueriesSchemes</key>
<array>
    <string>url_scheme1</string>
    <string>url_scheme2</string>
</array>
```

### Testing URL Handling and Validation

Developers should inspect specific methods in the source code to understand URL path construction and validation, such as `application:didFinishLaunchingWithOptions:` and `application:openURL:options:`. For instance, Telegram employs various methods for opening URLs:

```swift
func application(_ application: UIApplication, open url: URL, sourceApplication: String?) -> Bool {
    self.openUrl(url: url)
    return true
}

func application(_ application: UIApplication, open url: URL, sourceApplication: String?,
annotation: Any) -> Bool {
    self.openUrl(url: url)
    return true
}

func application(_ app: UIApplication, open url: URL,
options: [UIApplicationOpenURLOptionsKey : Any] = [:]) -> Bool {
    self.openUrl(url: url)
    return true
}

func application(_ application: UIApplication, handleOpen url: URL) -> Bool {
    self.openUrl(url: url)
    return true
}
```

### Testing URL Requests to Other Apps

Methods like `openURL:options:completionHandler:` are crucial for opening URLs to interact with other apps. Identifying usage of such methods in the app's source code is key for understanding external communications.

### Testing for Deprecated Methods

Deprecated methods handling URL openings, such as `application:handleOpenURL:` and `openURL:`, should be identified and reviewed for security implications.

### Fuzzing URL Schemes

Fuzzing URL schemes can identify memory corruption bugs. Tools like [Frida](https://codeshare.frida.re/@dki/ios-url-scheme-fuzzing/) can automate this process by opening URLs with varying payloads to monitor for crashes, exemplified by the manipulation of URLs in the iGoat-Swift app:

```bash
$ frida -U SpringBoard -l ios-url-scheme-fuzzing.js
[iPhone::SpringBoard]-> fuzz("iGoat", "iGoat://?contactNumber={0}&message={0}")
Watching for crashes from iGoat...
No logs were moved.
Opened URL: iGoat://?contactNumber=0&message=0
```


## References
* [https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0075/](https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0075/)

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