node-opcua is an OPC UA implementation for TypeScript and Node.js. Prior to node-opcua-client 2.145.0, the internal fieldsToJson method in packages/node-opcua-client/source/alarms_and_conditions/client_alarm.ts directly assigns unsanitized field names and allows a __proto__.pollutedKey path to modify Object.prototype. Successful exploitation requires an application to expose attacker-controlled event fields to fieldsToJson and may cause denial of service or application logic corruption. This vulnerability is fixed in 2.145.0.