It’s still unclear to us why Facebook servers would do this when all the other apps put a limit on how much data gets downloaded.”Īccording to Mysk, “the servers need to open the links and download what's in there. “The moment the link was sent, several Facebook servers immediately started downloading the file from our server… 24.7GB of data was downloaded from our server by Facebook servers. While others stopped at 20 to 50MB, the researchers saw Facebook download a 2.6GB file onto its servers. But only Facebook’s platforms were seen downloaded massive files, beyond the size needed for a preview. So that secret design document that you shared a link to from your OneDrive, and you thought you had deleted because you no longer wanted to share it? There might be a copy of it on one of these link preview servers.”Ī number of messaging platforms take this approach-Facebook Messenger and stablemate Instagram, LinkedIn, Slack, Twitter, Zoom and Google Hangouts among them. With this approach, the server will need to make a copy (or at least a partial copy) of what’s in the link to generate the preview. “Say you were sending a private Dropbox link to someone,” Mysk and Haj Bakry warn, “and you don’t want anyone else to see what’s in it. This goes way beyond links to public domain websites. Are the servers downloading entire files, or only a small amount to show the preview? If they’re downloading entire files, do the servers keep a copy, and if so for how long? And are these copies stored securely, or can the people who run the servers access the copies?”

Although these servers are trusted by the app, there’s no indication to users that the servers are downloading whatever they find in a link.

This could be bills, contracts, medical records, or anything that may be confidential. This somehow shows that Facebook admits that the way link previews are treated in the normal chats may impact user privacy.”Īs the researchers explain in their report, “links shared in chats may contain private information intended only for the recipients. “All the vulnerabilities we discovered in Facebook Messenger occur in normal chats. As the report explains, “when you send a link, the app will first send it to an external server and ask it to generate a preview, then the server will send the preview back to both the sender and receiver.” But this is a potential security nightmare. “Facebook Messenger doesn't provide link previews at all in its secret conversations, which are end-to-end encrypted,” Mysk told me. Which brings us to the final option, the Facebook Messenger approach-server-side link previews.

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