I'm uh, still reading about it, and even just as I was reading and trying to find out which version I have - without actually opening the program because holy crappola - Windows Defender displayed behind-the-scenes scan results saying it found an infection...in CCleaner. I'm not sure what to tell anyone as far as "what to do" because I don't know the answer to that.
ETA, 9-20-17: Current recommendations are to either use System Restore to roll back your system to a time before CCleaner 5.33 (32-bit) was installed, or else to "refresh" (Windows 10) or completely re-install your copy of Windows.
It's a backdoor called Backdoor:Win32/Floxif that's been in the wild since CCleaner v.5.33 released in August of 2017. ETA, 9-20-17: But it affects 32-bit versions of CCleaner only. You can tell which version you have by opening CCleaner and looking in the top left corner, where it will say if it's 32-bit or 64-bit.
Time to reinstall my OS, if the fucking backdoor hasn't already destroyed our ability to do so...ETA, 9-20-17: and I did just that. It didn't destroy our ability to do so, at least not as far as I can tell.