When using unencrypted WLAN (Open Authentication), your wireless network is not protected in any way. Everyone, who is located in the surrounding of the AP and can hear a signal, can join and use the network. The whole authentication process is very simplified and consists of authentication/association exchanges as shown below −
In the lab setup, I have prepared a WLAN with SSID of "LAB-test" with open authentication. As an attacker, you first need to make some passive scanning to detect such a network, so let's do it! In the first step, I will enable my wireless card and create a WLAN monitoring interface, using airmon-ng utility.
The next step it to check the WLANs that are heard by the wireless card using "airmon-ng mon0".
My wireless card was able to see the "LAB-test" SSID on channel 1, broadcasted by AP with MAC address (BSSID) of 00:18: 0A:6D:01:30. Under encryption column, you can see the letter "OPN" – it means that there is Open Authentication (in fact, it means no authentication at all).
On the Windows PC, the WLAN that has Open Authentication is marked with exclamation mark as a warning of unsecure WLAN environment, as shown below (comparing to the lack of additional sign next to protected WLANs) −
We can try to simulate if the wireless client would be able to connect to this SSID. We can make it using aireplay-ng utility.
As you can see, the authentication and association process went smooth, and any wireless client is able to join the network.
The only mechanism that you can use to improve a security of this unsecure environment is to implement MAC filtering. This feature was already described earlier, so I will jump directly to practice.
On the AP itself, I will implement a MAC filter, allowing only a client with MAC address of 98:0d: 2E:3C:C3:74 to be able to join the wireless network (this is my smartphone).
Then, when I repeat the authentication process using aireplay-ng, and this time it fails.
After I change the MAC address of the mon0 interface to the one that my smartphone has - I got successful authentication again.
It is very unlikely, that you will meet open authentication WLAN nowadays. But it is a very good to be aware of all those older deployment types as well.