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CCleaner 5.33 infected with malware
Apparently someone managed to hack Avast's server, and replace CCleaner 5.33 with an infected version. See discussion at https://it.slashdot.org/story/17/09/18/140251/avasts-ccleaner-free-windows-application-infected-with-malware
More details here:
https://www.techpowerup.com/237111/piriform-hacked-ccleaner-august-versions-v5-33-6162-injected-compromised. Apparently, the attack was still in its 'early days' and the hackers were still mainly gathering user information rather than downloading more malicious malware.