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Downloading or uploading controversial information can put users at risk, making them hesitant to access or share such information. While anonymous communication networks (ACNs) are designed to hide communication meta-data, already connecting to an ACN can raise suspicion. In order to enable plausible deniability while providing or accessing controversial information, we design CoverUp: a system that enables users to asynchronously upload and download data. The key idea is to involve visitors from a collaborating website. This website serves a JavaScript snippet, which, after user's consent produces cover traffic for the controversial site/content. This cover traffic is indistinguishable from the traffic of participants interested in the controversial content; hence, they can deny that they actually up-or downloaded any data.