Phishing and updates of the popular M.E.Doc software (reports to Ukraine's Fiscal Service, electronic correspondence) were the main two means used by the new Petya virus, which infected hundreds of Ukrainian companies and banks, the cyberpolice unit of Ukraine's National Police said on its Facebook page on June 28.

"We are not accusing M.E.Doc, just stating facts, which should be re-checked. We recommend not updating [M.E.Doc] software in the interim," the cyberpolice unit said.

The developer of M.E.Doc software, meanwhile, refuted reports that updates of its software was the cause for the fast spread of the virus.

"Such conclusions are certainly erroneous. The actual version of the update package was issued on June 22, 2017 and was checked for any virus files," a message from M.E.Doc's website said.

M.E.Doc developers said they also suffered from the cyberattack.

"This was the reason for temporarily halting services, including exchanging original documents on our server. We are currently taking active measures to restore all services," the statement says.

The M.E.Doc developer said the software works and servers with updates have not been affected. The developer said SOTA, the web service used for accountability and document flow, would resume operations during the second half of the day on Thursday.

The message said the developer of M.E.Doc had contacted the cyberpolice unit in order to facilitate cooperation in determining possible infection of software and that a meeting attended by technical specialists [from the cyberpolice unit and software developer] was held.

The cyberpolice unit said it received more than 1,000 reports from computer users infected by the virus. Some 150 companies filed official complaints to the police, and 23 criminal cases were opened involving interference in computer network operations.

Phishing is a form of fraud which involves attempts to obtain sensitive information such as usernames, passwords, and credit card details and, indirectly, money, often for malicious reasons, by disguising as a trustworthy entity in an electronic communication.