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I am aware that 'hiding' a wifi network/SSID does not make the network MORE secure, but Apple's warning on IOS 10:
Using a hidden network can expose personally identifiable information
seems to indicate that hidden networks are LESS secure.
Clicking the 'learn more' link does not give any further info supporting the claim.
Exactly how can hiding an SSID make the network any LESS secure (by "expose[ing] personally identifiable information")
Clients that connect to known networks automatically will advertise “hidden” SSIDs in all of their probe requests. This results in your device broadcasting those SSIDs everywhere you go, to anyone who’s listening.
This behavior is dependent on the client’s operating system. For instance, you can configure Windows 7 and later to not connect to hidden networks automatically (only “visible” ones). That prevents such broadcasts from happening, but then you have to connect to hidden networks manually every time.
On the other hand, iOS and macOS always connect to known networks, hidden or not. The fact that iOS 10 warns about this would indicate that Apple has no plans to add the kind of toggle switch that Microsoft added in Windows 7, or to force the user to connect manually. Therefore, iOS and macOS constantly broadcast all the hidden SSIDs they are capable of connecting to.
Microsoft explains this behavior on TechNet:
A non-broadcast network is not undetectable. Non-broadcast networks are advertised in the probe requests sent out by wireless clients and in the responses to the probe requests sent by wireless APs. Unlike broadcast networks, wireless clients running Windows XP with Service Pack 2 or Windows Server® 2003 with Service Pack 1 that are configured to connect to non-broadcast networks are constantly disclosing the SSID of those networks, even when those networks are not in range.
Therefore, using non-broadcast networks compromises the privacy of the wireless network configuration of a Windows XP or Windows Server 2003-based wireless client because it is periodically disclosing its set of preferred non-broadcast wireless networks.
Now as to why this is a privacy issue:
1 and 2 show how trying to hide an SSID makes the privacy of its network much worse. 3 and 4 show how that extends to your personal privacy as well.
Sound far-fetched? Criminals/advertisers/jealous exes/the government have done worse things. In fact, MAC addresses were once used to track shoppers’ movements through malls. Apple subsequently randomized MAC addresses in probe requests.
Thankfully, no one I know has used a hidden SSID in well over a decade, and I haven’t seen that practice recommended in even longer long.
Bottom line: don’t hide your SSID. It achieves the exact opposite of what you think it does.
Update: Since there seems to be some confusion as to why you can’t connect to a hidden network without broadcasting it to the world, as well as about security vs. privacy, let’s make a fun analogy.
Imagine a driver (the AP) is picking you up from the airport. They don't know you, and you don’t know them. So they hold up a sign that reads, “John Doe.” When you find them, you (the client) go and tell them, “I’m John Doe.” This is what happens when connecting to a broadcast network.
Now, imagine that driver is trying to be super covert, and doesn’t hold up that sign. What happens now is you have to walk around yelling, “Who’s picking up John Doe?” over and over, until finally the driver steps forward and responds, “I'm picking up John Doe.”
In either case, you then exchange credentials, make sure you’re each who you think you’re dealing with. What happens after authentication is just as secure either way. But every step leading up to it compromises your privacy.
Apple representatives stated that iOS 10 passively listens for known access points and broadcasts SSIDs related to hidden access points but no other "personal" information. Other experts, however, claim to have been able to easily collect information on a phone's trusted wifi access points, hidden or not. Though hiding your SSID cannot prevent an attack (nothing can), it probably doesn't increase your risk of attack in a measurable way unless iOS is broadcasting other data that wouldn't be available to the attacker otherwise, that could inform an attacker about your identity or other information that could aid their efforts to target you. Apple should clarify exactly what they mean by "personally identifiable information".
Concerns are raised that your phone will broadcast the SSID of your hidden network while you're not connected to it. Broadcasting the name of an access point in locations where the access point is NOT seems insignificant. An attacker would still have to find the actual access point which seems pretty resource intensive unless you AND the attacker are nearby the access point at the same time.
A "hidden" SSID can reduce your exposure to someone with malicious intent war driving or stumbling upon your network by chance. The fact that someone can still discover your SSID even if it's hidden seems less concerning because it means an attacker may have to take extra steps to get that information making it more resource and time intensive for them, even if just by a bit. You never know when someone might get deterred and move on to another target.
A couple points.
Any wireless device is broadcasting (searching 1st) for its "preferred network SSID's" when out of range regardless if SSID hidden or broadcast. Apple has a list of protocol priorities of what and how are connected.
Security (handshake to the AP) isn't any different. The false sense of hidden SSID is possibly a huge security issue depending on the AP's password or even lack of. One might think "hidden" is making their AP more secure yet everywhere they travel all their/families wireless devices are searching for those hidden/preferred AP's when out of range.
It would seem like the best approach to resolve this situation, while still being able to benefit from the added security of using a hidden network, would be to geocode the location of the access point, and then supply an option in the AP config on the endpoint to enable geofencing for broadcast of the SSID query. The reason that I say added security, is only meant in the general sense; because I feel that it does enhance security for the average person. Most people out there do not have, or know how to use, wifi scanners that could be used to pickup and decode broadcast queries for AP location. The only additional security provided is that someone that doesn't know the name of the network, would be unable to try to guess its password. This obviously has no effect on a well-motivated and properly educated individual, but let's face it, your average person lacks both of those qualities. To anyone that intent on getting in, they'll find a way.
The only thing I see here is that if you are hiding your SSID, then the device (IOS or windows) must query for the SSID on a regular basis to determine if it is in range, even if it isn't. This means that your device will broadcast everywhere were the SSID cannot be reached.
If you however turn on your SSID as broadcast, then the device will passively 'listen' for the SSID instead of constantly broadcasting it.
I have problems with my wifes IPhone 5 on my home hidden SSID since she upgraded to iOS 10 and I think it is because Apple has changed it so that it does not continually broadcast for hidden SSIDs. I have no problems at all with any of the android devices I use or with any of the windows devices either... and after a quick scan, it appears they do not broadcast for the SSID (They broadcast for all SSID's and then choose from the list). It appears to be an Apple specific issue.... Placing a message like they have is BS IMHO because they are blatently blaming the issue on others when it is clearly they seem to lack the talent to seek for hidden SSID's in a secure manner like every other device does.
I think what Apple's message on your screenshot wanted to say is that hiding networks' SSID won't allow them to hid anything except that. So basically they're telling you hiding networks isn't safer it just doesn't broadcast the SSID.
I agree with you the message can be very tricky to interpret, but I see this as the only reasonable way to read that. But let's not desperate, it's still a Beta, Apple changes a lot in Beta versions, so hopefully they'll update and clarify this message as well!
Thus, as Apple wrote on that page it suggested you:
Hiding a network doesn't secure your Wi-Fi network, because the SSID is still available through other mechanisms. Security is enforced by a different setting.
Hiding networks' SSID won't make them safer or more secure.
