Blockchain clients are the fundamental element of blockchain network, each keeping a copy of the blockchain’s ledger. They play a crucial role in ensuring the network’s decentralization, integrity, and stability. As complex software systems, blockchain clients are not exempt from bottlenecks. Some bottlenecks create new attack surfaces, where attackers deliberately overload these weak points to congest client’s execution, thereby causing denial of service (DoS). We call them manipulatable bottlenecks. Existing research primarily focuses on a few such bottlenecks, and heavily relies on manual analysis. To the best of our knowledge, there has not been any study proposing a systematic approach to identify manipulatable bottlenecks in blockchain clients.

To bridge the gap, this paper delves into the primary causes of bottlenecks in software, and develops a novel tool named ThreadNeck to monitor the symptoms that signal these issues during client runtime. ThreadNeck models the clients as a number of threads, delineating their inter-relationship to accurately characterize client’s behavior. Building on this, we can identify the suspicious bottlenecks and determine if they could be exploited by external attackers. After applying ThreadNeck to four mainstream clients developed in different programming languages, we totally discover 13 manipulatable bottlenecks, six of which are previously unknown. At the time of writing, three CVEs have been assigned.