Peer-assisted delivery network (PDN) can significantly reduce the bandwidth cost incurred by traditional CDN services. However, it is unclear whether they have been deployed extensively and their security implications have never been investigated thoroughly. In this paper, we report the first effort to address this issue through an automatic pipeline to discover real-world PDN services and their customers, and a PDN analysis framework to test the potential security and privacy risks of these services. Our results have revealed the extensive adoption of PDN across the Internet, especially by Chinese video platforms. Most importantly, our analysis on these PDN services has brought to light a series of novel security vulnerabilities, i.e., free riding of PDN services, video segment pollution, and unreported privacy risks, i.e., resource squatting and extensive leakage of video viewers' IPs. We have responsibly disclosed these security risks to relevant PDN providers which in turn have well acknowledged our findings.