Why Public Wi-Fi Is Unsafe

We often hear warnings from security experts that public Wi-Fi is not secure and you should connect with caution.

These warnings are indeed correct, and a very large proportion of public Wi-Fi spots have security risks. A sample security survey once showed that 21% of the 80,000 public Wi-Fi spots in the country have risks.

However, security experts rarely explain the principle of the harm, and everyone can only know the result but not the reason. We know that various accounts may be stolen after connecting to public Wi-Fi, but we are not sure why.

Why? Well, this is a principle explanation. Generally speaking, after connecting to public Wi-Fi, we may face two types of attacks.

Intranet monitoring attack

Simply put, in a common network, attackers can easily eavesdrop on your online content, including photos uploaded to Baidu Netdisk, newly posted Weibo posts, etc.

There are two ways. One is ARP attack, which is more familiar to people who have used 1Mb or 2Mb broadband a few years ago. Some people use p2p speed limit software immediately after downloading with Thunder. This software uses ARP attack. It pretends to be a transfer station between your mobile phone/computer and the router, which can not only see all the traffic passing through, but also limit the speed of the traffic.

Another is the promiscuous mode monitoring of the network card, which can receive all broadcast traffic in the network. This is conditional, that is, there is a broadcasting device in the network, such as a HUB. You can often see a HUB in a company or Internet cafe, an expansion device with "one network cable in and dozens of network cables out". If there is such a facility in the public Wi-Fi, your Internet traffic may be eavesdropped.

The protection methods for the above two methods are also very mature, namely ARP firewall and traffic encryption (buy an SSL VPN). If you also encounter these problems in your personal network, you can refer to the article "Set up the route in this way, 99.9% of hackers cannot break through" published by Leifeng.com.

Fake Wi-Fi Attack

The above is about the possible attacks if you are connected to a public Wi-Fi. But there is another situation where you can be attacked after you have connected to a public Wi-Fi.

Let's take a (fictitious) case. Xiao Wang once connected to China Mobile's CMCC network to surf the Internet. One time, he found that his phone automatically connected to the "CMCC" Wi-Fi on the subway, but there is no CMCC signal on this subway normally? And he could still surf the Internet. He felt very strange and immediately turned off the Wi-Fi, but all kinds of software installed on his phone were connected to the Internet in the background. When he went home and logged into Baidu Netdisk, he found a txt document called "Hacking by Helen".

Sounds amazing, right? The principle is very simple. With a built-in 3G router, make a "CMCC" password-free Wi-Fi, configure it to let all traffic go through your monitoring software, and then do the same as before. Here is a small loophole, your phone/computer will automatically connect to all Wi-Fis that have been connected before, and CMCC happens to be password-free, so it is very easy to forge.

As for Baidu Netdisk, the App actively synchronizes with the Internet, and many of them use cookies values ​​and some specific values ​​for verification. With these data, you can forge the App ID to log in on the computer. Here I just take Baidu Netdisk as an example. If it really hits the mark, it is just an accidental injury.

Fake Wi-Fi attacks use the "auto-connect" setting, which can be protected by "not auto-connecting". However, this seems to be more troublesome. The editor has three devices. Windows has a separate auto-connection configuration for each Wi-Fi; MIUI is similar; iOS is "ask whether to join the network", but it will automatically connect whether it is turned on or not, which is strange. Therefore, the easiest way is to turn off Wi-Fi when you go out, which also saves power.

postscript

You may still hear about some secure Wi-Fi, which is a plan launched by Tencent, 360, and Xiaomi in cooperation with public Wi-Fi and commercial Wi-Fi providers. I have used Tencent's secure Wi-Fi, and the mobile phone manager will automatically check the DNS, ARP and other items of this network, which has a certain protective effect. However, whether it can really block the above-mentioned attack methods still needs to be tested, and these companies do not have public information.

The above are several possibilities of public Wi-Fi being unsafe. We can actually see that it is quite difficult to achieve "safety". At least, it is a big project to set up each password-free Wi-Fi to not connect automatically. Leifeng.com can only provide security knowledge, and the rest is up to everyone to do.

As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity.