Even with a VPN, open Wi-Fi exposes users

Image:Ken Hawkins

Larry Seltzer is the former editorial director of BYTE, Dark Reading, and Network Computing at UBM Tech and has spent over a decade consulting and writing on technology subjects, primarily in the area of security. Larry began his career as a Software Engineer at the now-defunct Desktop Software Corporation in Princeton, New Jersey, on the team that wrote the NPL 4GL query language.

By now, any sentient IT person knows the perils of open Wi-Fi. Those free connections in cafes and hotels don’t encrypt network traffic, so others on the network can read your traffic and possibly hijack your sessions. But one of the main solutions to this problem has a hole in it that isn’t widely appreciated.

Large sites like Twitter and Google have adopted SSL broadly in order to protect users on such networks. But for broader protection, many people use a virtual private network (VPN). Most people, if they use a VPN at all, use a corporate one. But there are public services as well, such as F-Secure’s Freedome and Privax’s HideMyAss. Your device connects with the VPN service’s servers and establishes an encrypted tunnel for all your Internet traffic from the device to their servers. The service then proxies all your traffic to and from its destination.

It’s a better solution than relying on SSL from websites for a number of reasons: with a VPN, all of the traffic from your device is encrypted, whether the site you are visiting has SSL or not. Even if the Wi-Fi access point to which you are connected is malicious, it can’t see the traffic. Any party that is in a position to monitor your traffic can’t even see the addresses and URLs of the sites with which you are communicating, something they can do with SSL over open Wi-Fi.

But there is a hole in this protection, and it happens at connect time. The VPN cannot connect until you connect to the Internet, but the VPN connection is not instantaneous. In many, perhaps most public Wi-Fi sites, your Wi-Fi hardware may connect automatically to the network, but you must open a browser to a “captive portal,” which comes from the local router, and attempt to gain access to the Internet beyond. You may have to manually accept a TOS (Terms of Service) agreement first.

In this period before your VPN takes over, what might be exposed depends on what software you run. Do you use a POP3 or IMAP e-mail client? If they check automatically, that traffic is out in the clear for all to see, including potentially the login credentials. Other programs, like instant messaging client, may try to log on.

I tested this scenario at a Starbucks with Google Wi-Fi while running Wireshark. Thousands of packets went back and forth on the open network before the VPN attempted to connect. A quick scan of the list found nothing that looked dangerous, and in fact the software on my system used TLS 1.2 in almost all cases, which was quite a relief. But your configuration may be different from mine, and even if your software attempts to use HTTPS, it could be vulnerable to attacks like SSLStrip, which tricks the software into using open HTTP anyway.

This gap in coverage may only be a matter of seconds, but that’s enough to expose valuable information like logon credentials. Try running a network monitoring tool like Microsoft’s TCPView for Windows or Little Snitch for Mac before you establish your Internet connection and see what happens in those first few seconds. The information may be protected by encryption, but it can carry details about your system configuration that could be used to identify it—or provide clues for an attacker

Read More: Even with a VPN, open Wi-Fi exposes users | Ars Technica.