The IAAF said today it believes the hack "has compromised athletes' Therapeutic Use Exemption (TUE) applications stored on IAAF servers" during an unauthorized remote access to its network on Feb. 21.
TUEs are permissions for athletes to take substances that would normally be banned, and are used by athletes around the world.
"Our first priority is to the athletes who have provided the IAAF with information that they believed would be secure and confidential," IAAF President Sebastian Coe said in a statement. "They have our sincerest apologies and our total commitment to continue to do everything in our power to remedy the situation."
Context Information Security, a British security company, said in a statement released by the IAAF that it discovered the attack.
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"In January 2017, the IAAF contacted Context Information Security to conduct a proactive and thorough technical investigation across its systems, which led to the discovery of a sophisticated intrusion," the company said. "Throughout the investigation, the IAAF have understood the importance and impact of the attack and have provided us comprehensive assistance."
Coe, speaking in Aarhus, Denmark, ahead of a conference of global Olympic sports officials, said the IAAF is using the "world's best people" to make the organization's computers safe.
WADA has previously said Fancy Bears originate from Russia, citing information from law enforcement agencies.
Russian officials have denied any links with Fancy Bears, but have praised the group's previous publications, which they say undermined Western countries' criticism of widespread use of banned substances by Russians. The IAAF banned Russia's team from competing internationally in 2015 after investigations by WADA found evidence of state-sponsored doping.
As of today, Fancy Bears' website contained no mention of IAAF information.