In my Android app there is a security vulnerability that my app can be opened by a malicious application. I am using the following intent filter in the Launcher Activity.
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
This intent filter makes the Main Activity exposed to other apps. Is there any way to expose the Main Activity only to the Android OS and not to other applications. As far as I understand We can't use "exported=false".
Is there any way to expose the Main Activity only to the Android OS and not to other applications.
Not as you are defining the terms.
Every activity, exported or not, is "exposed" to "the Android OS". Otherwise, they would be unusable, even by the app itself. The difference between an exported activity and one that is not is whether a third-party app can start the activity.
The home screen itself is just an app. A device ships with at least one home screen pre-installed, and users can install others from the Play Store or elsewhere. Hence, from the standpoint of your app, the home screen is a third-party activity.
In my Android app there is a security vulnerability that my app can be opened by a malicious application
Having an exported activity is not a "security vulnerability" in its own right. Your argument is akin to saying "there is a security vulnerability in my Web site — how do I allow my home page to only be opened from a Google search result, but not by a bookmark or any other Web site?".
Having unnecessarily exported activities is bad from a security standpoint, but the launcher activity is exported by necessity.