WhatsApp and Telegram use end-to-end encryption to guarantee user privacy. This encryption is designed to ensure that only the people communicating can read the messages and nobody else in between. The vulnerability, if exploited, would have allowed attackers to completely take over users accounts on any browser, and access victims personal and group conversations, photos, videos and other shared files, contact lists, and more. WhatsApp & Telegram have patched this vulnerability and developed a fix for web clients worldwide soon after.”]
Source: https://blog.checkpoint.com/2017/03/15/check-point-discloses-vulnerability-whatsapp-telegram/