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New Research Reveals the Hidden Downsides of Link Previews

Link previews are a ubiquitous feature found in just about every chat and messaging app, and with good reason. They make online conversations easier by providing images and text associated with the file that’s being linked.

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This story originally appeared on Ars Technica, a trusted source for technology news, tech policy analysis, reviews, and more. Ars is owned by WIRED’s parent company, Condé Nast.

Unfortunately, they can also leak our sensitive data, consume our limited bandwidth, drain our batteries, and, in one case, expose links in chats that are supposed to be end-to-end encrypted. Among the worst offenders, according to research published on Monday, are messengers from Facebook, Instagram, LinkedIn, and Line. More about that shortly. First a brief discussion of previews.

When a sender includes a link in a message, the app will display the conversation along with text (usually a headline) and images that accompany the link.

For this to happen, the app itself—or a proxy designated by the app—has to visit the link, open the file there, and survey what’s in it. This can open users to attacks. The most severe are those that can download malware. Other forms of malice might be forcing an app to download files so big they cause the app to crash, drain batteries, or consume limited amounts of bandwidth. And in the event the link leads to private materials—say, a tax return posted to a private OneDrive or DropBox account—the app server has an opportunity to view and store it indefinitely.

The researchers behind Monday’s report, Talal Haj Bakry and Tommy Mysk, found that Facebook Messenger and Instagram were the worst offenders. As the chart below shows, both apps download and copy a linked file in its entirety—even if it’s gigabytes in size. Again, this may be a concern if the file is something the users want to keep private.

It’s also problematic because the apps can consume vast amounts of bandwidth and battery reserves. Both apps also run any JavaScript contained in the link. That’s a problem because users have no way of vetting the security of JavaScript and can’t expect messengers to have the same exploit protections modern browsers have.

Haj Bakry and Mysk reported their findings to Facebook, and the company said that both apps work as intended. LinkedIn performed only slightly better. Its only difference was that, rather than copying files of any size, it copied only the first 50 megabytes.


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