Avast-owned Piriform, which makes the computer-cleaning program, says no harm was done to any users but encourages people to check which version they have.

Hackers penetrated computer-optimization software CCleaner in a way that could have let them control millions of devices, Piriform, the company that makes the program, said Monday.

A malicious program was planted inside the popular, free software, which is downloaded as often as 5 million times per week and is used to delete cookies and junk programs to make computers and Android phones run faster.

Piriform said it estimates that 2.27 million people used the infected software, but sought to calm consumers who might be inclined to panic.

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