Distributed denial-of-service attacks against the infrastructure that supports the Bitcoin have resulted in wild swings in the virtual currency's value, McAfee said. Law enforcement cracked down on some of the cybercriminals behind the attacks. But, malware is available on the black market that can be used to infect PCs and servers to use the computing power to mine Bitcoins without the owner's knowledge. Account holders are at an increased risk of attack from phishers and attackers attempting to drain accounts, McAfee said. Other forms of e-currency are available and also come with serious risks, the security firm said.