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A task is a collection of activities that users interact with when performing a certain job. The activities are arranged in a stack—the _**back stack**_)—in the order in which each activity is opened.
The activity that is **displayed** on the screen is called a **foreground****activity** and its **task** is called the **foreground****task**. At a time, only **one foreground task is visible on the screen**.
This is some simple activity flow:
* There's only Activity 1 in the foreground.
* Activity 2 is started which pushes Activity 1 to the Back Stack. Now Activity 2 is in the foreground.
* Activity 3 is started which pushes both Activity 1 and 2 to the Back Stack.
* Now when Activity 3 is closed. The previous activity i.e., 2 is brought automatically to the foreground. This is how task navigation works in Android.
**Task affinity** is an attribute that is defined in each `<activity>` tag in the `AndroidManifest.xml` file. It describes which Task an Activity prefers to join.\
**Launch modes** allow you to define how a new instance of an activity is associated with the current task. The [`launchMode`](https://developer.android.com/guide/topics/manifest/activity-element#lmode) attribute specifies an instruction on how the activity should be launched into a task.\
When the launchMode is set to `singleTask`, the Android system evaluates three possibilities and one of them is the reason why our attack is possible. Here they are -
Android resumes the existing instance instead of creating a new one. It means that there is at most one activity instance in the system under this mode.
* **If creating a new activity instance is necessary**:\
The Activity Manager Service (AMS) selects a task to host the newly created instance by finding a “**matching**” one in all existing tasks. **An activity “matches” a task if they have the same task affinity**. This is the reason why we can **specify the same task affinity as the vulnerable app in our malware/attacker's app so it launches in their task instead of creating it's own**.
* **Without finding a “matching” task**:\
The AMS creates a new task and makes the new activity instance the root activity of the newly created task.
The victim needs to have the **malicious****app****installed** in his device. Then, he needs to **open****it****before** opening the **vulnerable****application**. Then, when the **vulnerable** application is **opened**, the **malicious****application** will be **opened****instead**. If this malicious application presents the **same****login** as the vulnerable application the **user won't have any means to know that he is putting his credentials in a malicious application**.
**You can find an attack implemented here:** [**https://github.com/az0mb13/Task\_Hijacking\_Strandhogg**](https://github.com/az0mb13/Task\_Hijacking\_Strandhogg)
Setting `taskAffinity=""` can be a quick fix for this issue. The launch mode can also be set to **singleInstance** if the app does not want other activities to join tasks belonging to it. A custom **onBackPressed()** function can also be added, to override the default behaviour.
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