One of the most popular freeware Windows products, CCleaner, was compromised by unknown parties. Supposedly only the 32-bit (5.33.6162) and Cloud (1.07.3191) versions were affected, but given that Piriform, now owned by Avast, was informed of the intrusion by two outside parties, users should be wary of all other versions. The only trustworthy cleaner is the one already in Windows, Disk Cleanup: use the Start menu to search for Disk Cleanup and then run it. This would be an excellent time to run full scans with Malwarebytes and then your regular antivirus.
Cisco Talos stated: "Uninstalling the tool will not remove the malware. To remove the malware you should restore from a previous backup that is known to be clean or try a virus removal tool." According to Piriform, "the code executed ... was heavily obfuscated to make its analysis harder (encrypted strings, indirect API calls, etc.)," so it's possible that the malware is actually worse than we know. And contrary to Piriform's assertions, updating to the latest version of CCleaner does not completely remove the infection, specifically with respect to the registry.