**Research taken from** [**https://blog.oversecured.com/Android-Access-to-app-protected-components/**](https://blog.oversecured.com/Android-Access-to-app-protected-components/)
This vulnerability resembles **Open Redirect in web security**. Since class `Intent` is `Parcelable`, **objects belonging to this class** can be **passed** as **extra****data** in another `Intent` object. \
Many developers make **use** of this **feature** and create **proxy****components** (activities, broadcast receivers and services) that **take an embedded Intent and pass it to dangerous methods** like `startActivity(...)`, `sendBroadcast(...)`, etc. \
This is dangerous because **an attacker can force the app to launch a non-exported component that cannot be launched directly from another app**, or to grant the attacker access to its content providers. **`WebView`** also sometimes changes a **URL from a string to an `Intent`** object, using the `Intent.parseUri(...)` method, and passes it to `startActivity(...)`.
`AuthWebViewActivity` is an example of **hidden app functionality that performs certain unsafe actions**, in this case passing the user’s authentication session to a URL obtained from the `url` parameter.
Export restrictions mean **the attacker cannot access `AuthWebViewActivity` directly**. A direct call
and no security violation will arise, because t**he app that is under attack does have access to all its own components**. Using this code fragment, the attacker can bypass the Android system’s built-in restrictions.
In order to escalate the impact of this vulnerability you need to **find other vulns/missconfigurations that could allow to increate the impact of the vulnerability** (as the vulnerability by it's own isn't creating any risks).
Besides access to arbitrary components of the original app, the **attacker can attempt to gain access to those of the vulnerable app’s Content Providers** that satisfy the following conditions:
* it must have the **`android:grantUriPermissions`** flag set to **`true`**.
*`android:grantUriPermissions="true"` indicates that your Java code can use `FLAG_GRANT_READ_URI_PERMISSION` and `FLAG_GRANT_WRITE_URI_PERMISSION` for **any `Uri` served by that `ContentProvider`**.
*`android:grantUriPermissions="false"` indicates that **only the `Uri` values specified by child `<grant-uri-permission>`** elements can be used with `FLAG_GRANT_READ_URI_PERMISSION` and `FLAG_GRANT_WRITE_URI_PERMISSION`.
The attacker must set itself as the recipient of an embedded intent and set the following flags
*`Intent.FLAG_GRANT_PREFIX_URI_PERMISSION` permits URI access by prefix – for example, instead of repeatedly obtaining separate access using a complete path such as `content://com.victim.provider/image/1` the attacker can grant access to all the provider’s content using the URI `content://com.victim.provider/` and then use `ContentResolver` to address `content://com.victim.provider/image/1`, `content://com.victim.provider/image/2`, etc.
An example of a typical provider where an attacker can gain access to it and perform regular operations like `query`, `update`, `insert`, `delete`, `openFile`, `openAssetFile`
This vulnerability also makes it possible for the attacker to **steal app files** located in directories that the developer predetermined. For a successful attack, the malign app needs to **obtain access rights to Android File Provider and then read content from the file provider using Android ContentResolver**.
Example file provider (for more details see [https://developer.android.com/reference/android/support/v4/content/FileProvider](https://developer.android.com/reference/android/support/v4/content/FileProvider))
Each tag specifies a root directory with a `path` value relative to the root. For instance, the value `external_files` will correspond to `new File(Environment.getExternalStorageDirectory(), "images")`
The value `root-path` corresponds to `/`, i.e. provides access to arbitrary files.
Let us say we have some secret data stored in the file `/data/data/com.victim/databases/secret.db`: the theft of this file may look something like this `MainActivity.java`
InputStream i = getContentResolver().openInputStream(getIntent().getData()); // we can now do whatever we like with this stream, e.g. send it to a remote server
An Intent object can be cast to a string with a call to `Intent.toUri(flags)` and back from a string to an Intent using `Intent.parseUri(stringUri, flags)`. This functionality is often used in WebView (the app’s built-in browser): the **app can verify an `intent://` scheme, parse the URL into an Intent and launch the activity**.
**This vulnerability can be exploited both via other vulnerabilities** (e.g. the ability to open arbitrary links in-app in WebView directly via exported activities or by way of the deeplink mechanism) in the client app and also remotely, including cross-site scripting on the server side or MitM on the client side
The point here is that the `shouldOverrideUrlLoading(...)` method of class `WebViewClient` is called each time WebView tries to load a new link, but gives the app the option of adding a custom handler.
To exploit this vulnerability the attacker needs to create a WebView redirect to a specially prepared intent-scheme URL. Example of URL creation
* Embedded `Parcelable` and `Serializable` objects cannot be cast to string (they will be ignored)
* The insecure flags `Intent.FLAG_GRANT_READ_URI_PERMISSION` and `Intent.FLAG_GRANT_WRITE_URI_PERMISSION` are **ignored** when `Intent.parseUri(...)` is called. The parser will only leave them if the `Intent.URI_ALLOW_UNSAFE` (`startActivity(Intent.parseUri(url, Intent.URI_INTENT_SCHEME | Intent.URI_ALLOW_UNSAFE))` flag is set, which is very rare
But even complete filtering does not guarantee complete protection, because an attacker can create an implicit intent corresponding to the `intent-filter` of some non-exported activity. Example of an activity declaration:
Some app developers implement their **own intent parsers** (often to handle **deeplinks** or push messages), using e.g. **JSON** objects, strings or byte arrays, which either do not differ from the default or else present a great danger, because they may expand **`Serializable`** and `Parcelable` objects and they also allow insecure flags to be set. The security researcher may also encounter more exotic versions of intent creation, such as casting a byte array to a `Parcel` and then reading an intent from it
