If you share a WiFi connection with an attacker at a coffee shop, for example, there are certain attacks they can execute to see the unencrypted parts of your Internet communications (e.g., the domain names of the websites you visit) and interfere with your communications to carry out other advanced attacks against you. Typically, security experts recommend the use of a VPN to protect against attackers with whom you share a WiFi connection. Our research reveals that using a VPN opens you up to similar attacks from other VPN users with whom you share your VPN server. In the same way that the WiFi radio signal is a shared resource that makes users vulnerable to attacks, there is a shared resource on VPN servers called a port (each connection through the VPN server is assigned to a port). By carefully crafting packets from within the attacker’s own connection to the VPN server and from a remote Internet location controlled by the attacker, it is possible to carry out attacks on other VPN users who are using the same VPN server in a manner that is very similar to the attacks that could be carried out on shared WiFi. We call this attack primitive a port shadow because the attacker shadows their own information on a victim’s port as a shared resource, and this attack primitive can lead to snooping of unencrypted data, port scans, or connection hijacking.