Blocking people on social media and other sites is a popular privacy and security option for online users, but security researchers in Japan have discovered a way to abuse that feature and access sensitive information about users who deploy it. At least a dozen social media and other online sites – including Facebook, Instagram, Tumblr, Google+, Twitter, eBay, PornHub, Medium, Xbox Live, Ashley Madison, Roblox, and Xvideos – were vulnerable to the side-channel attack found by researchers at NTT Secure Platform Laboratories and Waseda University.

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