When an attacker gains control over the URL used by an application to fetch content, it introduces a critical security risk. By manipulating the URL, the attacker can redirect the application to a malicious server or inject their own content into the response. This can lead to various security vulnerabilities and potential consequences. For instance:

* Code Execution: The attacker may alter the URL to point to a server they control, enabling them to deliver malicious code to the application. This code could exploit vulnerabilities in the application, execute arbitrary commands, or install malware on the user's system.
* Data Theft: By redirecting the application to a fraudulent server, the attacker can trick users into entering sensitive information such as login credentials, credit card details, or personal data, which can then be captured and misused.
* Content Manipulation: The attacker can modify the content retrieved by the application, altering the displayed information or injecting malicious scripts. This can lead to various consequences such as displaying misleading information, defacing webpages, or conducting phishing attacks.
* Supply Chain Attacks: If the application fetches content from external sources, such as libraries or plugins, controlling the URL can allow the attacker to replace legitimate resources with compromised or malicious versions. This can compromise the security of the entire system.

Below is a sample code of an application fetching attacker controlled input:


=== "Dart"
	```dart
	import 'package:flutter/material.dart';
	import 'package:http/http.dart' as http;
	
	class MainActivity extends StatefulWidget {
	  final String url;
	
	  MainActivity({required this.url});
	
	  @override
	  _MainActivityState createState() => _MainActivityState();
	}
	
	class _MainActivityState extends State<MainActivity> {
	  String? content;
	
	  @override
	  void initState() {
	    super.initState();
	    fetchContent(widget.url);
	  }
	
	  Future<void> fetchContent(String url) async {
	    try {
	      final response = await http.get(Uri.parse(url));
	
	      if (response.statusCode == 200) {
	        setState(() {
	          content = response.body;
	        });
	        // TODO: Process the fetched content here
	      } else {
	        // Handle error response
	      }
	    } catch (e) {
	      // Handle network or API call error
	    }
	  }
	
	  @override
	  Widget build(BuildContext context) {
	    return Scaffold(
	      appBar: AppBar(
	        title: Text('Fetching URL Content'),
	      ),
	      body: Center(
	        child: content != null
	            ? Text(content!)
	            : CircularProgressIndicator(),
	      ),
	    );
	  }
	}
	```



=== "Swift"
	```swift
	import UIKit
	
	class AppDelegate: UIResponder, UIApplicationDelegate {
	
	    var window: UIWindow?
	
	    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
	        // Handle deep link URL if the app was launched from a URL
	        if let url = launchOptions?[.url] as? URL {
	            handleDeepLink(url)
	        }
	        return true
	    }
	
	    func application(_ application: UIApplication, open url: URL, options: [UIApplication.OpenURLOptionsKey : Any] = [:]) -> Bool {
	        // Handle deep link URL when the app is already running
	        handleDeepLink(url)
	        return true
	    }
	
	    private func handleDeepLink(_ url: URL) {
	        // Check if the URL scheme is "ostorlab"
	        if url.scheme == "ostorlab" {
	            // Extract the URL from the deep link
	            if let deepLinkURL = URL(string: url.absoluteString.replacingOccurrences(of: "ostorlab://", with: "")) {
	                // Fetch the content from the provided URL
	                fetchContent(deepLinkURL)
	            }
	        }
	    }
	
	    private func fetchContent(_ url: URL) {
	        URLSession.shared.dataTask(with: url) { (data, response, error) in
	            if let error = error {
	                print("Error fetching content: \(error.localizedDescription)")
	                return
	            }
	            if let data = data, let content = String(data: data, encoding: .utf8) {
	                // Process the fetched content here
	                print("Fetched content: \(content)")
	            }
	        }.resume()
	    }
	}
	```


=== "Kotlin"
	```kotlin
	import android.content.Intent
	import android.net.Uri
	import android.os.Bundle
	import androidx.appcompat.app.AppCompatActivity
	import kotlinx.coroutines.Dispatchers
	import kotlinx.coroutines.GlobalScope
	import kotlinx.coroutines.launch
	import java.io.BufferedReader
	import java.io.InputStreamReader
	import java.net.HttpURLConnection
	import java.net.URL
	
	class MainActivity : AppCompatActivity() {
	    override fun onCreate(savedInstanceState: Bundle?) {
	        super.onCreate(savedInstanceState)
	        setContentView(R.layout.activity_main)
	
	        // Get the URL from the intent
	        val intent: Intent = intent
	        val url: String? = intent.getStringExtra("url")
	
	        if (url != null) {
	            // Fetch the content from the provided URL
	            GlobalScope.launch(Dispatchers.IO) {
	                val content: String? = fetchContent(url)
	                // Process the fetched content as needed
	                if (content != null) {
	                    // TODO: Process the fetched content here
	                }
	            }
	        }
	    }
	
	    private fun fetchContent(urlString: String): String? {
	        var connection: HttpURLConnection? = null
	        var reader: BufferedReader? = null
	
	        try {
	            val url = URL(urlString)
	            connection = url.openConnection() as HttpURLConnection
	            connection.requestMethod = "GET"
	
	            // Check if the response code is successful
	            if (connection.responseCode == HttpURLConnection.HTTP_OK) {
	                reader = BufferedReader(InputStreamReader(connection.inputStream))
	                val response = StringBuilder()
	                var line: String?
	
	                // Read the response line by line
	                while (reader.readLine().also { line = it } != null) {
	                    response.append(line)
	                }
	
	                return response.toString()
	            }
	        } catch (e: Exception) {
	            e.printStackTrace()
	        } finally {
	            // Close the connection and reader
	            connection?.disconnect()
	            reader?.close()
	        }
	
	        return null
	    }
	}
	```
