If we have a folder shared between the host and guest OS in virtual box so that the guest can write in this folder, then it can drop any file (including malware) into such folder. But is there a way for the guest OS to run the file in the context of the host machine? In fact can a malware both copy itself into this folder and make the host system run it? If not, solely having write permission for only the shared folder is not a big issue. As long as there is not a path traversal flaw which could allow the guest OS to access folders beyond what is explicitly shared with it.
A 0-day attack could take advantage of this. No one knows how safe you would be. Consider instead using an encrypted cloud storage to move files. Unfortunately you can never be completely sure with virtualization. Necessary security precautions on the host and guest go a long way. You are probably fine, but keep in mind you are trusting the guest to share more with the host.