Federal Reserve chair Janet Yellen has finally weighed in on cryptocurrencies, giving some guarded commentary about what some say is one of the biggest financial bubbles of all time.

"Bitcoin at this time plays a very small role in the payment system. It is not a stable source of value and it does not constitute legal tender. It is a highly speculative asset," said Yellen, answering a question by CNBC's Steve Liesman.

Yellen has offered little commentary on the surging crytpocurrency during her tenure. She did say back in 2014 that the Fed did not have the authority to regulate bitcoin and reiterated that point on Wednesday.

"The Fed doesn't really play any regulatory role with respect to bitcoin, other than assuring that banking organizations that we do supervise are attentive, that they are appropriately managing any interactions they have with participants in that market, and appropriately monitoring anti-money laundering, bank secrecy act responsibilities that they have," Yellen said.