  Frequency-multiplexing is an effective method to achieve resource-efficient
superconducting qubit readout. Allowing multiple resonators to share a common
feedline, the number of cables and passive components involved in the readout
of a qubit can be drastically reduced. However, this improvement in scalability
comes at the price of a crucial non-ideality -- an increased readout crosstalk.
Prior works have targeted building better devices and discriminators to reduce
its effects, as readout-crosstalk-induced qubit measurement errors are
detrimental to the reliability of a quantum computer. However, in this work, we
show that beyond the reliability of a system, readout crosstalk can introduce
vulnerabilities in a system being shared among multiple users. These
vulnerabilities are directly related to correlated errors due to readout
crosstalk. These correlated errors can be exploited by nefarious attackers to
predict the state of the victim qubits, resulting in information leakage.
