The World Anti-Doping Agency confirmed the hack of its database by a Russian group on Tuesday.
WADA said in a statement that the Russian cyber espionage group Tsar Team (APT28), also known as Fancy Bear, had broken into its Anti-Doping Administration and Management System (ADAMS) database, AFP reported.
A Russian hacker group had claimed to have accessed World Anti-Doping Agency's (WADA) confidential database and published information on four prominent US athletes earlier on Tuesday.
"WADA deeply regrets this situation and is very conscious of the threat that it represents to athletes whose confidential information has been divulged through this criminal act," WADA director general Olivier Niggli said in a statement.
The group alleged that athletes like Williams sisters and Rio Olympics gold medallist Simone Biles were allowed to consume banned substances by WADA.
"After detailed studying of the hacked WADA databases we figured out that dozens of American athletes had tested positive," the Fancy Bears' group claimed on its website.
Certain illnesses or conditions may require the athlete to take particular drugs. If those drugs fall under WADA's prohibition list, a therapeutic use exemption can be given, allowing the athlete to take the needed medicine.
The group alleged that WADA and International Olympic Committee 's (IOC) Medical and Scientific Department were corrupt as they approved the athletes' certificates for therapeutic use.
"As predicted, the USA dominated the 2016 Olympics medal count with 46 gold, 37 silver, 38 bronze for 121 total. The US team played well but not fair," the group said.
According to the Fancy Bears' group, Serena was allowed to take banned substances such as oxycodone, hydromorphone, prednisone and methylprednisolone in 2010, 2014 and 2015, Press Trust of India reported.
Venus was allowed to take prednisone, prednisolone, and triamcinolone among others in 2010, 2011, 2012 and 2013.
"The IOC strongly condemns such methods, which clearly aim at tarnishing the reputation of clean athletes," a spokeswoman for the IOC told AFP. "The IOC can confirm however that the athletes mentioned did not violate any anti-doping rules during the Olympic Games Rio 2016."
This is that second attack on the organisation since August. Last month, Russian whistleblower Yuliya Stepanova's file had been accessed by hackers.