A successful phishing attack at SMS services company Twilio may have exposed the phone numbers of roughly 1,900 users of the secure messaging app Signal—but that’s about the extent of the breach, says Signal, noting that no further user data could be accessed.
In a Twitter thread and support document, Signal states that a recent successful (and deeply resourced) phishing attack on Twilio allowed access to the phone numbers linked with 1,900 users. That’s “a very small percentage of Signal’s total users,” Signal writes, and all 1,900 affected users will be notified (via SMS) to re-register their devices. Signal, like many app companies, uses Twilio to send SMS verification codes to users registering their Signal app.
With momentary access to Twilio’s customer support console, attackers could have potentially used the verification codes sent by Twilio to activate Signal on another device and thereby send or receive new Signal messages. Or an attacker could confirm that these 1,900 phone numbers were actually registered to Signal devices.