  We study communication over a Multiple Access Channel (MAC) where users can
possibly be adversarial. The receiver is unaware of the identity of the
adversarial users (if any). When all users are non-adversarial, we want their
messages to be decoded reliably. When a user behaves adversarially, we require
that the honest users' messages be decoded reliably. An adversarial user can
mount an attack by sending any input into the channel rather than following the
protocol. It turns out that the $2$-user MAC capacity region follows from the
point-to-point Arbitrarily Varying Channel (AVC) capacity. For the $3$-user MAC
in which at most one user may be malicious, we characterize the capacity region
for deterministic codes and randomized codes (where each user shares an
independent random secret key with the receiver). These results are then
generalized for the $k$-user MAC where the adversary may control all users in
one out of a collection of given subsets.
