With over 3 billion users globally, mobile instant messaging apps have become indispensable for both personal and professional communication. Besides plain messaging, many services implement additional features such as delivery and read receipts informing a user when a message has successfully reached its target. This paper highlights that delivery receipts can pose significant privacy risks to users. We use specifically crafted messages that trigger delivery receipts allowing any user to be pinged without their knowledge or consent. By using this technique at high frequency, we demonstrate how an attacker could extract private information such as the online and activity status of a victim, e.g., screen on/off. Moreover, we can infer the number of currently active user devices and their operating system, as well as launch resource exhaustion attacks, such as draining a user’s battery or data allowance, all without generating any notification on the target side. Due to the widespread adoption of vulnerable messengers (WhatsApp and Signal) and the fact that any user can be targeted simply by knowing their phone number, we argue for a design change to address this issue.
wow:
We use specifically crafted messages that trigger delivery receipts allowing any user to be pinged without their knowledge or consent
That makes think that 1st, perhaps it would be a good idea to avoid “return receipts” on any messenger, though that breaks ability to know if the destination has actually received, and if the destination has actually read the message.
Perhaps another thing, even though your messenger doesn’t identify users with phone numbers at all, still block the messenger to have access to your contact list. Not sure if this affects, for example if a xmpp client has access to a broader contact list, if it can only relate to xmpp addresses it wouldn’t pay attention to phone numbers, but I can’t really tell.
And of course, don’t use any messenger which tights users with phone numbers, no matter if to share among contacts now usernames are used instead of the phone number, when the phone number is still the way to identify the user.
