The World Anti-Doping Agency has confirmed it was targeted by a Russian cyber espionage group during the Olympic Games.
The group, known as Tsar Team (APT28) or Fancy Bear, gained access to WADA's Anti-Doping Administration and Management System (ADAMS) via an International Olympic Committee (IOC) account.
WADA confirmed on Tuesday that the group had been able to access athlete data, including confidential medical reports, related to the Rio Olympics and subsequently released it into the public domain.
A number of athletes had details of previous Therapeutic Use Exemptions (TUE) published, including American tennis stars Serena and Venus Williams and gymnast Simone Biles..
Venus said: "I was disappointed to learn today that my private, medical data has been compromised by hackers and published without my permission.
"The applications for TUEs under the Tennis Anti-Doping program require a strict process for approval which I have adhered to when serious medical conditions have occurred.
"I am one of the strongest supporters of maintaining the highest level of integrity in competitive sport and I have been highly disciplined in following the guidelines set by WADA, USADA (US Anti-Doping Agency), the ITF (International Tennis Federation) and collectively the Tennis Anti-Doping Program."
The leak of a TUE with information about four-time Rio gold medallist Biles prompted her to disclose on Twitter that she has ADHD.
"I have ADHD and I have taken medicine for it since I was a kid," Biles said. "Please know, I believe in clean sport, have always followed the rules, and will continue to do so as fair play is critical to sport and is very important to me."
US women's basketball player Elena Delle Donne also had her information hacked and she laughed off the revelations on Twitter.
"I'd like to thank the hackers for making the world aware that I legally take a prescription for a condition that I've been diagnosed with, which WADA granted me an exemption for. Thanks guys!" she wrote.
WADA's director general Olivier Niggli said he was angered by the hack, which the Russian Government said on Tuesday it had no part in.
Niggli said: "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.
"We are reaching out to stakeholders, such as the IOC, international federations and national anti-doping organisations regarding the specific athletes impacted.
"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system.
"WADA has been informed by law enforcement authorities that these attacks are originating out of Russia.
"Let it be known that these criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia further to the outcomes of the Agency's independent McLaren Investigation Report."
Meanwhile, WADA President Sir Craig Reedie defended the use of Therapeutic Use Exemptions, as long as they are properly monitored.
"This system has been in place and has worked, in the main, extremely well for many years," Reedie told Sky Sports News HQ.
"If you say to an athlete who has a health problem, 'No, you may not take that medicine which would physically enabled you to take part,' then you deny these people access to sport and that must be wrong.
"The system works well with proper medical records at the start of the process and the Therapeutic Use Exemptions is granted by a responsible international federation.
"In the case of the two Williams sisters, it is the National Tennis Federation who do this extremely well and extremely responsibly and I think the system works.
"Clearly it has been a matter of concern for some time when drugs appear which some people claim are performance-enhancing.
"If there is any evidence of bad behaviour we put it on a monitoring list and, eventually if this is sufficiently serious, it goes onto the prohibited list."