A Twitter app bug was used to match 17 million phone numbers to user accounts

A security researcher said he has matched 17 million phone numbers to Twitter user accounts by exploiting a flaw in Twitter’s Android app.

Ibrahim Balic found that it was possible to upload entire lists of generated phone numbers through Twitter’s contacts upload feature. “If you upload your phone number, it fetches user data in return,” he told TechCrunch.

He said Twitter’s contact upload feature doesn’t accept lists of phone numbers in sequential format — likely as a way to prevent this kind of matching. Instead, he generated more than two billion phone numbers, one after the other, then randomized the numbers, and uploaded them to Twitter through the Android app. (Balic said the bug did not exist in the web-based upload feature.)

Over a two-month period, Balic said he matched records from users in Israel, Turkey, Iran, Greece, Armenia, France and Germany, he said, but stopped after Twitter blocked the effort on December 20.

Balic provided TechCrunch with a sample of the phone numbers he matched. Using the site’s password reset feature, we verified his findings by comparing a random selection of usernames with the phone numbers that were provided.

In one case, TechCrunch was able to identify a senior Israeli politician using their matched phone number.

While he did not alert Twitter to the vulnerability, he took many of the phone numbers of high-profile Twitter users — including politicians and officials — to a WhatsApp group in an effort to warn users directly.

It’s not believed Balic’s efforts are related to a Twitter blog post published this week, which confirmed a bug could have allowed “a bad actor to see nonpublic account information or to control your account,” such as tweets, direct messages and location information.

A Twitter spokesperson, when reached, did not immediately comment outside of business hours.

It’s the latest security lapse involving Twitter data in the past year. In May, Twitter admitted it gave account location data to one of its partners, even if the user had opted-out of having their data shared. In August, the company said it inadvertently gave its ad partners more data than it should have. And just last month, Twitter confirmed it used phone numbers provided by users for two-factor authentication for serving targeted ads.

Balic is previously known for identifying a security flaw breach that affected Apple’s developer center in 2013.