user40956: Is it possible to connect to an SSID that does not require a password?
My project using ESP8266-01 modules will sometimes be used in non-password protected guest networks.

user41546: Yes, it works! You can use:
WiFi.begin(ssid, NULL)

or even leave the passphrase empty, because it is optional:
WiFi.begin(ssid)

Reference: https://www.arduino.cc/en/Reference/WiFiBegin

40956: <p>Is it possible to connect to an SSID that does not require a password?
My project using ESP8266-01 modules will sometimes be used in non-password protected guest networks.</p>
 41546: <p>Yes, it works! You can use:</p>
<pre><code>WiFi.begin(ssid, NULL)
</code></pre>
<p>or even leave the passphrase empty, because it is optional:</p>
<pre><code>WiFi.begin(ssid)
</code></pre>
<p>Reference: <a href="https://www.arduino.cc/en/Reference/WiFiBegin" rel="nofollow noreferrer">https://www.arduino.cc/en/Reference/WiFiBegin</a></p>
