(CBS Local) — Bitcoin’s roller coaster ride may be about to take a disturbing new turn after researchers claim the cryptocurrency’s digital ledger contains child pornography.

Bitcoin’s blockchain is the distributed document that records every bitcoin transaction however, it also stores some non-financial data and files. According to German researchers, someone has used the ledger to store files with links to child porn and other malicious content. In their paperA Quantitative Analysis of the Impact of Arbitrary Blockchain Content on Bitcoin, the writers warn that anyone using the blockchain could be unknowingly participating in a crime.

“Our analysis shows that certain content, e.g., illegal pornography, can render the mere possession of a blockchain illegal,” researchers from RWTH Aachen University wrote. The study found at least eight files in the open-source blockchain that contain sexual content; three of those files reportedly contain material that would be illegal in “almost all jurisdictions.” RWTH Aachen also found 274 links to child abuse material in the files, about half of those are allegedly tied to the dark web.

“Since all blockchain data is downloaded and persistently stored by users, they are liable for any objectionable content added to the blockchain by others,” the study explains. Unfortunately, the illegal content is reportedly stuck within the Bitcoin blockchain because removing the files would unravel the entire ledger.

As Mashable explains:

“The problem with this is Bitcoin’s immutability — you simply cannot remove a block from the blockchain. Imagine laying single bricks on top of one another — brick after brick after brick. Take one out and the entire thing crumbles. This is good for financial transactions, as it makes it impossible to double-spend your money, but it’s problematic when you have an illegal image or link in there.”

Bitcoin has been warned in the past about attempts to corrupt the popular online currency. In 2015, Interpol issued a warning that hackers may try to inject malware into cryptocurrency blockchains which would infect the computers of people mining Bitcoin. The German study did not find any malware in the Bitcoin blockchain at the moment.