Adversaries may make, forward, or block phone calls without user authorization. This could be used for adversary goals such as audio surveillance, blocking or forwarding calls from the device owner, or C2 communication.
Several permissions may be used to programmatically control phone calls, including:
ANSWER_PHONE_CALLS- Allows the application to answer incoming phone calls
CALL_PHONE- Allows the application to initiate a phone call without going through the Dialer interface
PROCESS_OUTGOING_CALLS- Allows the application to see the number being dialed during an outgoing call with the option to redirect the call to a different number or abort the call altogether
MANAGE_OWN_CALLS- Allows a calling application which manages its own calls through the self-managed
BIND_TELECOM_CONNECTION_SERVICE- Required permission when using a
WRITE_CALL_LOG- Allows an application to write to the device call log, potentially to hide malicious phone calls
When granted some of these permissions, an application can make a phone call without opening the dialer first. However, if an application desires to simply redirect the user to the dialer with a phone number filled in, it can launch an Intent using
Intent.ACTION_DIAL, which requires no specific permissions. This then requires the user to explicitly initiate the call or use some form of Input Injection to programmatically initiate it.
Application vetting services could provide further scrutiny to applications that request permissions related to phone calls.
Users should be encouraged to be very careful with what applications they grant phone call-based permissions to. Further, users should not change their default call handler to applications they do not recognize.
Users can view their default phone app in device settings. Users can review available call logs for irregularities, such as missing or unrecognized calls.