  We study the electronic structure near impurities in the one-dimensional
chains of YBa$_2$Cu$_3$O$_{6+x}$. Assuming chain superconductivity below $T_c$
due to a proximity coupling, we show that only a magnetic impurity induces
resonance states in the local density of states. Its spatial structure reflects
the particle-hole nature of chain superconductivity and thus distinguishes it
from other broken symmetry phases. For a sufficiently large scattering
strength, or due to interference effects arising from a second impurity, the
chains undergo a quantum phase transition to a polarized state.
