Russian doping whistleblower Yulia Stepanova's electronic account at the World Anti-Doping Agency (WADA) has been hacked, it has emerged.
The revelation comes as the International Olympic Committee (IOC) disclosed that it has been fending off "massive" cyber attacks during the Rio Olympics.
Stepanova's account on the ADAMS database - which normally traces the location of athletes so random drug tests can be carried out - has been illegally accessed, the watchdog said.
The 800m runner was a key source of information on accusations that the Russian government organised doping on a massive scale. She now lives in hiding in the US.
Stepanova, whose husband also provided evidence, has been barred from competing in Rio because she served a past doping ban.
WADA said it had "immediately locked Stepanova's account to prevent further access and notified her of the situation".
Stepanova's password for her account had been "illegally obtained, which allowed a perpetrator to access her account", the watchdog said.
It said an investigation had determined no other athlete's accounts on ADAMS had been accessed, but some had reported receiving so-called phishing emails, disguised as official WADA communications.
"The agency confirmed that some users had received illegitimate emails that look as though they come from WADA, which ask users to click on a link and enter their personal credentials," a WADA statement said.
"We continue monitoring the situation to determine whether or not users have acted (or act) on these e-mails to ensure that data remains secure."
The ADAMS database is used by athletes to enter so-called "whereabouts" information which they are obliged to give so they can make themselves available for drug testing outside competitions.
Someone with an athlete's credentials can possibly alter that information, sending testers to the wrong location - and potentially leading to athletes being wrongly blamed for missing a test.
Meanwhile, IOC spokesman Mark Adams said revealed the organisation is coming under "regular attack".
"Throughout the Games, the same in London (in 2012) when you are the center of attention of the world, there are massive attempts everyday all day long to break our security," Adams said.
"We have it all year round anyway but it's particularly intense at the moment."