Security experts from Checkpoint have discovered a vulnerability that was present in both messaging services. The hack only affected the browser-based versions of both WhatsApp and Telegram.

The flaw affected the way both apps process images and multimedia files, in both cases they lack verifying the presence of malicious code inside the media files.

This flaw, if exploited, would have allowed hackers to take over victims’accounts on any browser. The attackers would have exploited the issue to access victims’ personal and group conversations, videos, photos, and other shared files, contact lists, and much more.

The attack starts by sending an image containing a hidden malicious code, the flaw is triggered when the victim clicks on the picture.

“The exploitation of this vulnerability starts with the attacker sending an innocent looking file to the victim, which contains malicious code.” reads the blog post published by security firm CheckPoint. “In WhatsApp, once the user clicks to open the image, the malicious file allows the attacker to access the local storage, where user data is stored. In Telegram, the user should click again to open a new tab, in order for the attacker to access local storage. From that point, the attacker can gain full access to the user’s account and account data. The attacker can then send the malicious file to the all victim’s contacts, opening a dangerous door to a potentially widespread attack over the WhatsApp and Telegram networks.”

Below the video PoC of the attacks on both Telegram and WhatsApp:

The vulnerability in both messaging apps was difficult to discover because they implement end-to-end encryption for their messages, this means that the companies offering the services were not able to analyze the malicious image used to trigger the issue.

“Since messages were encrypted without being validated first, WhatsApp and Telegram were blind to the content, thus making them unable to prevent malicious content from being sent.” continues the analysis.

“Check Point disclosed this information to WhatsApp’s and Telegram’s security teams on March 7th. Both companies have verified and acknowledged the security issue and developed a fix for web clients worldwide soon after.”

Both companies quickly fixed the issue, WhatsApp has done it in just 24 hours.

WhatsApp confirmed that the vulnerability was not exploited by attackers in the wild.