Hi,
I don’t know if you have already encountered this problem but here is what happened:
Out of the blue, when trying to open my Microsoft Word, I kept receiving a sandbox alert for this RarExt64.dll. At first i didn’t take it too serious as I hadn’t installed or downloaded anything, so I just followed the recommended instructions. Despite this, I kept getting the alert and I reached the conclusion that something was fishy. I tried googling it and I wasn’t able to find too many details about it. So I tried uninstalling it. After doing that I ended up with 2 files in my WinRAR folder: a registry key which I had no problem in deleting it and the dll that turned into a tmp file, something like “RarExt64.dll.0.tmp” which I couldn’t delete (although I did take ownership of both the .dll file and the folder). My concern is that Avast wasn’t able to detect any threat whatsoever, and that’s a problem. I managed to delete the file while running safe mode. When I restarted my computer the sandbox alert was gone. However I am still worried that maybe the files were somehow able to hide themselves somewhere and are still running undetected. Before removing the files, I also tried looking for suspicious processes that were running at that time, but my search came out empty. On the internet I have also found references to a RarExtLoader.exe causing similar problems and acting the same (topics on other forums posted by people using avast-this was undetectable for avast too).
Hope it’s not too serious and hope this post helps making avast better. I would also be very grateful is someone having any kind of information regarding this problem could shed some light in this matter. Thank you!