If you’ve downloaded the Bitcoin blockchain, according to a new paper, then you’ve downloaded a list of URLs to child pornography — and maybe even an image showing a nude underage girl, which would be illegal to possess in many countries and could, theoretically, leave anyone who’s downloaded the distributed ledger in the U.S. open to possession charges.

In the paper, which was presented earlier this month at the Financial Cryptography and Data Security conference in Curaçao, German researchers described finding at least eight files on the blockchain related to “sexual content,” including two backup lists of 247 links to child porn and the nude image. Bitcoin forums discussed the image as early as 2014, but its nature is not well-known since actually viewing it could legally implicate the viewer. The research raises a provocative possibility: Bad actors could use blockchains, which by design are downloaded by users across the world, to share harmful material, roping in innocent users in the process.

The Bitcoin blockchain was created to store financial transactions, but enthusiasts have long used it to share other types of content, ranging from Catholic prayers to the Wikileaks diplomatic cable dump and many hidden images. That capability, the author’s paper points out, could be used to distribute copyrighted material, malware, banned religious iconography, or even child porn — perhaps even rendering the blockchain itself illegal in some jurisdictions.

The legal implications of the German paper are unclear. Most people who download the blockchain, usually for the purpose of mining a cryptocurrency, never delve deeply into its contents. Simply sharing a link can count as distribution of child porn under some circumstances, but in 2013, when it emerged that links to child porn had been stored on the blockchain, a U.S. Justice Department spokesperson suggested to CNN at the time that it would only be an issue if a person “knowingly” obtained the links.

Even if the content were legal, of course, the ethical implications of the paper are grotesque. Companies from Reddit to Periscope have acted quickly to ban accounts that spread child porn — but on a distributed blockchain, which by design lack a central authority, it’s not clear how the content could be removed. And even though the researchers weren’t able to show that the image they found definitely depicted an underage person, there would be nothing to stop someone from uploading one.

It also raises the possibility that saboteurs could upload objectionable material to a blockchain specifically to make it illegal, perhaps to manipulate the price of a cryptocurrency or simply to sow chaos.

Child porn isn’t the only worrisome content that can spread on a blockchain. In 2014, Microsoft Security Essentials flagged the Bitcoin blockchain as containing an outdated virus. In 2015, INTERPOL warned that hackers could use a blockchain to spread malware.

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