Intent redirection

Overview

An intent redirection occurs when an attacker can partly or fully control the contents of an intent used to launch a new component in the context of a vulnerable app.

The intent used to launch the new component can be supplied in several ways, most commonly either as a serialized intent in an extras field, or marshaled to a string and parsed. Partial control of parameters can also lead to the same result.

Impact

The impact can vary. An attacker might execute internal functionality in the vulnerable app, or it might access private components like unexported ContentProvider objects.

Mitigations

General

In general, don’t expose functionality related to redirecting nested intents. In cases where it’s unavoidable, apply the following mitigation methods:

  • Properly sanitize the bundled information. It’s important to remember to check or clear flags (GRANT_URI_PERMISSIONS), and to check where the intent is being redirected. IntentSanitizer can help with this process.
  • Use PendingIntent objects. This prevents your component from being exported and makes the target action intent immutable.

Apps can check where an intent is being redirected using methods such as ResolveActivity:

Kotlin

val intent = getIntent()
// Get the component name of the nested intent.
val forward = intent.getParcelableExtra<Parcelable>("key") as Intent
val name: ComponentName = forward.resolveActivity(packageManager)
// Check that the package name and class name contain the expected values.
if (name.packagename == "safe_package" && name.className == "safe_class") {
    // Redirect the nested intent.
    startActivity(forward)
}

Java

Intent intent = getIntent()
// Get the component name of the nested intent.
Intent forward = (Intent) intent.getParcelableExtra("key");
ComponentName name = forward.resolveActivity(getPackageManager());
// Check that the package name and class name contain the expected values.
if (name.getPackageName().equals("safe_package") &&
        name.getClassName().equals("safe_class")) {
    // Redirect the nested intent.
    startActivity(forward);
}

Apps can use IntentSanitizer using logic similar to the following:

Kotlin

val intent = IntentSanitizer.Builder()
     .allowComponent("com.example.ActivityA")
     .allowData("com.example")
     .allowType("text/plain")
     .build()
     .sanitizeByThrowing(intent)

Java

Intent intent = new  IntentSanitizer.Builder()
     .allowComponent("com.example.ActivityA")
     .allowData("com.example")
     .allowType("text/plain")
     .build()
     .sanitizeByThrowing(intent);

Common mistakes

  • Checking if getCallingActivity() returns a non-null value. Malicious apps can supply a null value for this function.
  • Assuming that checkCallingPermission() works in all contexts, or that the method throws an exception when it is actually returning an integer.

Debugging features

For apps that target Android 12 (API level 31) or higher, you can enable a debugging feature that, in some cases, helps you detect whether your app is performing an unsafe launch of an intent.

If your app performs both of the following actions, the system detects an unsafe intent launch, and a StrictMode violation occurs:

  • Your app unparcels a nested intent from the extras of a delivered intent.
  • Your app immediately starts an app component using that nested intent, such as passing the intent into startActivity(), startService(), or bindService().

Resources