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

# Webview Attacks

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## Simplified 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](../../.gitbook/assets/image%20(718).png)

### **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.

### **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](../../.gitbook/assets/image%20(719).png)

Exploitation example using adb:

```bash
adb.exe shell am start -n com.tmh.vulnwebview/.SupportWebView –es support_url "https://example.com/xss.html"
```

### 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.

### Important Considerations

- **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)

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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>

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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your 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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