Sarahah, a new app that lets people sign up to receive anonymized, candid messages, has been surging in popularity; somewhere north of 18 million people are estimated to have downloaded it from Apple and Google’s online stores, making it the number three most downloaded free software title for iPhones and iPads.

Sarahah bills itself as a way to “receive honest feedback” from friends and employees. But the app is collecting more than feedback messages. When launched for the first time, it immediately harvests and uploads all phone numbers and email addresses in your address book. Although Sarahah does in some cases ask for permission to access contacts, it does not disclose that it uploads such data, nor does it seem to make any functional use of the information. Sarahah did not respond to requests for comment.­

"Zachary Julian, a senior security analyst at Bishop Fox, discovered Sarahah's uploading of private information when he installed the app on his Android phone, a Galaxy S5 running Android 5.1.1. The phone was outfitted with monitoring software known as BURP Suite, which intercepts internet traffic entering and leaving the device, allowing the owner to see what data is sent to remote servers. When Julian launched Sarahah on the device, BURP Suite caught the app in the act of uploading his private data.

"As soon as you log into the application, it transmits all of your email and phone contacts stored on the Android operating system," he said. He later verified the same occurs on Apple's iOS, albeit after a prompt to "access contacts," which also appears in newer versions of Android. Julian also noticed that if you haven't used the application in a while, it'll share all of your contacts again. He did some testing on the app on a Friday night, and when he booted the app on a Sunday morning, it pushed all of his contacts again."