# Denial-of-Service Attacks on Permissioned Blockchains: A Practical Study
## 1 Introduction  
Blockchain technology represents a paradigm shift in how digital trust is established across various industries. Initially developed to support cryptocurrencies, blockchain has evolved into a foundational technology for decentralized applications (dApps), smart contracts, and enterprise systems [1]. At its core, a blockchain operates as a distributed, immutable ledger maintained through consensus between a network of nodes. This decentralized structure ensures tamper resistance, transparency, and fault tolerance, making it vital for applications requiring integrity and trust. Blockchain networks are broadly categorized into public (permissionless) and permissioned types. Public blockchains, such as Bitcoin and Ethereum, use open participation and consensus mechanisms like Proof of Work (PoW) [2] and Proof of Stake (PoS) [3]. However, they suffer from privacy limitations, as transaction data is visible to all participants [4]. Permissioned blockchains address this issue by restricting access to authorized users, thereby offering confidentiality and regulatory compliance. Furthermore, permissioned systems can employ more efficient consensus mechanisms, achieving higher throughput and lower latency compared with their public counterparts [5]. They may be further categorized into private and consortium blockchains, depending on the governance model [6]. These features have made permissioned blockchains particularly attractive for enterprise applications in finance, supply chains, and healthcare [7]. Despite these advantages, permissioned blockchain systems remain vulnerable to cyber threats. Denial-of-Service (DoS) attacks, in particular, pose a serious risk to network availability. Public blockchains derive resilience from their decentralization and scale, while permissioned networks, with their relatively smaller and more centralized infrastructure, may become single points of failure under attack [8]. DoS attacks may be executed through communication flooding, invalid transactions, or resource exhaustion, thereby degrading the performance or availability of the network [9]. Although DoS attacks have been widely studied in the context of public blockchains, the literature on permissioned platforms is sparse and fragmented. Most prior studies focus on public-blockchain-specific attack vectors, such as incentive manipulation in PoW systems [10], speculative execution in smart contracts [11], or selfish mining coupled with Sybil identities [12]. These rely on assumptions that are not applicable to permissioned systems, such as anonymous validators, open participation, and financial incentives, which limits their relevance. A key research gap, therefore, lies in systematically analyzing DoS threats in permissioned environments, where system architecture, access control, and consensus models differ fundamentally from public networks. To address this gap, we focused on Hyperledger Fabric (HLF), a widely adopted, modular permissioned blockchain framework. Used in projects such as IBM Food Trust [13], Walmart's supply chain traceability [14], and healthcare data sharing with HealthVerity [15], HLF is a highly relevant case study for assessing the real-world impact of DoS threats in enterprise settings. While some prior work has explored performance bottlenecks and consensus behavior in HLF [16], few studies have designed and executed controlled experiments to demonstrate specific DoS vulnerabilities in different HLF components. Moreover, existing literature lacks insight into how configurations (e.g., Raft consensus tuning, MSP settings) may be exploited to launch targeted attacks, leaving a blind spot for both researchers and practitioners. The contribution of this paper is thus twofold: (1) we experimentally demonstrated a series of DoS attack vectors that targeted HLF's orderers, peers, consensus mechanism, and CouchDB state database, and (2) we analyzed the impacts of these attacks on key performance metrics.