Trigger.dev is a platform for building and deploying fully managed AI agents and workflows. Prior to 4.5.6, POST /api/v1/deployments/:deploymentId/background-workers calls CreateDeploymentBackgroundWorkerServiceV4.call() in apps/webapp/app/v3/services/createDeploymentBackgroundWorkerV4.server.ts, where workerDeployment.findFirst() selects a deployment by friendlyId without an environmentId predicate. A caller with a valid API key for one project can submit another project's deployment identifier, link an attacker-owned background worker to the victim deployment, and move the victim deployment from BUILDING to DEPLOYING. This issue is fixed in version 4.5.6.