  In this paper, we analyze pilot contamination (PC) attacks on a multi-cell
massive multiple-input multiple-output (MIMO) network with correlated pilots.
We obtain correlated pilots using a user capacity-achieving pilot sequence
design. This design relies on an algorithm which designs correlated pilot
sequences based on signal-to-interference-plus-noise ratio (SINR) requirements
for all the legitimate users. The pilot design is capable of achieving the SINR
requirements for all users even in the presence of PC. However, this design has
some intrinsic limitations and vulnerabilities, such as a known pilot sequence
and the non-zero cross-correlation among different pilot sequences. We reveal
that such vulnerabilities may be exploited by an active attacker to increase PC
in the network. Motivated by this, we analyze the correlated pilot design for
vulnerabilities that can be exploited by an active attacker. Based on this
analysis, we develop an effective active attack strategy in the massive MIMO
network with correlated pilot sequences. Our examinations reveal that the user
capacity region of the network is significantly reduced in the presence of the
active attack. Importantly, the SINR requirements for the worst-affected users
may not be satisfied even with an infinite number of antennas at the base
station.
