Q-Defender: Admission-time mitigation of persistent coordinated sybil attacks in Semantic-ISAC healthcare systems

Semantic Integrated Sensing and Communication (Semantic-ISAC) is emerging as a key enabler of next-generation Smart Healthcare by supporting on-demand coordination and resource-efficient operation in dense IoMT environments. As these systems grow in scale and complexity, they face increasingly sophisticated threats, including fabricated identities, replayed credentials, impersonation, and Advanced Persistent Coordinated Sybil (APC-Sybil) attacks that can undermine clinical reliability. This paper presents Q-Defender, an admission-time security framework that combines PQC-supported anonymous identity attestation with semantic-temporal coordination analysis to detect persistent Sybil behavior before network access is granted. Q-Defender provides hardware-rooted device onboarding, unlinkable anonymous admission, and lightweight secure communication for medical telemetry. By analyzing the timing and behavioral characteristics of attestation requests, the Semantic-ISAC gateway detects coordinated Sybil formations, identity forgery, replay attempts, and anomalous device behavior before admission. Experimental results show gateway-side decisions within 19.6–32.4 ms and approximately 228 ms end-to-end session establishment for admitted devices, with 1.6 KB communication overhead, a 99.89% Authentication Success Rate (ASR), and a 99.43%–99.53% Sybil Detection Rate (SDR) across default-threshold and mixed APC-Sybil evaluations. Q-Defender therefore enables semantically informed admission with post-quantum credential and session protection, improving the reliability of Semantic-ISAC Smart Healthcare Ecosystems.