I have a guest house with a wireless network. My guests are given a password to our "Guest Network". Is it possible for someone on this "Guest Network" to see my mac address, spoof it and then use my mac address to sign in to my email account without me knowing?
No. Security and IT Professionals realize that Mac Addresses can be spoofed. They are not and never were intended to be used for security per-se, but rather as a convenient unique identifier. Your email account, assuming it is a normal email like yahoo, google, etc. does not and will not use your Mac address as a form of authentication akin to a username or password.
As far as your guest WiFi network goes, people could see your mac address if you were both using "Guest Network", and they could spoof your mac address. But in and of itself it would be pretty harmless. And under no circumstances would it be used to compromise your email account.
Granted, there are risks associated with using open or guest WiFi networks, such as DNS spoofing or man-in-the-middle attacks, but that's more likely to be a concern at coffee shops as opposed to your house. And even then, if you use your secure/password protected WiFi network and outsiders only ever have access to "Guest Network", they shouldn't be able to see any of your devices and it shouldn't be a problem.
No. It is not possible to use a MAC address to access an email account, unless the email server is on your local network and somehow uses MAC addresses to authenticate - I imagine this would not be the case.
MAC addresses are never broadcast over the internet, and they should never be used for authentication purposes, however I cannot guarantee that someone, somewhere might have attempted it!
I would have just commented but whatever, guess I can't do that.
- I too live in a small town and have dealt with the point to point isps that use insanely massive broadcast domains. risebroadband (google it), has such a screwy setup I've positioned myself into mitm their inside routers. I told them that their wiki page internally was exposed, so they spent almost 6 months trying to fix routing issues.
- If your system is behind a router before going out to the broadcast domain of the crappy isp, your system's mac doesn't matter. But, arp spoofing both to hijack traffic and to trash connections is a thing. It has been done on many levels. If someone could position themselves right, they could become your next level router and mitm all your traffic.
- Good part is, this is the purpose for ssl, including ssl on email. Most email services provide this and most corporate emails require it. But people have posed in place of ssl mitm attacks before as well.
So, basically my suggestion is using tunnels for your traffic. The biggest risk would be your isp being a mitm point. Arp wouldn't matter as much, unless you're in their broadcast domain. If a company has appropriate domains that can't be overlapped, then there's no risk anyways and several routers out mitm wouldn't be worth it because too much traffic.
You mentioned vpn, vpns work as private networks. Meaning you'd be part of another broadcast domain, which (if not properly segregated or firewalled) could allow takeovers within the vpn service. This is one of the big reasons why people advocate AGAINST free vpn services unless you control the service yourself and can manage it appropriately.
To answer the direct question: If two people on the same network are able connected, they can both get arp resolution (ip->mac|mac->ip) of eachother. This has been used in many ways in many old attacks and there's several things you can do to stop it, but the biggest risk again however is mitm. Most host-based firewalls (hids or generic security suites (like mcafee or comodo)) monitor for arp changes in your router, multiple ips using the same mac address, and monitors for dns spoofing attempts.
So at least you have that. Your best defense in public networks, is to have a decent host intrusion detection platform and to use external (ones you trust) dns providers, as well as tls/ssl or really any encryption scheme anywhere possible.
A MAC address just represents the physical address of a device. They are defined by the manufacturer to uniquely identify a network device.
It is easy to find someone else's MAC address if you are both on the same network, however it is not linked to your email at all and it does not harm you in any way. Just like Verbal Kint stated, Your email account does not and will not use your MAC address as a form of authentication akin to a username or password.
Hence, knowing someone's MAC address does not tell you that much about them but it is sometimes the first step needed to hack someone (if you both share the same network).