WiFi Sense - the Microsoft functionality that allows you to share your Wifi password with a friend. 

How it works. Your Windows 10 system allows you to turn Wifi sense on or off. By default it is ON.  General FUD is that means all your friends will get access to all your wifi immediately. Not so.

In order for Wifi to be shared you need to select the specific WiFi network to share and  allow sharing. Here is where it all starts to go wrong (IMHO)

I may be wrong - please correct me if I am - I'm keen to hear from any experts out there. 

1 - You cannot choose a single user to share your wifi with out of your Outlook / MSHotmail / Offce 365 contacts book, they all get connection ability.
2 - You can also share via Facebook - again it's an all or nothing situation.

Two examples using two people called Bob and Chris
 - Bob selects the home network for sharing. 
 - The password for the wifi is sent up to Microsofts servers, encrypted and stored, where Chris can access it
 - Chris now can connect to Bob's wifi - no other password required. 
 - Chris cannot share Bob's connection with any one else - a friend of a friend cannot receive that password, only because Microsoft says so - it's policy.

Second scenario.
 - Bob selects the home network for sharing. 
 - The password for the wifi is sent up to Microsofts servers, encrypted and stored, where Chris can access it
 - However, Chris has already been to Bob's place, or visits Bob's place where Bob puts in the password to Chris's device manually. 
 - Chris now can connect to Bob's wifi - no other password required. 
 - Chris is at a party at bob's house and clicks share the network on bob's connection
 - All Chris's contacts, facebook friends etc can now access Bob's network. 

That is how i understand it works or can work. Can anyone confirm that? 

Without wanting to share FUD -... if number two scenario is correct or number one is wrong then:

I don't know about your contacts book and Facebook - but there is something a little creepy about me sharing my password with my mum (who is in my contact book) AND everyone else who is in there including suppliers, clients, accidental auto emailed people, ex-wife, old lovers, my psychiatrist and the drug addled nut job from the end of the street who got emailed once with a restraining order to stop attacking me with a machete  :)   (some of those people may be fictional)

Also while the password is held by MS in an encrypted format on an Ms server ready to dish it out to your friends:  
   - The only thing that stops it being shared with Friends of Friends is Microsoft's policy of not sharing with friends of friends.
   - I also assume that server is secured by the same system that secures the MS login for Windows 8.1, Hotmail, outlook.com, Skype and a range of other services, some of which are regularly hacked.
     Breaking into one persons account will unleash the ability to spy on a lot of peoples networks, breaking into many is a license to spam and crack on an unprecedented scale. 

Yes I know that the log onto the wireless is theoretically running a form of wireless isolation, secured by policy in such a way to ensure you cant see other peoples info on the network BUT that must be done at the PC  / connected device level, not the router level and if it is only policy stopping a device seeing widely on the lan, what's to stop it doing so? Any wireless isolation at a connected device level is surely flawed. 

While people might say you can prevent the wrong people getting on the network via MAC address permission, you can spoof mac addresses. But that aside, why would you use a system that lets you easily share your network then put in place lots of security making it far harder to setup and connect.   Hey Chris, What's you MAC address so I can log you on?   'Blowed if I know!'

Could you change your password after sharing it? Yup! But as a father of four children, each with tablets / laptops / phones etc and more importantly a wife who is a self confessed IT challenged person - that's more pain than it's worth. businesses would be worse and don't even ask about Brother printers, wireless setup and having to fix control Centre 3/4 again when it loses track of the printer.

Could I set up a guest network and only share that? Yes - but again, when the police knock at my door about the jihadist literature or porn downloaded, it's my goose that's cooked as the owner of the wireless access point, not the drive by pervert / jihadist. ( please note I put a slash between those two people types ... denoting no commonality between those two words. ) 

The security implications are more difficult with kids. how many 'friends' have they got? How likely is it my kids might 'share' their network. Its surely higher than if they have to type in the password for a friend while standing near the router in the house Password = i'ma ReallyLong and Annoying String with 5468rjf 43-098u 0u430u [80 t4[80  in the middle of me".

Lastly - work - One misconfigured device (windows mobile, Windows 10 tablet, PC, laptop, device) and it's game over. Even worse if you dont run a guest network.

So FUD or real concern.?

Me: I'm :
Tying down any and all Windows 10  / mobile devices
Ensuring no Windows 10 devices will be able to share in my clients networks where possible (Mac address authentication PLUS password. you can also change the SSID to include _optout_nomap.). 
Setting up guest network at home and work with rotating password.
Setting up wireless isolation on my home network at a router level for guest networks.
Using Mac authentication AND a password for my home networks. 
Logging, logging and more logging.
Occasionally finding time to look at the logging