Until recently, cybercriminals could have hijacked the account of any GroupMe user simply by knowing the phone number connected to the targeted account, a researcher reported on Thursday.
GroupMe is a popular mobile group messaging application that works on Android, iOS, Windows Phone, and even older phones via SMS. In 2011, GroupMe was acquired by Skype, which became a Microsoft company in the same year.
When users register a GroupMe account via the iOS app, they’re first instructed to enter their name, email address and password. In the next phase of the registration process, they’re asked to verify their phone number either by using the number of the iPhone on which the registration takes place, or a different phone number.
Users who select the “different phone number” option will receive an SMS containing a 4-digit code, which they must enter in the next screen in order to complete the registration process.
In late August, New York-based security researcher Dylan Saccomanni discovered a serious vulnerability in this verification feature. The expert found that an attacker could enter an existing phone number when registering a new account and selecting the “different phone number” option.
GroupMe would send the 4-digit code to the entered phone number and present the attacker with the screen requesting the code for validation. However, since the app’s developers hadn’t implemented any type of rate limiting or security lockout mechanism, finding the correct code wasn’t difficult because there are only 10,000 possible combinations.
“It was therefore trivial to automate this process and enter any user’s account knowing nothing but their phone number,” Saccomanni wrote in a blog post.
The name, email address and password of the targeted account would be changed to the ones set by the attacker during the registration process. However, all the data from the victim’s account, including messages, groups, and account history, remained unchanged, the researcher said.
“The password and email address would be changed without any confirmation by, or notification to the user, which combined with complete account takeover made this vulnerability very dangerous,” Saccomanni noted.
The flaw was reported to GroupMe on August 28 and it was patched on September 17 with the release of version 5.0 of the iOS application. The researcher says there is no evidence that the vulnerability was exploited in the wild before being fixed.