The messaging service Telegram has come under fire again. This time, accounts of dozens of Iranian Telegram users have been compromised. Hackers were also able to access more than 15 million phone numbers and accounts exploiting the API of the service.

Independent researchers Claudio Guarnieri and Collin Anderson discovered that the Telegram breach in Iran was conducted through SMS messages. Telegram uses SMS messages to send a verification code when a new device is set up. Hackers gained access to a target’s SMS message and then obtained code to add their device to the target’s account. Once the device was added to the account, they gained complete access to tthat account, including the chat history.

According to various reports, Rocket Kitten, an Iranian hacking group, is behind this breach. Researchers also hinted at the involvement of the Iranian government and cellular companies, because the service is heavily used by activists, journalists, and citizens who want to bypass the government’s control. Iran has more than 20 million Telegram users.

Telegram downplayed this hack and told users to “stay calm and Telegram.” In a blog post, the company confirmed the hack: “Certain people checked whether some Iranian numbers were registered on Telegram and were able to confirm this for 15 million accounts. As a result, only publicly available data was collected and the accounts themselves were not accessed. Such mass checks are no longer possible since we introduced some limitations into our API this year. However, since Telegram is based on phone contacts, any party can potentially check whether a phone number is registered in the system. This is also true for any other contact-based messaging app (WhatsApp, Messenger, etc.).”

The company also reminded users to use two-factor authentication instead of SMS verification. SMS verification is not trustworthy, and if you rely on end-to-end encryption of your messaging, this breach is a reminder to be aware of other factors, such as SMS authentication, that can compromise your account.

This is not the first time Telegram has been under fire. Last year, security researchers pointed out many security issues affecting the service.