Abstract
Federated learning (FL) systems are vulnerable to malicious clients that submit poisoned local models to achieve their adversarial goals, such as preventing the convergence of the global model or inducing the global model to misclassify some data. Many existing defense mechanisms are impractical in real-world FL systems, as they require prior knowledge of the number of malicious clients or rely on re-weighting or modifying submissions. This is because adversaries typically do not announce their intentions before attacking, and re-weighting might change aggregation results even in the absence of attacks. To address these challenges in real FL systems, this paper introduces a cutting-edge anomaly detection approach with the following features: i) Detecting the occurrence of attacks and performing defense operations only when attacks happen; ii) Upon the occurrence of an attack, further detecting the malicious client models and eliminating them without harming the benign ones; iii) Ensuring honest execution of defense mechanisms at the server by leveraging a zero-knowledge proof mechanism. We validate the superior performance of the proposed approach with extensive experiments.
Qifan Zhang
Senior Staff Researcher
Dr. Qifan Zhang (张起帆) is now a Senior Staff Researcher of Palo Alto Networks. His research focuses on safeguarding critical internet infrastructure and addressing emerging threats in networked systems. His work centers on Network Security, with deep expertise in the Domain Name System (DNS)—the backbone of internet communication. By combining protocol analysis, fuzzing techniques, and formal methods, he designs automated tools to uncover high-risk vulnerabilities in DNS implementations and standards.
One of his flagship projects, ResolverFuzz, is a novel testing framework that exposed critical flaws in widely deployed DNS resolvers, including protocol-level security gaps (e.g., cache poisoning) and implementation errors (e.g., memory corruption). These discoveries have directly strengthened cybersecurity practices for the industry, open-source communities, and public infrastructure providers, earning recognition from organizations like CERT/CC and CVE.
Beyond DNS, he also explores the intersection of AI and Security, investigating risks in real-world machine learning deployments. My research, published in ACSAC 2022, demonstrated the first practical model extraction attacks against autonomous vehicle (AV) systems, using gradient-descent-based methods to reverse-engineer proprietary AI models. This work underscores the urgent need for robust defenses in safety-critical AI applications.
Prior to Palo Alto Networks, he earned his Ph.D. in Computer Engineering from University of California, Irvine advised by Prof. Zhou Li in 2025, and B.Eng. in Computer Science and Technology from ShanghaiTech University in 2020, complemented by a summer session at the University of California, Berkeley in 2017.
Pronunciation of his name: Chee-Fan Jang.
His Curriculum Vitae (last updated on March 14, 2025)