# iOS Universal Links
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## Introduction
Universal links offer a **seamless redirection** experience to users by directly opening content in the app, bypassing the need for Safari redirection. These links are **unique** and secure, as they cannot be claimed by other apps. This is ensured by hosting a `apple-app-site-association` JSON file on the website's root directory, establishing a verifiable link between the website and the app. In cases where the app is not installed, Safari will take over and direct the user to the webpage, maintaining the app's presence.
For penetration testers, the `apple-app-site-association` file is of particular interest as it may reveal **sensitive paths**, potentially including ones related to unreleased features.
### **Analyzing the Associated Domains Entitlement**
Developers enable Universal Links by configuring the **Associated Domains** in Xcode's Capabilities tab or by inspecting the `.entitlements` file. Each domain is prefixed with `applinks:`. For example, Telegram's configuration might appear as follows:
```xml
com.apple.developer.associated-domains
applinks:telegram.me
applinks:t.me
```
For more comprehensive insights, refer to the [archived Apple Developer Documentation](https://developer.apple.com/library/archive/documentation/General/Conceptual/AppSearch/UniversalLinks.html#//apple_ref/doc/uid/TP40016308-CH12-SW2).
If working with a compiled application, entitlements can be extracted as outlined in [this guide](extracting-entitlements-from-compiled-application.md).
### **Retrieving the Apple App Site Association File**
The `apple-app-site-association` file should be retrieved from the server using the domains specified in the entitlements. Ensure the file is accessible via HTTPS directly at `https:///apple-app-site-association`. Tools like the [Apple App Site Association (AASA) Validator](https://branch.io/resources/aasa-validator/) can aid in this process.
### **Handling Universal Links in the App**
The app must implement specific methods to handle universal links correctly. The primary method to look for is [`application:continueUserActivity:restorationHandler:`](https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623072-application). It's crucial that the scheme of URLs handled is HTTP or HTTPS, as others will not be supported.
#### **Validating the Data Handler Method**
When a universal link opens an app, an `NSUserActivity` object is passed to the app with the URL. Before processing this URL, it's essential to validate and sanitize it to prevent security risks. Here's an example in Swift that demonstrates the process:
```swift
func application(_ application: UIApplication, continue userActivity: NSUserActivity,
restorationHandler: @escaping ([UIUserActivityRestoring]?) -> Void) -> Bool {
// Check for web browsing activity and valid URL
if userActivity.activityType == NSUserActivityTypeBrowsingWeb, let url = userActivity.webpageURL {
application.open(url, options: [:], completionHandler: nil)
}
return true
}
```
URLs should be carefully parsed and validated, especially if they include parameters, to guard against potential spoofing or malformed data. The `NSURLComponents` API is useful for this purpose, as demonstrated below:
```swift
func application(_ application: UIApplication,
continue userActivity: NSUserActivity,
restorationHandler: @escaping ([Any]?) -> Void) -> Bool {
guard userActivity.activityType == NSUserActivityTypeBrowsingWeb,
let incomingURL = userActivity.webpageURL,
let components = NSURLComponents(url: incomingURL, resolvingAgainstBaseURL: true),
let path = components.path,
let params = components.queryItems else {
return false
}
if let albumName = params.first(where: { $0.name == "albumname" })?.value,
let photoIndex = params.first(where: { $0.name == "index" })?.value {
// Process the URL with album name and photo index
return true
} else {
// Handle invalid or missing parameters
return false
}
}
```
Through **diligent configuration and validation**, developers can ensure that universal links enhance user experience while maintaining security and privacy standards.
## Tools
* [GetUniversal.link](https://getuniversal.link/): Helps simplify the testing and management of your app's Universal Links and AASA file. Simply enter your domain to verify AASA file integrity or use the custom dashboard to easily test link behavior. This tool also helps you determine when Apple will next index your AASA file.
## References
* [https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0070/#static-analysis](https://mas.owasp.org/MASTG/tests/ios/MASVS-PLATFORM/MASTG-TEST-0070/#static-analysis)
* [https://mobile-security.gitbook.io/mobile-security-testing-guide/ios-testing-guide/0x06h-testing-platform-interaction#testing-object-persistence-mstg-platform-8](https://mobile-security.gitbook.io/mobile-security-testing-guide/ios-testing-guide/0x06h-testing-platform-interaction#testing-object-persistence-mstg-platform-8)
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Support HackTricks
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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