KIEV (Reuters) – The Ukrainian software firm at the center of a cyber attack that spread around the world last week said on Wednesday that computers which use its accounting software are compromised by a so-called “backdoor” installed by hackers during the attack.

The backdoor has been installed in every computer that wasn’t offline during the cyber attack, said Olesya Bilousova, the chief executive of Intellect Service, which developed M.E.Doc, Ukraine’s most popular accounting software.

Last week’s cyber attack spread from Ukraine and knocked out thousands of computers, disrupting shipping and shut down a chocolate factory in Australia as it reached dozens of countries around the world.

Ukrainian politicians were quick to blame Russia for a state-sponsored hack, which Moscow denied, while Ukranian cyber police and some experts say the attack was likely a smokescreen for the hackers to install new malware.

The Ukrainian police have seized M.E.Doc’s servers and taken them offline. On Wednesday morning they advised every computer using M.E.Doc software to be switched off. M.E.Doc is installed in around 1 million computers in Ukraine, Bilousova said.

“… the fact is that this backdoor needs to be closed. There was a hacking of servers,” Bilousova told reporters.

“As of today, every computer which is on the same local network as our product is a threat. We need to pay the most attention to those computers which weren’t affected (by the attack). The virus is on them waiting for a signal. There are fingerprints on computers which didn’t even use our product.”

(Reporting by Jack Stubbs; writing by Matthias Williams; Editing by Toby Chopra)