  We study the problem of quantum hacking, which is the procedure of
quantum-information extraction from and installation on a quantum network given
only partial access. This problem generalizes a central topic in contemporary
physics -- information recovery from systems undergoing scrambling dynamics,
such as the Hayden--Preskill protocol in black-hole studies. We show that a
properly prepared partially entangled probe state can generally outperform a
maximally entangled one in quantum hacking. Moreover, we prove that finding an
optimal decoder for this stronger task is equivalent to that for
Hayden--Preskill-type protocols, and supply analytical formulas for the optimal
hacking fidelity of large networks. In the two-user scenario where Bob attempts
to hack Alice's data, we find that the optimal fidelity increases with Bob's
hacking space relative to Alice's user space. However, if a third neutral
party, Charlie, is accessing the computer concurrently, the optimal hacking
fidelity against Alice drops with Charlie's user-space dimension, rendering
targeted quantum hacking futile in high-dimensional multi-user scenarios
without classical attacks. When applied to the black-hole information problem,
the limited hacking fidelity implies a reflectivity decay of a black hole as an
information mirror.
