DD4BC, the Bitcoin extortionist group, has increased its attacks in 2015 and is taking on corporations and banks.
The group has carried out 114 attacks since April 2015 and 141 since September 2014 on Akamai customers, according to a report by the company.
DD4BC has been conducting the campaigns since 2014.
Targeted websites receive an email with the threat and a demand for the payment of Bitcoin.
"Your site is going under attack unless you pay 25 Bitcoin," a typical message reads.
Below the Bitcoin address the payment should be sent to, the message says:
"Please note that it will not be easy to mitigate our attack, because our current UDP flood power is 400-500 Gbps, so don’t even bother."
The group promises if payment is received the target will "never hear from [them] again", saying "we do bad things, but we keep our word."
The attacks have been measured at up to 50 Gbps, rising from the 15-20 Gbps observed in May; this is lower than the group claims.
According to the report, between September 2014 and July 2015, 58 percent of the attacks targeted the financial services sector.
Banking and credit unions took up 35 percent of the total, currency exchanges 13 percent and payment processing 10 percent.
Most of the attacks against banks and credit unions occurred in June.
"DD4BC has been using the threat of DDoS attacks to secure Bitcoin payments from its victims for protection against future attacks," said Stuart Scholly, Senior Vice President & General Manager, Security Division at Akamai.
Scholly added: "The latest attacks – focused primarily on the financial service industry – involved new strategies and tactics intended to harass, extort and ultimately embarrass the victim publically."
Akamai recommended that organisations deploy anomaly and signature-based DDoS detection methods to detect the attack before the site becomes unavailable.
It also suggested that they distribute resources to increase resiliency and implement Layer 7 DdoS mitigation appliances on the network in strategic locations.
