After informing lawmakers of the breach in a letter, the Treasury Department declared it to be a “major incident”.
The US agency claimed to have been examining the effects of the attack in collaboration with the FBI and other authorities.
China said it “consistently opposes all forms of hacking” and labeled the accusation “baseless” in rejection of any involvement.
It is the most recent in a string of ignominious and well-publicized security lapses in the United States that have been attributed to China.
Large segments of American society may have had their phone record data compromised in a December breach of telecom providers.
In its letter to legislators, the Treasury Department claimed that in this most recent attack, actors based in China were able to circumvent security by using a key that was utilized by a third-party service provider.
