Child abuse imagery has been detected in Bitcoin’s blockchain, according to a new study. The discovery potentially puts cryptocurrency users at risk, as anyone who downloads a blockchain could be held liable for illegal content.

Researchers from RWTH Aachen University in Germany and the Data Protection Research Institute at Goethe University in Frankfurt found 1600 files, including texts and images, on blockchain. Among the data were instances of illegal pornography and serious privacy violations, the team says.

The researchers extracted content from the blockchain, converted it into readable files and presented their findings at the Financial Cryptography and Data Security conference in Curaçao earlier this month. Eight files with sexual content were detected, two of which were backups of link lists to child pornography, containing 274 links to websites, some 142 of which referred to Tor hidden services.

Yesterday, we gave our talk on unwanted blockchain content in #bitcoin at #fc18. Had great discussions afterwards! Now we're looking forward to presenting follow-up results at #ic2e#bta in April!FC'18 Paper: https://t.co/bWsrDcZzRzBTA paper: https://t.co/SDvT8Tr87spic.twitter.com/fYw8nqJrDE — Roman Matzutt (@matzutt) March 1, 2018

The remainder reportedly included an image depicting mild nudity of a young woman and five files which showed, described, or linked to mildly pornographic content. The origin of all this content is not clear. “Among these files there is clearly objectionable content such as links to child pornography, which is distributed to all Bitcoin participants,” the researchers’ paper states.

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“Although court rulings do not yet exist, legislative texts from countries such as Germany, the UK, or the USA suggest that illegal content such as child pornography can make the blockchain illegal to possess for all users,” it concluded.

While spending bitcoin does not not necessarily require a copy of the blockchain, other transactions, such as some mining techniques, require that the user downloads either the full blockchain or chunks of it.

“As of now, this can affect at least 112 countries in which possessing content such as child pornography is illegal. This especially endangers the multi-billion dollar markets powering cryptocurrencies such as Bitcoin,” the researchers said.

The authors identified five categories of content that may cause problems for anyone storing the blockchain: copyright violations; malware; privacy violations; politically sensitive content; and illegal and condemned content. According to the researchers, this is the first categorization of objectionable content, and survey of potential risks for users, if such content enters the blockchain.

Among the other files reportedly found were seven copyright violations and 609 transactions containing online public chat logs, emails, and forum posts discussing Bitcoin, including topics such as money laundering. In two instances, the researchers found the complete disclosure of another person’s personal information including phone numbers, addresses, bank accounts, passwords, and multiple online identities.

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While the researchers didn’t actually find any malware, they noted its potential to spread via blockchain. Interpol previously warned that malware could be injected and hosted on the blockchain, with the chain potentially becoming a ‘safe haven’ for sharing child pornography.

Roman Matzutt, a member of the research team, told The Register that there is also potential for these issues to arise on other blockchains that allow content to be inserted, such as Litecoin and Ethereum.