Can a virus from a VirtualBox VM affect the host computer?

"Unless there's a security flaw in Virtualbox that allows you to break out of the VM (and you've not patched), then no. However, it's worth remembering that if there's any network connection between the two, there's the possibility of it moving to the host, as it would move between normal machines on the same network.

Edit: In terms of checking for connections, the simplest way is to nmap the host from the guest. Use the -PN switch, incase your host blocks ping. If there is any response, then you've got a connection there. Even if you don't, there's still the possibility of a connection through another machine, if that one is connected to both the host and the guest."

"I can confirm that it is possible for the host to get affected while browsing in the guest.

I have Windows 7 as the host and Ubuntu 12.04 as the guest system. I also have ESET Smart Security in Windows 7.

I was working on the guest system and suddenly wanted to browse. I opened Firefox in the guest system and while searching clicked on an ad. That ad seemed to be malware as ESET (in the host) popped up a warning saying something has been blocked from installing.

So, an ad clicked at guest seemed to affect host. Though it was blocked, hopefully, by the host antivirus. I was thinking that shared folder was the only link untill now. But a simple network connection via NAT allows for a virus to spread between systems.

Hope this clears the air a bit. This is just my experience yesterday, as I don't know the full technical details of how it is possible."

A little late to the answer party, but affect can have multiple meanings. The virus can be contained and have no risk of spreading from the VM. However, it can still consume resources such as CPU, RAM, disk, and the network of the host machine.

technically yes because the VM saves the files to the host PC so its risky and it depends on the virus