Communication company Twilio provides Signal with phone number verification services, and recent security breach it has suffered had also impacted some users of the popular instant-messaging app.
Twilio hackers could have attempted to re-register the number of Signal users to another device or learned that their number was registered to Signal.
“For about 1,900 users, an attacker could have attempted to re-register their number to another device or learned that their number was registered to Signal. This attack has since been shut down by Twilio. 1,900 users is a very small percentage of Signal’s total users, meaning that most were not affected.” reads the advisory published by Signal.