Attacking Broadcast receiver:
A Broadcast Receiver is an Android Component which allows you to register for system or application events.We recommend reading Android Penetration Testing – Part 5 for more details with Broadcast Receiver
Let’s examine android manifest
Broadcast receivers are generally registered in the following format. As we can see in android manifest file exported value = True for broadcast
The code seems insecure since the receiver is exported.
The parameters that are passed to the Broadcast Receiver can be seen below
The Goal is to send spoofed broadcast intents and see if the application is accepting them. If yes, we will exploit the application to send an SMS to some random mobile numbers using the fake broadcasts.
- Attacking vulnerable broadcast receivers
- Securing the applications
Attacking Vulnerable Broadcast Receivers
Let’s now try to send some spoofed broadcasts to the above receiver.
Using am tool available in adb
- Get an adb shell on the device and type the following command to send a spoofed broadcast.
- Navigate back to the “platform-tools” folder and enter the below mentioned command:
. /adb shell
- Enter the following command in the shell:
am broadcast -a theBroadcast -n com.android.insecurebankv2/com.android.insecurebankv2.MyBroadCastReceiver –es phonenumber 5554 –es newpass [email protected]!
- Back on the emulator, navigate to the “Messages”. The above-entered command automatically makes a call to the mentioned Broadcast Receiver and an SMS text with the passwords is sent.
Now, a broadcast event is sent in the background, and since the application is not validating the input source, an SMS will be sent to emulator-5554 without user intervention.
Let’s see how we can secure application-
Securing the Applications:
- Setting android: exported attribute’s value to false
In the AndroidManifest.xml file of our application, we should add the following attribute to the receiver to be secured.
- Limiting access to custom permissions
We can also impose permission-based restrictions by defining custom permissions for each receiver. This is helpful if the developer wants to limit the access to his app’s components to those apps which have permissions.
This is the reason why we need to specify “android.permission.RECEIVE_SMS” permission in the AndroidManifest.xml file if any app wants to listen for the Receive SMS event using the action “android.provider.Telephony.SMS_RECEIVED“
Check out our previous parts of Android Penetration Testing
Android Application Penetration Testing – Part 1
Android Application Penetration Testing – Part 2
Android Application Penetration testing – Part 3
Android Application Penetration Testing – Part 4
Android Application Penetration Testing – Part 5
Android Application Penetration Testing – Part 6
Android Application Penetration Testing – Part 7
Android Application Penetration Testing – Part 8
Android Application Penetration Testing – Part 9