Mutable implicit ``PendingIntent``s should be avoided, as they allow malicious applications to modify the underlying intent and redirect
it to their own components, potentially gaining unauthorized access to the sender app's permissions. This vulnerability enables
privilege escalation attacks, where an attacker can perform operations using the victim app's identity.

== Why is this an issue?

Mutable implicit ``PendingIntent``s create a serious security vulnerability by essentially lending the Android app's identity and permissions
to other applications. Because such a ``PendingIntent`` does not have an exact component specified to receive it (implicit) and can be altered,
other apps that are able to receive this ``PendingIntent`` can modify its destination and contents. Since ``PendingIntent``s lend permissions to
other apps, an attacker could abuse this to execute actions the user never gave permission for.

The real-world impact for users can be severe. An attacker exploiting this vulnerability could access sensitive data that the app has permission
to use (contacts, location, photos, etc.) For example, if the app has the ``READ_CONTACTS`` permission and creates a mutable implicit ``PendingIntent``,
a malicious app could redirect that intent to steal all the user's contacts. This completely bypasses Android's permission system and violates user
trust, potentially leading to privacy breaches, data theft, or even financial fraud depending on what permissions the app holds.

== How to fix it

=== Code examples

==== Noncompliant code example

On Android 12 and higher, ``PendingIntent``s are only mutable if ``PendingIntent.FLAG_MUTABLE`` is set.

[source,kotlin,diff-id=1,diff-type=noncompliant]
----
val intent = Intent("com.example.ACTION_VIEW")
val pendingIntent = PendingIntent.getActivity(
    context,
    0,
    intent,
    PendingIntent.FLAG_MUTABLE  // Noncompliant
)
----

On Android versions 11 and below, ``PendingIntent``s are mutable by default. So even if ``PendingIntent.FLAG_MUTABLE`` is not set,
the Android app might be using mutable ``PendingIntent``s on some end user devices.

[source,kotlin,diff-id=2,diff-type=noncompliant]
----
val intent = Intent("com.example.ACTION_VIEW")
val pendingIntent = PendingIntent.getActivity(
    context,
    0,
    intent,
    PendingIntent.FLAG_UPDATE_CURRENT  // Noncompliant
)
----

==== Compliant solution

Explicitly marking the ``PendingIntent`` using ``PendingIntent.FLAG_IMMUTABLE`` resolves this issue.

[source,kotlin,diff-id=1,diff-type=compliant]
----
val intent = Intent("com.example.ACTION_VIEW")
val pendingIntent = PendingIntent.getActivity(
    context,
    0,
    intent,
    PendingIntent.FLAG_IMMUTABLE
)
----

In addition, it is recommended for the ``Intent`` within to be explicit instead of implicit.

[source,kotlin,diff-id=2,diff-type=compliant]
----
val intent = Intent("com.example.ACTION_VIEW")
intent.setClass(context, MyActivity::class.java)
val pendingIntent = PendingIntent.getActivity(
    context,
    0,
    intent,
    PendingIntent.FLAG_IMMUTABLE
)
----

=== How does this work?

There are two ways to prevent mutable implicit ``PendingIntent``s from being abused: making the ``PendingIntent`` immutable and making the ``Intent`` explicit.

By making the ``PendingIntent`` immutable, an attacker is not able to alter it to redirect to their own components. This is done by setting the ``PendingIntent.FLAG_IMMUTABLE``
flag when creating the ``PendingIntent``. This flag is available on Android 6.0 and higher. If a lower version of Android is targeted, it is recommended to make the wrapped
``Intent`` explicit.

An ``Intent`` can be made explicit by setting the component that should receive the ``Intent``. This way, the ``Intent`` can only be received by the specified component and
cannot be received by an attacker. This is done by calling ``Intent.setClass()`` or ``Intent.setComponent()`` with the target component.

Note that mutable implicit ``PendingIntent``s are explicitly blocked when an application is targeting Android 14 or higher.

== Resources
=== Documentation

* Android Documentation - https://developer.android.com/privacy-and-security/risks/implicit-intent-hijacking[Implicit intent hijacking]
* Android Documentation - https://developer.android.com/reference/android/app/PendingIntent[``PendingIntent`` API reference]

=== Articles & blog posts

* Google - https://support.google.com/faqs/answer/10437428[Remediation for Implicit PendingIntent Vulnerability]
* Dimitrios Valsamaras - https://valsamaras.medium.com/pending-intents-a-pentesters-view-92f305960f03[PendingIntents: A Pentester's View]

=== Standards

* OWASP Mobile Application Security Testing Guide - https://mas.owasp.org/MASTG/0x05h-Testing-Platform-Interaction/#pending-intents[Android Platform APIs]
* OWASP Mobile Application Security Testing Guide - https://mas.owasp.org/MASTG/tests/android/MASVS-PLATFORM/MASTG-TEST-0030/[MASTG-TEST-0030: Testing for Vulnerable Implementation of PendingIntent]
* CWE - https://cwe.mitre.org/data/definitions/927[CWE-927 - Use of Implicit Intent for Sensitive Communication]