1. What a fake or «evil twin» wifi network is
A fake wifi network, also called an «evil twin», is a wireless network created by someone with the intent to deceive, using a name (SSID) identical or very similar to that of a known, trusted network. It's common to find them in cafes, airports, hotels or shopping centers, places where a lot of people look to connect to free wifi without thinking too much about it. When you connect to that fake network, all the traffic you generate can pass through the equipment of whoever created it, who could then see the pages you visit or intercept data you enter.
2. Warning signs to detect a fake wifi network
There are several signs that can make you suspect that a network isn't the official one at the place you're in:
Two networks appear with the same name. If you open the list of available wifi networks and see two with the same or almost the same name (for example, "Cafe_Central" and "Cafe_Central_2"), that's a clear sign that one of them could be a copy created to deceive.
The network doesn't ask for a password when it always used to. If you've used that place's wifi before and it normally asked for a password, and suddenly an open network appears with the same name, be suspicious: it could be an evil twin created so you connect without friction.
The browser warns of an invalid security certificate. If while browsing you get warnings that the connection isn't secure or the certificate is invalid on pages where that normally doesn't happen, it's a symptom that something is intercepting your traffic.
3. How to protect yourself from a fake wifi network
Ask the venue's staff for the exact name of the official network before connecting, especially if you see more than one network with similar names. Avoid logging into important accounts, such as your bank, email or social media, or making payments while on public wifi, even if it looks legitimate. And if you have doubts about the network you've connected to, use mobile data instead, especially for anything that requires entering passwords or personal data.
4. What to do if you've already connected to a suspicious network
If you think you've connected to a fake wifi network, disconnect as soon as possible and switch to another trusted network or to your mobile data. If you logged into any important account while on that network, change that account's password as soon as you can from a secure connection, and turn on two-step verification if you don't already have it enabled.