A side-channel tracking technique leveraging WhatsApp and Signal allows an attacker to infer a target phone's device activity state (screen on/off, offline) by sending silent probe reactions to invalid message IDs and measuring delivery receipt round-trip times. This method does not generate visible notifications or messages on the victim's device, making it stealthy. Over time, attackers can profile user behavior such as when they are home, sleeping, or mobile. The attack requires the ability to send messages to the target number, which can be mitigated by restricting unknown contacts in privacy settings. Although no direct data breach occurs, this privacy-invasive tracking poses significant risks. The threat is relevant to European users given WhatsApp and Signal's widespread adoption. No known exploits in the wild exist yet, but the proof-of-concept is publicly available.
