This paper examines the problem of ensuring the security of distributed ledger systems in digital infrastructures of a smart city. The following properties are characteristic of such smart city subsystems: blurring of the information perimeter, self-organization and mobility of network nodes, as well as low computing resources and disk space for most devices. In such conditions, to protect distributed ledger systems from attacks related to transaction forgery, it is proposed to use a directed acyclic graph supplemented by the developed trust model. The main feature of the proposed trust model is the maintenance of stable operation of the distributed ledger of a smart city even in conditions when more than half of the nodes in the network are malicious. The paper also presents a demonstration of the proposed solution using the example of a distributed ledger of a smart intersection.