The New Zealand-headquartered security firm Emsisoft claims the group behind the attack is known as HelloKitty.

CD Projekt, which is also behind the digital distribution service GOG.com, said in a statement the attackers had gained access to its systems, downloaded a lot of data, encrypted all information and left a ransom note.

The amount of people that are thinking this was done by a disgruntled gamer is laughable. Judging by the ransom note that was shared, this was done by a ransomware group we track as "HelloKitty". This has nothing to do with disgruntled gamers and is just your average ransomware. https://t.co/RYJOxWc5mZ

— Fabian Wosar (@fwosar) February 9, 2021

"We will not give in to the demands nor negotiate with the actor, being aware that this may eventually lead to the release of the compromised data," the company statement said.

Emsisoft chief technology officer Fabian Wosar said: "The amount of people that are thinking this was done by a disgruntled gamer is laughable. Judging by the ransom note that was shared, this was done by a ransomware group we track as 'HelloKitty'. This has nothing to do with disgruntled gamers and is just your average ransomware.

"Before CDPR the same group has hacked the Brazilian energy company CEMIG late last year. Earliest victims date back to November 2020, way before CyberPunk was even released. I can see the appeal of the 'this was revenge by a former fan' [explanation], but life is much more boring than that."

The ransom note claimed that the attackers had dumped the complete source code for Cyberpunk 2077, Witcher 3, Gwent and an unreleased version of Witcher 3.

People who say this is "fair" or "to be expected" because of the problems of Cyberpunk 2077 should take a good look in the mirror and ask themselves how "fair" their lives would be if they got death threats, hacked and things of that nature for each time they messed something up.

— Alex Moukala (@alex_moukala) February 9, 2021

Additionally, it was claimed that all accounting, administration, legal, HR and investor relations documents had been exfiltrated.

The attackers acknowledged that it would be possible for CD Projekt to recover by using back-ups. They warned that if the company did not reach a deal with them within 48 hours, then the source code would be sold or leaked online, and the stolen documents would be sent to gaming journalists.

CD Projekt has suffered financial issues after the release of Cyberpunk 2077 for the PS4 and Xbox One on 10 November last year, due to the number of bugs. Shares of the company fell by a third in the first six days after the launch.

Commenting on the incident, global security firm Kaspersky said in a statement that it supported CD Projekt's decision not to bow to the attackers and pay the ransom.

"The incident might be just another fast-disappearing news item about a ransomware attack if not for the company’s reaction to the attack," Kaspersky said.

"CD Projekt says it does not plan to give in to any demands, or even to negotiate with the ransomware operators. Instead, the company plans to focus on mitigation, in particular, by working with potentially affected third parties. In addition, CD Projekt published the ransom note.

"We support the decision not to pay, as well as such transparency in communications about the incident. Any payment to extortionists makes their ransomware business more profitable and supports the development of more and better malicious tools – but does not guarantee the criminals won’t publish the stolen data anyway."