As the report mentions, once they find the right one, it could allow someone to take the number and then use it to hijack a victim’s SIM card and phone number. Once you have that, it’s easy to hack any service that relies on text messages to send two factor-authentication codes as a primary method or backup, which is what we’ve seen happen to people like @Deray, or owners of a slew of Instagram accounts. That’s some of the reasons why, if possible, it’s better to use a two-factor system that relies on a code generated by an app, sent via push notifications or, even better yet, is tied to a physical key.
For AT&T, a similar vulnerability occurred on one of the pages where customers could file insurance claims via Asurion that asked for a PIN, but allowed unlimited attempts and did not put any limit on the rate. Apple and Asurion told Buzzfeed News that the issues have been fixed now that they’ve been brought to their attention, but that they existed at all shows why it’s important to have multiple layers of account security — vulnerabilities can exist even in places that you wouldn’t expect.