Modern software systems are increasingly relying on dependencies from the ecosystem. A recent estimation shows that around 35% of an open-source project’s code come from its depended libraries. Unfortunately, open-source libraries are often threatened by various vulnerability issues, and the number of disclosed vulnerabilities is increasing steadily over the years. Such vulnerabilities can pose significant security threats to the whole ecosystem, not only to the vulnerable libraries themselves, but also to the corresponding downstream projects. Many Software Composition Analysis (SCA) tools have been proposed, aiming to detect vulnerable libraries or components referring to existing vulnerability databases. However, recent studies report that such tools often generate a large number of false alerts. Particularly, up to 73.3% of the projects depending on vulnerable libraries are actually safe. Aiming to devise more precise tools, understanding the threats of vulnerabilities holistically in the ecosystem is significant, as already performed by a number of existing studies. However, previous researches either analyze at a very coarse granularity (\eg~without analyzing the source code and constraints) or are limited by the study scales. This study aims to bridge such gaps. In particular, we collect 44,450 instances of <CVE, upstream, downstream> relations and analyze around 50 million invocations made from downstream to upstream projects to understand the potential threats of upstream vulnerabilities to downstream projects in the Maven ecosystem. Our investigation makes interesting yet significant findings with respect to multiple aspects, including the reachability of vulnerabilities, the complexities of the reachable paths as well as how downstream projects and developers perceive upstream vulnerabilities. We believe such findings can not only provide a holistic understanding towards the threats of upstream vulnerabilities in the Maven ecosystem, but also can guide future researches in this field.