Open TCP/UDP ports are traditionally used by servers to provide application services, but they are also found in many Android apps. In this paper, we present the first open-port analysis pipeline, covering the discovery, diagnosis, and security assessment, to systematically understand open ports in Android apps and their threats. We design and deploy a novel on-device crowdsourcing app and its server-side analytic engine to continuously monitor open ports in the wild. Over a period of ten months, we have collected over 40 million port monitoring records from 3,293 users in 136 countries worldwide, which allow us to observe the actual execution of open ports in 925 popular apps and 725 built-in system apps. The crowdsourcing also provides us a more accurate view of the pervasiveness of open ports in Android apps at 15.3%, much higher than the previous estimation of 6.8%. We also develop a new static diagnostic tool to reveal that 61.8% of the open-port apps are solely due to embedded SDKs, and 20.7% suffer from insecure API usages. Finally, we perform three security assessments of open ports: (i) vulnerability analysis revealing five vulnerability patterns in open ports of popular apps, e.g., Instagram, Samsung Gear, Skype, and the widely-embedded Facebook SDK, (ii) inter-device connectivity measurement in 224 cellular networks and 2,181 WiFi networks through crowdsourced network scans, and (iii) experimental demonstration of effective denial-of-service attacks against mobile open ports.