SMB is a well-known network file sharing protocol, and I assume it's supposed to be used internally only. At my company I found someone connecting to a share on a server on the Internet over port 445 using SMB. Is there any risk in allowing such connections? I'm thinking if someone were able to MitM they be able to capture sensitive data.
I'm not sure what operating system you are using, or if/what firewalls you have implemented, but exposing the SMB service with unfiltered access from the internet is asking to be compromised.
Also, a Man in the Middle attack would be the least of your worries. Someone could easily gain root access to your computer and consequently your entire network. Famously, the ms08_067_netapi exploits the SMB service on Windows XP on port 445 using a relative path stack corruption. I could easily execute this and gain root access in less than a minute assuming I know an IP address. For any random attacker, port scans are done across the internet continuously. Someone will find your open port.
I would recommend that you close that port and find a different solution for what you're trying to accomplish.
There is a risk of exposing your credentials via broken SMB NTLMSSP_NEGOTIATE implementation, as descibed here: https://www.cybersecurity-help.cz/blog/167.html
Yes it is risky.
While I don't know about any "exploits" that may be out there in the wild, what I do know is that anybody with the right username and password could get access to the disk drives that are exposed.
Presumably there is an administrator-level account on the computer that would allow full access.
I don't believe SMB limits the speed at which someone could run a dictionary of most-used passwords to try to get into Administrator, but it is possible to do it slowly, even if it does.
All in all, it seems like a bad idea to do it without first setting up an encrypted tunnel of some kind like a VPN. Or like SCP in Linux.