Abstract
The advent of shared, cloud-based quantum computers introduces critical security vulnerabilities. This work identifies and demonstrates two novel attacks against the important HHL algorithm. The two attacks are the Improper Initialization Attack (IIA) and the Higher Energy Attack (HEA), and this work shows that both can be abused to cause HHL to output incorrect results. To address this new threat, this work presents design and implementation of a novel, low-overhead defense circuit for HHL. By adding a single ancilla qubit and minimal gates, the proposed defense reliably detects both IIA and HEA regardless which qubits (ancilla, clock, b) they target. The proposed defense is validated in simulation and on IBM quantum hardware, demonstrating its effectiveness and resilience to noise, providing a practical pathway to securing HHL against the two types of attacks.
