  We investigate the security of generalized quantum multiple-access channels.
We provide the formula for the achievable rate region of secure communication
in the scenario of two senders and a single receiver. We explicitly specify a
protocol for secure communication in this scenario, which employs superdense
coding. The protocol is based on the distribution of a tripartite GHZ state. It
allows for both symmetric and asymmetric key distribution whereby one of the
senders can have twice the capacity of the other sender. We prove the security
of the protocol against general quantum attacks, analyze different strategies
of the eavesdropper and compute the key rate for a range of noisy quantum
channels.
