BEIJING—A recent cyberattack on a U.S. coding website suggests China has broader capabilities to tamper with global web traffic than previously recognized, a new report said.

Toronto-based human rights research group Citizen Lab said Friday the attack, widely believed to have come from China, showed the country has developed an offensive cyber tool that it could potentially use for a broad range of purposes, such as to send viruses to targeted individuals or intercept messages.

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