Bungee-spoofing attack

Bungee spoofing is an attack against unfirewalled Bungeecord in which a player sends falsified player forwarding data (sent as part of the host field in the Handshake packet) to the backend server. Common targets for falsification are server owners, staff members and others with special permissions.

Solutions

BungeeGuard is a BungeeCord, Spigot and Sponge plugin developed by lucko which attempts to solve this problem by adding a secret token to the end of the player forwarding data.

This works for most setups, but if the backend and frontend (the proxy) servers communicate over the Internet, it can be intercepted via a sniffing attack as the Handshake packet is not encrypted and the token is sent in plaintext. It may additionally be possible to perform a timing-based side-channel attack, as the function responsible for checking the token may not always execute in constant time.

As such, for backend and frontend servers which may only communicate over the Internet, it is recommended that a firewall is configured for every backend server that listens on a public network interface, or communications are done exclusively virtual private network providing encryption.

Another possible solution is to use Velocity's modern forwarding, but it is most secure to use a virtual private network or local network to facilitate communication between frontend and backend servers.