CCleaner is a very popular maintenance utility for cleaning registry and removing unnecessary files. However, according to the producer of this software – the Avast's Piriform – the CClener version 5.33 was infected with malware.

The experts from the Cisco Talos company have discovered a malicious bit of code injected by the hackers that could have affected more than 2 million users who downloaded the most recent update. The security researcher estimates that CCleaner attracts more than 5 million new downloads a week.

Avast claims that its download servers were compromised between 15th of August and 12th of September, when it updated the servers with a new 5.33 version. During that time, a trojan was loaded into the download package. The malware was designed to collect sensitive data from the infected computers and transfer them to a hacker’s server.

Fortunately, it looks like the malware only affected the CCleaner users who were running the 32-bit version of the application (not the 64-bit version).

Piriform recommends deleting the compromised CCleaner version and downloading a clean 5.34 version from its website. It is also wise to perform an antivirus scan on the computer, or even better to use a backup to restore the PC to a state prior to 15th of August.