A vulnerability found in the web versions of WhatsApp and Telegram could have been exploited to hijack accounts by sending the targeted user a malicious HTML file disguised as an image or a video.
The flaw was discovered by researchers at Check Point earlier this month and it was quickly patched by both Telegram and WhatsApp on the server side.
According to experts, an attacker could have leveraged the vulnerability to take complete control of a user’s account, including personal and group conversations, photos, videos and contact list. The hacker could have not only stolen information, but also interact with the victim’s contacts (e.g. send spam, hijack their accounts using the same method).
Telegram and WhatsApp allow users to send various types of files to their contacts, including documents, audio files, videos and images. Users are normally blocked from sending unauthorized file types, but researchers have found a way to bypass restrictions and upload a malicious HTML file by manipulating its MIME type and making it appear as an authorized file.
Experts demonstrated their findings using a fake image file in the case of WhatsApp and a fake video in the case of Telegram. Video demos have been made available for both attacks:
Once the user opens the fake image or the fake video in a new browser tab, local storage data associated with the instant messaging applications is sent to the attacker, allowing them to take control of the account.
“In order to do that, the attacker creates a JavaScript function that will check every 2 seconds if there is new data in the backend, and replace local storage with the victim’s local storage,” researchers explained.
In the case of WhatsApp, the victim is normally alerted if there is more than one active session. However, the attacker’s malicious code could have caused the victim’s browser window to get stuck. The hacker could have maintained access to the victim’s account until they logged out – simply closing the browser did not lock the attacker out.
Telegram allows multiple active sessions, which means the victim is not alerted if an unauthorized user logs in to their account at the same time.
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