Distributed Ledger Technology (DLT) is one of the most promising technology at present with applications in different domains, such as Cyber-Physical Systems (CPS) and Internet of Thing (IoT). IOTA is an example of Directed Acyclic Graph (DAG)-based DLTs applicable to IoT, addressing the issues of scalability, consensus complexity, and mining incentives of blockchains. In order to join the network and to later disseminate fresh information, nodes exploit IOTA's autopeering module. The security of this module is hence of fundamental importance for the network liveness, however this requirement has never been analyzed in the literature. In this paper, we provide the first security analysis of the autopeering module of the Coordicide, IOTA's main framework. We divide the contribution in two parts: i) we show that the peer-discovery process in the autopeering module is vulnerable to flooding attack, a type of Denial of Service (DoS), and ii) we propose Time Constrained Identity Match (TID), a novel protocol to mitigate the flooding attack. In particular, we compare two implementations of TID: leaky-TID based on leaky bucket, and token-Tid based on token bucket. We discuss their suitability to IOTA both in terms of effectiveness in mitigating the flooding attack and in terms of resource consumption (memory and CPU). Based on the results, we observe that leaky-TID shows the better performance in preventing flooding attacks, reducing the traffic under attack by 67% and reducing memory consumption by 12.5%.

Blockchains are currently deployed in a large number of different scenarios, such as cryptocurrencies, communications, factory automation, and vehicular networks. Both academia's and industry's interest in this technology is motivated by its distributed nature and its inherent security in managing information. However, traditional blockchains are limited in terms of the transaction rate they can approve. To overcome this limitation, IOTA proposed a novel blockchain based on a Directed Acyclic Graph (DAG) structure (the Tangle). It handles a larger number of transactions to target high-traffic scenarios such as Internet of Things (IoT). However, the security of the IOTA's blockchain depends on the design of its underlying communication protocols. In this paper, we analyze the security of the IOTA Tangle from a transport layer perspective. In particular, we show that well-known port-based attacks can jeopardize the integrity and availability of the IOTA services. We focus on the IOTA's TCP-based ports, and provide a discussion of all the possible attacks and their effects. We assess their validity via experimental evaluation showing their effect on the network operations. Results show that SYN flooding affects the gossip protocol by fully disrupting the service. We also find that 16% of the nodes are unable to create a neighbours list, leading to a disruption of the information exchange process. On the other side, we also show that some components are not affected by these attacks. To the best of our knowledge, we are the first to identify and validate these vulnerabilities in IOTA. Lastly, we discuss some possible countermeasures.