Plex Media Server systems are actively being abused by DDoS-for-hire services as a UDP reflection/amplification vector in Distributed Denial of Service (DDoS) attacks. Netscout says that amplified PMSSDP DDoS attacks observed since November 2020 have been abusing. The total number of attacks from Jan 1, 2020, to present day, clocked in at approximately 5,700 (compared to the more than 11 million attacks in total we saw during the same time frame)
Source: https://www.bleepingcomputer.com/news/security/plex-media-servers-actively-abused-to-amplify-ddos-attacks/