The malware allows Hackers to potentially get access to the user's computer, and other connected systems, to steal personal data or credentials.
A spokeswoman additionally told Business Insider that Piriform estimated 2.27 million users had installed the infected software.
That could mean an external hacker infiltrated the build process itself, or an insider "intentionally included" the malicious code.
Piriform said: "At this stage, we don’t want to speculate how the unauthorized code appeared in the CCleaner software, where the attack originated from, how long it was being prepared and who stood behind it."
- Hackers compromised free CCleaner software, Avast's Piriform saysReuters.com
- Hackers hid malware in CCleaner PC tool for nearly a monthZDNet
- Hackers Hid Backdoor In CCleaner Security App With 2 Billion Downloads -- 2.3 ...Forbes
- Hackers hid malware in CCleaner softwareThe Verge
- CCleaner was hacked to spread malware to millions of users for a monthTNW
- Hackers hid a backdoor in a security app that has more than 2 billion downloadsBGR
- Hackers target 'vast number' of devices in CCleaner Cloud software attackRT
- CCleaner Compromised to Distribute Malware for Almost a MonthBleepingComputer
- Security warning: Hackers compromised CCleaner and installed a backdoorBetaNews