The online accounts of Virgin Mobile USA subscribers are vulnerable to brute force attacks because the company forces customers to use weak passwords on its website, according to a software developer.

Virgin Mobile has simplified its post-paid mobile plans, reducing the number of options it offers from 19 to less than 10, and allowing users to roll over their unused monthly call and text credit.

Virgin Mobile is redefining itself, and a part of that new identity involves partnering with Apple and turning its business into the first “iPhone-only” carrier. The carrier’s new program is called ...